Difference between revisions of "Science: Topics Electromagnetism"
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Revision as of 06:43, 24 August 2012
Scope of this document
The following note is a background document for teachers. It summarises the things we will need to know. This note is meant to be a ready reference for the teacher to develop the concepts in electricity and magnetism from Class 6 onwards to Class 10.
This document attempts to cover all the topics identified in the concept map. To plan the actual lessons, the teacher must use this in connection with the theme plan.
- 1 Concept Map
- 2 Theme Plan
- 3 Syllabus
- 4 Curricular Objectives
- 5 Electric Charge, Conductors and Insulators
- 5.1 Structure of the atom
- 5.2 Charging an object, flow of charges, conductors and insulators
- 5.3 Static Electricity in action
- 5.4 Electrostatic force and Coulomb's Law
- 5.5 Inverse Square Law
- 5.6 Key Vocabulary
- 5.7 Additional web resources
- 6 Electric Field
- 7 Current Electricity
- 8 Magnetism
- 8.1 Electromagnetism
- 8.2 Nature of the magnetic forces
- 8.3 The Earth's Magnetism
- 8.3.1 Features of Earth's Magnetic Field
- 8.3.2 Magnetic Elements
- 8.3.3 Global and Temporal Variation in Earth's Magnetic Field
- 8.3.4 Magnetisation and Magnetic Intensity
- 8.3.5 Intensity of magnetising field (H)
- 8.3.6 Intensity of magnetisation (I)
- 8.3.7 Magnetic induction
- 8.3.8 Magnetic susceptibility
- 8.4 Magnetic Properties of Material
- 8.5 Diamagnetic Substance
- 8.6 Paramagnetic Substance
- 8.7 FerromagneticSubstance
- 8.8 Hysteresis
- 8.9 Key Vocabulary:
- 8.10 Additional Web Resources
- 9 Magnetic Effects of Electric Current
- 10 Electromagnetic induction
- 10.1 Faraday's law
- 10.1.1 Magnetic flux
- 10.1.2 The Experiment of Faraday and Henry
- 10.1.3 Faraday's laws of electromagnetic induction.
- 10.1.4 Lenz's Law
- 10.1.5 Methods of producing induced emf
- 10.1.6 Motional e.m.f and Faraday's Law
- 10.1.7 Lenz's Law and Energy Conservation
- 10.1.8 Fleming's Right Hand Rule
- 10.1.9 Eddy Currents
- 10.1.10 Application of Eddy Currents
- 10.1.11 Induction Furnace
- 10.1.12 Electronic Brakes
- 10.2 Self Induction
- 10.3 Alternating Currents
- 10.4 AC Generator or Dynamo
- 10.5 Transformer
- 10.1 Faraday's law
- 11 Activities
- 11.1 Activity 1: Repelling Strings
- 11.2 Activity 2: Charge of the light balloons
- 11.3 Activity 3: Rub the tube
- 11.4 Activity 3: Electroscope
- 11.5 Activity 4: Conductors
- 11.6 Activity 5: Conductivity
- 11.7 Activity 6: A current is a magnet; a magnet is a current
- 11.8 Activity 7: Electromagnet
- 11.9 Activity 8: Make a magnet, break a magnet
- 11.10 Activity 9: Totally tubular magnets
- 11.11 Activity 10 – PhET Magnetism
- 11.12 Activity 11: Simple Battery
- 11.13 Activity 12: Electromagnetism
- 11.14 Activity 13: Faraday's Electromagnetic Lab – AC/DC Current and Electromagnetism
Charges, static electricity, electric current, conductors, insulators, simple circuits
Charge is intrinsic to matter. There are two types of charges. The charges at rest constitute static current, and the charges in motion constitute electric current. Electricity needs a medium to travel, which is called as conductor. The path of flow of charges is circuit.
1.The students recall the two types of charges. 2.The students recognise the accumulation of charges in the activity, 3.The students can recognise the necessity of a condctor to the flow of charges. 4.The students can differentiate between conductors and insulators.
1.The students can perform activities to accumulate charges. 2. They can identify the conductors and insulators in the given materials 3.They can draw the diagrams to represent simple circuits.
ACTIVITY 1 ACTIVITY 2 ACITIVITY 3
The important effects of electricty, Electric appliances,non contact forces-charges And magnets
When current flows through the substances, it shows chemical magnetic effect and heating effect. Magent attracts magnetic materials. Force of magnets is non- contact force .
1.The students identify the different effects shown by electricity in some situations 2.The students recall the different effects of electricity 3.The students give examples for magnetic materials 4.The studetns can idntify the non contact forces such as magnetic force
1.The students perform activities show the heating effect of electricity. 2.The students differentiate between magnetic and nonmagnetic materials In the given objects. 3.The students can perform experiment to show the attractive propery of magnets 4. The students show the existance of non contact forces
ACTIVITY4 ACTIVITY5 ACTIVITY 6
Magnets and their properties, magnetic field lines Magnetic materials,some fields where magnets are used,power and electric power, sources of electricity,unit of electric energy
Magnets attract magnetic materials,it sets itself in N-S direction when suspended freely. It has two poles, like poles repel and unlike poles attract, poles cannot be separated. Both poles are equal In strength.The strength of the magnet is more near the poles, we use magnets in many fields of life,power is rate of work done and as electricity does work, electric power is rate at which it does Work. Electricity energy consumed is the product of power and time
1.The students recall the properties of the magnets. 2.The students understand the applications of magents. 3.They will be able to identify the strentgh of magnets at various palces around the magents 4.They will be able to recall definitions of work and electric energy consumed 5.They will be able to recall the units of work and electric energy consumed 6.They will recall the different sources of electricity
1.The students can perform activities to find out the properties of magnets. 2. The students differentiates between magnets between the magnets and Non magnetic materials using the properties of the magnets 3. Can design simple devices which work on the properties of the magnets
ACTIVITY7 ACTIVITY8 ACTIVITY 9 ACTIVITY10
Magnetic field lines and the property of magnetic field lines,relation between electricity and magnetic Field. Oersted's experiment,pattern of magentic Fields in various situations,elecric potential and Potential diference, electric current necessity of electriomagnetic force in the flow of current.
Magnetism is due to charges in motion. Unlike charges, magnetic monopoles do ot exist.
1. The students identify magnetic field lines around diffetn shapes of magents. 2. The students recall the properties of magnetic lines of force 3. The students can identify the relation between magentism and electricity 4. They can recall Ohm's law 5. They can define potential,potential difference,electric current and emf.
1. Calculate voltage, current and resistance using Ohm’s law. 2. Analyze circuit diagrams for series circuits and parallel circuits. 3. Can calculate the electric energy consumed in various situations 4. Solve equations that relate electric power to current, voltage and electrical Energy.
ACTIVITY11 ACTIVITY12 ACTIVITY 13 ACTIVITY14
Ohm's law, concept of resistance and resistivity, combination of resistances,electric energy consumed,heat produced, steps to save electricity.
The flow in unit time is called electric current. In a conductor the flow also depends on the resistance of the wire. The resistance of a Wire is made useful in many situations,specially to produce heat. As electric current is the best form of energy, it should be saved.
6.The students list the factors on which resistance depeds 7.The students will identify the heat produced when current flows through resistances 8.They describe resistivity. 9.They students briefly expain the methods to save electric current
5. Describe devices and procedures for maintaining electrical safety. 6.They draw simple circuits . 7. The students perform activity to prove Ohm's law. 8.They obtaining the skill of calculating the problems on Ohm's law
Electromagnetic Induction,Faraday' laws,AC and DC, fleming's rules,AC and DC dynamo, Types of Electromagnetic induction,Eddy currents and Lenz' law.
When magnetic field inked with the conductor changes an emf is induced in thre conductor. The induced emf depends on rate of change magnetic field and no of turns in the coil.
1 .The students observe the experimental evidence for electromagnetic induction. 2.They understand the circumstances under which changing magnetic fields lead to
induced currents. 3.They understand how the movement of a conductor through a magnetic field leads to an induced emf.
4.They understand and use Lenz’s law for induced currents.
1.The students can perform activities to prove Farady's experiment on
ACTIVITY15 ACTIVITY16 ACTIVITY 17 ACTIVITY18 ACTIVITY19 ACTIVITY20 ACTIVITY 21
Induction, solenoid, field lines in solenoid,motor and Its applications.Domestic circuits, Steps to save electricity
The direction of emf is perpendicular to magnetic field. The advantages of AC in power generation; why we do not generate using DC. The possibility of voltage step up and down.
5.They use Lenz’s law and Faraday’s law to determine the direction and size of Induced currents. 6.They recall the laws of electromagnetic Induction, fleming's rules. 7. The students differentiate between AC And DC; a motor and a dynamo 9. The students will mention the use of dynamos and motors 10.The students will be able to describe the methods to save electricity.
2. The students can draw the diagrams of AC dynamo,DC dynamo, 3. They can represent AC and DC with graph. 4.They can construct simple Dynamo 5. They can construct simple motor 6. They can explain the working of Dynamo and Motor 7. They can perform activity to expain lenz's law
- Insulators and Conductors – Meaning, types, functions and examples; will understand the concept of conductor.
- Pattern of domestic wiring and how it is done
- Electrolysis of water and diagram of an electrolytic cell
- Simple and complex electrical circuits and understanding safety measures
- Elecricity, Ohm's law
- Effects of electric energy
- Electromagnetic radiation, spectrum and uses
- Electromagnetic induction, Faraday's laws, understand the functioning of a DC motor and AC dynamo
- The children are introduced to the following concepts:
- Forces can act at a distance, and the idea of force fields to explain forces act at a distance. The presence of four fundamental forces is introduced. They have already studied the gravitational force; now the electromagnetic force will be introduced.
- There is another intrinsic property of matter - that of charge. (Earlier, they were introduced to the intrinsci property of mass). There are two types of charges – positive and negative.
- Just like mass has two aspects – inertial and gravitational, charge also exhibits two kinds of effects – when they are stationary and when they are moving. Static charges result in electrostatic force and when a charge is moving, it exerts a magnetic force. They are also introduced to the fact that while gravitational force is always attractive, electrostatic force can be repulsive as well as attractive.
- Our understanding is based on the atomic structure - what constitutes positive and negative charge. Charge is conserved and is quantized. They will also learn about movement of charges, conductors, insulators and how to charge objects.
- They will be introduced to the mathematical representation of Coulomb's law and the equation that describes how the electrostatic force acts.
- The children are introduced to the idea of work done in an electric field and this is what we call as the potential difference between two points. They will learn about the electric potential at a point.
- Flow of charges is electric current and there is resistance to the flow of current in a conductor. They will learn the relationship between potential difference and flow of current in an ohmic conductor (Ohm's Law). They will also be able to calculate the effective resistance when resistors are connected in series and parallel
- They will be able to explain the working of a battery operated simple circuit. They will also be introduced to the idea of EMF and be able to distinguish it from the terminal voltage when there is a current flowing in the circuit.
- There are objects, called magnets, that have the ability to repel and attract other similar objects. Magnetic forces are also forces that act at a distance and we need the idea of force fields to explain magnetic forces. The properties of magnetic field lines and the Earth's magnetic field are discussed.
- The magnetic effect of electric current will be discussed and the objective is to introduce the idea that electricity and magnetism are not two different kinds of forces. Rather, they are different aspects of the same force. For example, a current carrying wire behaves like a magnet.
- The children are introduced to properties of electromagnets, behaviour of current carrying conductors in a magnetic field and the operation of the motor.
- The opposite effect is also explained – the change in magnetic field resulting in the generation of a potential difference – electro magnetic induction. This is at the core principle of power generation. They are briefly introduced to the idea of alternating EMF, peak voltage and transformers.
- The students must be able to understand the consumption of electric energy, units of energy consumed (kWh) and the cost of electricity.
Electric Charge, Conductors and Insulators
Charge is an intrinsic property of matter, just as mass is. All matter is made up of charge, in fact a vast quantity of it. The only reason we do not notice this charge is because there are two kinds of charges and most objects contain an equal amount of these two kinds of charges. These two kinds of charges are equal and opposite in nature, such that an equal amount of these two types of charges neutralize each other. These two types of charges are called, rather arbitrarily, positive and negative charges. (Benjamin Franklin, of lightning fame, coined these terms).
When these two charges are equal, the object is electrically neutral and there is no net charge on the object. When this balance is disturbed, a net charge develops on the object. In one of the earlier discoveries, people noticed that when similar objects with a net charge were brought together, they repelled each other; while when dissimilar objects with a net charge were brought together, they attracted each other. This led to one key finding about electric charges – like charges repel each other and unlike charges attract each other.
This attraction or repulsion is a result of forces that these charges exert on each other. This force, operating between two charges, when they are not in contact with each other, is called electrostatic force. The charges exert an influence on each other through force fields; this explains how forces act at a distance. This electrostatic force operates very similar to the gravitational force, in as much that it is directly proportional to the magnitude of charge and inversely proportional to the distance between them. One key difference between gravitational force and electrostatic force is that gravitational force is always attractive while electrostatic force may be attractive or repulsive. This follows obviously because there is only one kind of mass whereas there occurs in matter two kinds of charge – positive and negative.
Structure of the atom
The origin of these electric charges lies in the atom.
The smallest particle of any element, and of all mass, is the atom. An atom consists of three main sub-atomic particles – electrically neutral neutrons, positively charged protons and negatively charged electrons. The neutrons and positively charged protons are held together in the nucleus (held together by strong and weak nuclear forces) and the electrons revolve around this nucleus in fixed orbits. The magnitude of charge on the proton and electron are equal. The number of protons is equal to the number of the electrons in an atom and this results in the atom being electrically neutral.
The electonic charge has a magnitude of 1.6X10-19 C. This is the smallest amount of charge that has been identified and all other charges are multiples of this charge. Charge is not continuous but is quantized. Since “e” is so small, 1.6 x 10-19 C, we see flow of charges as continuous.
'Charge is quantized as a multiple of the electron or proton charge':
Electrons are the smallest and lightest of the particles in an atom. Electrons are in constant motion as they circle around the nucleus of that atom. Electrons are said to have a negative charge, which means that they seem to be surrounded by a kind of invisible force field. This is called an electrostatic field.
Protons are much larger and heavier than electrons. Protons have a positive electrical charge. This positively charged electrostatic field is exactly the same strength as the electrostatic field in an electron, but it is opposite in polarity. Notice the negative electron (pictured at the top left) and the positive proton (pictured at the right) have the same number of force field lines in each of the diagrams. In other words, the proton is exactly as positive as the electron is negative.
To help express the mass of these subatomic particles, we take the example of the simplest atom – that of hydrogen. A hydrogen atom consists of a single proton and a single electron. The hydrogen atom does not contain a neutron. The mass of the proton and neutron are almost the same and is equal to the mass of one hydrogen atom. The mass of the electron is negligible and is equal to (1/1837)th of the mass of one hydrogen atom. The contribution of electrons to the mass of the atom is negligible.
We saw earlier that like charges repel each other and unlike charges attract each other. This property is at the core of the forces that hold an atom together. The protons held in the nucleus will try to repel each other. However, they are prevented from being thrown apart by the strong nuclear forces which overcome these repulsive forces and hold the nucleus together. These electrons are also held in orbit around the nucleus by electrostatic forces exerted by the positively charged nucleus.
The electrostatic force exerted by the positive nucleus on the negatively charged electrons is what keeps the electrons as a part of the atom. Otherwise, the force that the electron will develop when it is moving might take it outside of the atom. (This is, again, very similar to the gravitational force because of which planetary objects remain in orbits). However, if an electron is situated in an orbit far away from the nucleus, then this electrostatic force is much weaker (because it is inversely proportional to the distance). In such cases, these outermost electrons can be readily “removed” from the atoms and are called free electrons.
Charging an object, flow of charges, conductors and insulators
In one of the early discoveries, some objects were found to have developed attractive and repulsive properties when they were rubbed with one another. It was discovered later that this attractive or repulsive property was a function of charges redistributing/ moving from one object to another.
Charging by friction
When two bodies are rubbed against each other, the free electrons move from one object to another. They move from the atom of the element where the electrostatic force on the electrons is weak to the atom of the element with a higher electrostatic force of attraction. This movement of free electrons is what constitutes electrification of the body. In this process of charging by friction, the object which loses electrons develops a positive charge while the object which gains electrons develops a negative charge. The magnitude of the positive charge will be equal to the magnitude of the negative charge. It must be noted that only the electrons move and not the protons – the development of a positive charge is due to the deficit of electrons.
When a glass rod is charged with silk, the glass rod loses electrons which are transferred to the silk. This gives the glass rod a positive charge (due to deficit of electrons) and silk a negative charge (due to excess of electrons). Similarly, when ebonite is rubbed with fur, the ebonite rod gets negatively charged. Can you explain in terms of flow of electrons?
Movement of charges due to free electrons
We talked earlier of free electrons. Free electrons are free to move within the surface of the material between atoms and are not bound to one atom; they “escape” the orbit of one atom but generally drift around. Normally, these free electrons move about in all directions randomly and have no net flow.
However, when there are a large number of free electrons, they can be made to move in a particular direction by applying a potential difference. In these cases, a large movement of charges is possible. Materials where there are a large number of free electrons are called conductors. It is, therefore, easier to set up a flow of charges in a conductor. Insulators are materials where the number of free electrons is less and consequently, it is more difficult for charges to flow through them.
Ways of charging a conductor
An electrically neutral object may be charged (i.e., given a positive or a negative charge) by conduction or induction.
Charging by conduction:
When two objects are rubbed together, charges, electrons, move from one to another. This results in a deficiency of electrons in one object which gets positively charged. The object which received these electrons develops a negative charge. A conductor can be charged only if it is mounted on an insulated stand.
Charging by induction:
When an electrically charged object is brought near an uncharged object (the object must be a conductor), a distribution of charge happens in the uncharged object. The end of the uncharged object which is closest to the charged object will develop an opposite charge while the farthest end will develop a similar charge to that of the charged object.
For example, if a negatively charged ebonite rod is brought near the a conductor, the end of the conductor closest to the ebonite rod will develop a positive charge while the end of the conductor away from the ebonite rod will develop a negative charge.
These separated charges in the conductor are called induced charges.
The phenomenon due to which an insulated uncharged conductor gets charged when held near a charged body is called electrostatic induction and the charges so produced are called induced charges.
In all of these processes, 'charge is conserved'. It simply gets redistributed from one object to another.
Flow of charges and earthing
When there is a flow of charges, we talk of a current. Current is nothing but a flow of charges. When two objects are rubbed together, charges move from one object to another. But, we cannot usually notice this charging if we hold the two objects in our hand and rub together. While a net positive charge may be created on the object, there will be a flow of electrons from the earth through our hand to the glass rod, neutralizing the positive charge. Similarly, the negative charges on the silk will flow through our hand to the earth. In both the cases, the objects get “discharged” by flow of electrons into and out of them. The earth has acted both as a source and reservoir of charges.
The earth is always electrically neutral because of the huge number of protons and electrons it contains (because it is massive). If a few billion electrons are added or removed, it makes but a small difference to the total charge of the earth. Since, the total electric balance of the earth is not disturbed, it always remains neutral and at zero potential.
Direction of flow of charges
Before the model of the atom was understood, when an object was charged, the direction of movement of charges was not clear. It was assumed that charges flowed from positive to negative. The positively charged object was considered to be at a higher potential. Flow of charges was defined as current and it was assumed to flow from positive to negative. Positive charges were assumed to be at a higher potential than negative charges. After the atomic model was understood, it was clear that the flow of electrons was what constituted current and led to the development of net charge. In other words, electron flow is what constitutes current. While we still show the direction of conventional current in a circuit as positive to negative, the electron flow is in the opposite direction.
Static Electricity in action
One of the most spectacular displays of static electricity is during a thunderstorm. The cloud and the earth surface develop opposite charges due to induction. During a thunderstorm, electric discharge occurs between the negatively charged clouds and the oppositely charged ground or between two clouds which are oppositely charged.
When a charge build-up occurs between oppositely charged surfaces, electric discharge occurs. To prevent this, lightning rods are installed on tall buildings. Lightning rods, which are metal rods, collects the electrons and thereby prevents a large build-up of positive charge on the building. Even if lightning strikes, the metal rod will conduct the electricity quickly down to the Earth preventing any damage to the building.
Electrostatic force and Coulomb's Law
We saw earlier that charges attract or repel other charges due to the electrostatic force. This force can be defined as follows.
The electric force acting on a point charge q1 as a result of the presence of a second point charge q2 is given by Coulomb's Law, which is directly proportional to the magnitude of the charges and inversely proportional to the square of the distance between the charges.
where ε0 = permittivity of space.
The unit of charge is the coulomb, abbreviated C. 1C is the charge associated with 6.25*1018 electrons. The proportionality constant k has the value of 9*109 Nm2/C2. If we had two 1 coulomd charges, they would exert a force on each other equal mto 9*109 N. We do not see such charges in daily life.
Inverse Square Law
Coulomb's law of electrical forces, resembles the Newton's law of gravitation which is used to calculate the magnitude of gravitational force between two masses. Both are inverse-square laws, in which force is inversely proportional to the square of the distance between the bodies. Coulomb's Law has the product of two charges in place of the product of the masses, and the electrostatic constant in place of the gravitational constant. One important point of comparison is that the the value of the constant in Coulomb's law (for force between two charges of 1C separated by a distance of 1 m) is of the order of magnitude 109, which is 1000 billion billion times more than the gravitational constant. This means that the electrostatic force is a much stronger force than the gravitational force. Can you imagine what the mass would have to be for 2 masses at a distance of 1 m to exert a force of 9*109 N.
- Charge – Intrinsic property
- Quantization of charge – Charge occurs in multiples of electronic charge
- Conductors – Materials through which movement of charges is easier
- Insulators – Materials through which movement of charges is more difficult
- Conduction – Method by which a conductor is charged by touching it with a charged object
- Induction – Method by which a conductor is charged by bringing it near a charged object
- Earthing – Establishment of a path by which charges can be transferred to the ground
- Coulomb's Law – The law and the equation that describes the electrostatic force.
Additional web resources
- Basics of Static Electricity This is a good overview of the basics of static electricity.
- Science Object - Electricity This isa very good interactive session on electrostatics and current electricity. You can register at www.nsta.org for free and view all these science objects and many free materials in your online library.
- How lightning strikes This page describes how lightning strikes and how lightning conductors work.
We already saw that electric field operates at a distance, through a force field. Electric field has both direction and magnitude. Electric field at any point around a charge is defined as the electric force per unit charge. This is written as : E = F/ q; where F and E are vectors; in the same direction.
The direction of the field is taken to be the direction of the force it would exert on a positive test charge. The electric field is radially outward from a positive charge and radially in toward a negative point charge.
The strength of the field is given by the number of field lines through a given point. The greater the number of field lines, the stronger the field. And vice versa. The concept of electric field is important in understanding what happens when charges move. When a charged particle moves, it causes a disturbance in the space and this disturbance is communicated through the field. The information travels at the speed of light. This concept is at the core of understanding the electromagnetic force. We will discuss this more when we study the magnetic effects of electric current.
Electric field in a conductor and shielding
Electric fields can be shielded by various materials; this is an important differece between electric fields and gravitational fields. For example, metallic conductors will completely shield the field inside, regardless of the field outside. Let us understand how this happens.
Let us see we have a charged conductor in equilibrium; meaning the charges are not moving. This means that the net charge on the conductor has distributed itself in such a way that the replusive forces are all neutralized.
- In such a situation, there can be no field inside the conductor. Why? Because if there is a field in the conductor, the charges (electrons) would move to redistribute themselves. This violates the initial condition. Hence there can be no field inside the conductor.
- The second effect is that the electric field on the surface of the condutor is always perpendicular to the surface of the conductor. If this is not true, again there would be a movement of charges along the surface of the conductor. This violates our initial assumption.
These two results tell us that the field inside a conductor is zero. This allows us to build a cage, a metallic shield to keep out an electric field.
We saw that the electric field indicates the force that will be experienced by a positive charge. If “E” is the electric field at a point, the force experienced by a charge “q” would be
To move charge “q” from Point A to B in a field work will have to be done. This will result in the change in potential energy of the charged particle as it moves from A to B. The energy possesed by a particle by virtue of its position in an electric field is called electric potential energy. The difference in electric potential energy between two points is represented as a potential difference between the two points in the electric field.
In diagram A, we have to do work against the Electric Field, therefore, the electric potential energy of the charge will increase. By similar reasoning, we will see that in diagram B, the electric potential energy of the charge will decrease.
Let us say now we have A at infinity, where we assume that the charge will experience no force due to the electric field. Therefore, the potential energy of the charge will decrease gradually from B as it moves to A; where A is considered to be at infinity.
The test charge in this case will have electric potential energy at B, which is equal to UB relative to the zero potential energy at A. We must note here that what is meaningful is the difference in potential energy between points B and A. Work is done by or on the charge as it moves from A to B (depending on the direction of the field) and this results in a difference in the potential energy of the charge between these two points.
Now the electric potential energy is a measure dependent on the amount of charge. We will define a more useful term called electric potential. This is the potential energy per unit charge at a point in an electric field.
VB = UB / q
The unit of electric potential is joules/ coulomb and is given a special name, Volt, in honor of Alessandro Volta. The volt is abbreviated to 1 V = 1 J/ 1 C.
Like we mentioned before, what matters is the difference in potential energy.
The difference in potential energy is equal to the negative of the work done, WBA , as the charge moves from A to B.
UBA = UB - UA = WBA
Similarly we will be interested in finding the difference in electric potential between two points in an electric field.
VBA = VB - VA
'Why define electric potential and what is zero potential'
We define the electric potential because it is possible to assign a specific value to a given location in an electric field whether or not there is a charge present there. This is also useful when talking of voltages in a circuit.
We also said that at an infinite distance, the electric potential energy and hence, potential is zero. Often times, we also take the ground or a conductor connected directly to the ground to be at zero potential.
Electric Energy Storage
We saw that a charged particle has electric potential energy by virtue of its position. Electric energy can be stored in a device called a capacitor. When a pair of conducing plates is charged using a battery, it builds up an electric field between its plates. The capacitor plates develop equal and opposite and this acts as an energy storage device. A capacitor is discharged when a conducting path is provided between the plates.
Van de Graff generator
A Van de Graff generator is a hollow sphere that can hold a charge generated by a motor drive n belt. The charges lie on the surface of the sphere and this generator can be build to very high voltages. These voltages are used to accelerate charged particls that can be used as projectiles for penetrating the nuclei of atoms.
- Electric Field: Vector field – has magnitude and direction and gives the direction of force experienced by a unit positive charge.
- Electric Potential Energy: The energy possessed by a charge in an electric field by virtue of its position in the electric field.
- Electric Potential: The electric potential energy per unit charge.
Additional resources :
- Faraday's cage. This is a lecture by Walter Levin, Professor at MIT, demonstrating Faraday's cage
- MIT library This site shows you photographs of a Van de Graff generator
- Walter Levin explains how to build up charges in this video.
Rate of flow of charges, current
We have seen that in an electric field different points will be at different electric potentials. When there is a difference in potential, charges will flow. In this case, charged particles, electrons will flow.
This can be compared to water flow from a reservoir at higher pressure to a reservoir at lower pressure. Water will flow as long as there is a difference in the water levels.
'Electric current is simply defined as the flow of electric charge. The rate of flow of electric charge is measured in amperes, A.'
We saw before that 1 C of charge carries 6.25*1018 electrons. So if we have a wire carrying 5 A, we have 5C of charges passing in one second. A large number of electrons!
For charges to flow, there must be a potential difference maintained. It is possible to maintain a potential difference using two large charged spheres – one positively charged and the other negatively charged. This will not work because once a conducting path is provide, the charges will neutralize in one single discharge. For continuous current flow we need to maintain a steady potential difference.
Voltage sources – batteries and generators
File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html bb112d6.pngA battery or a generator does work to pull electrons from positive charges. In a battery, this is done using chemical reactions; where the energy of the chemical bonds is converted into electrical energy. A generator provides this voltage by electromagnetic induction. We will discuss this in greater detail in the section on electromagnetic induction.
The cell shown here uses dilute sulphuric acid as the electrolyte. One of the electrodes is carbon ; the other is zinc. The acid reacts with the zinc electrode which enters the solution as a positive ion. The zinc electrode becomes negatively charged and the electrolyte becomes positively charged. Electrons are pulled off the copper electrode which becomes positively charged. Thus a potential difference is maintained between the terminals and current can flow when the circuit is completed; when the electrodes are connected. If a charge is allowed to flow between the terminals, after a while, all the zinc will dissolve and the cell will be dead.
We have seen that a potential difference is necessary for a current to flow. But there is one more factor that determines how much current flows; that of the resistance. In the case of the water reservoir
Let us think of a crowded railway platform. How many children can go from one end of the platform to another in a given amount of time depends on how long the platform is and how wide the platform is. Why would the speed of the children be affected? The children will face obstacles – benches, luggage, people, etc. If the platform is wide, there will not be many collisions and the children can move faster. Also, if is a long platform, they will face obstacles for a long time and that will also affect the speed with which they will move.
A similar analogy holds for electric wires. The resistance offered to the flow of charges is due to the collisions with the molecules in the wires. If the wire is of a smaller cross- section, there are more collisions, and hence, higher resistance. If the wire is longer, the electrons will have to travel a longer distance and in that journey face more collisions. This also impacts the resistance of the wire.
The resistance R can be written as follows:
R = ρ L/ A
where L is the length of the wire, A is the cross-section of the wire and ρ is the specific resistivity of the material. Specific resistivity is defined as the resistance offered per unit length per unit cross-section.
Temperature also increases the resistance of a wire, except for carbon. This is so because temperature increases the movement of molecules in the wire and this increases the collisions (with the moving charges) and hence, the resistance increases.
The unit of electrical resistance is Ώ.
For ohmic conductors, and for not very high voltages, the current flowing through a conductor is directly proportional to the potential difference across its ends.
V = IR where R is the resistance of the wire
This holds for “ohmic” conductors where the voltage is not very high.
Resistors in series and parallel
The combination rules for any number of resistors in series or parallel can be derived with the use of Ohm's Law, the voltage law, and the current law.
Speed and source of electrons in a circuit
- Working of a cell This website has an interactive tutorial and an explanation of how cells work
- Introduction to current electricity This website gives an introduction to what typically happens in a wire when current starts flowing and explains very well all the elements of flow of charges – including drift velocity, resistance, Ohm's law and series and parallel circuits
From simple nails being drawn to a magnet to surgery to circuit breakers, magnetism is everywhere. The first magnetic phenomenon observed were those associated with naturally occurring magnets, fragments of iron ore found near the ancient city of Magnesia. These stones called lodestones These attracted unmagnetised iron. The attraction was maximum at certain regions of the magnet called the poles.
The Chinese have known to use Magnetic needles for navigation onFile:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 494fe551.jpg ships since the 12th century. In the 16th century, William Gilbert, Queen Elizabeth's physician made artificial magnets by rubbing pirces of iron against lodestone.
Since then, the magnetic materials have been playing an increasingly important role in our lives. It's therefore necessary to understand the structure of such material.
Shapes of Magnets
The natural magnets i.e., iron ore were irregular in shape and weak. Later it was found that iron or steel acquired magnetic properties on rubbing with a magnet. Such magnets were called artificial or man-made magnet. These magnets have a desired shape and strength.
Magnetism and elecricity were being pursued as independent subjects for a long time until a Danish physicist, Hans Chritian Oersted discovered that an electric current affects a magnetic compass. And this discovery was talked about in the scientific circles and came to Sir Humphrey Davy to investigate. Michael Faraday, a book binder by training was assigned this work to investigate. Not trained formally, Faraday visualized the magnetic force to be acting in the form of field lines.
Why should a current carrying wire deflect a compass? To answer this, we must go back to the fundamental property of charge. We said that charge is an intrinsic property of matter like mass. And just like mass is displayed in inertial and gravitational aspects, charge also possesess two properties – when stationary and when moving.
When there is a stationary charge, it produces around itself a certain effect – an information field called the electric field around it. When the charge is moving it produces an information field, called the magnetic field around it. We can think of electric and magnetic fields as information. We can understand this using an example:
Let us say you are standing on the bank of a river and are giving instructions to someone in next to me to cross the river. It does not matter which direction you are moving when you give the direction. The person receiving the instruction will cross the river.
Similarly, a magnetic field at a point gives informtion to a moving charge to move in a particular way, perpendicular to the velocity and the direction of the magnetic field. So, charges exhibit electrical force when they are not moving and magnetic force when they are moving. Both electrical and magnetic forces are different aspects of the same phenomenon of electromagnetism.
Nature of the magnetic forces
Just like electric forces, magnetic forces were also found to be attractive and replusive. The strength of the forces depends on the separation distance between the two magnets. Magnetic force can also act over a distance. It was found that magnetic poles where magnetic property appeared to be “concentrated” gave rise to these forces.
Poles of a magnet
If you suspend a bar magnet, it was found that it always came to rest in the North-South direction. One end of the magnet (the pole) was south-seeking and the other end of the magnet (the pole) was north-seeking. These were simply called the North and the South poles of the magnet.
When the north pole of one magnet is brought near the north pole of another magnet, they repel. The same is true of a south pole near a south pole. 'Like poles repel and opposite poles attract'.
This property is very similar to the forces of attraction and replusion of electric charges. There is one importtant difference though. An electric charge can be isolated - for instance, just as a positive or a negative charge, whereas magnetic poles cannot be. A magnet always has two poles – even the atom. This suggests that atoms themselves are magnets.
The ordinary magnetic effects in materials are determined by atomic magnetism. On continuing to cut a magnet into its smallest bit, we reach the level of a single atom. This is a tiny current loop in which the current corresponds to the circulation of the electrons in the atom. To this atomic current we associate a magnetic dipole moment. This tiny bit cannot be further divided and hence the dipole is the smallest fundamental unit of magnetism.
A magnetic material can be regarded as a collection of magnetic dipole moments, each with a north and a south pole. Microscopically, each dipole is actually a current loop that cannot be split into individual poles.
Magnetic field lines
The space around the magnet contains a magnetic field. They originate from one pole and return to another. By convention, magnetic field lines were assumed to originate from the North pole and end in the South pole.
The Bar Magnet
When a compass needle is brought near a magnet, the needle always lies along the direction of the field. The figure below shows the lines or pattern of the field, when the compass needle is placed at several places.
These field lines are developed to visualize the effect of the magnetic field. If we imagine a number of small compass needles around a magnet, each compass needle experiences a torque to the field of the magnet. The path along which this compass needles are aligned is known as magnetic field lines.
Properties of Magnetic field lines
Magnetic field lines form closed continuous curves.
- Outside the body of the magnet, their direction is from north to south pole.
- The tangent to these lines at any point gives the direction of the magnetic field at that point.
- No two lines can intersect each other. (Why?)
- The lines of force contract longitudinally and dilate laterally.
- Crowding of magnetic lines of force represents stronger magnetic field and vice-versa.
The following diagram depicts the magnetic lines of force between two north pole, two south pole; NortFile:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m378bcdfd.jpgh-South pole. File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 63b374f3.jpgFile:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 677bab2f.jpg
Solenoid as a bar magnet
The field due to a current in a long coil resembles that due to a bar magnet.
Inserting a soft iron core magnetises the iron and produces an electromagnet. The electromagnet can be made strong or weak by changing the current and the number of coils around the core.
The Earth's Magnetism
A magnetic compass was used to help the sailors for navigational purpose. But recently it has been discovered that some migrant birds have magnetic sensors in their heads, which help to guide them using the Earth's magnetic field.
William Gilbert suggested that Earth itself is a huge magnet from various observations he had made:
- On disturbing a freely suspended magnet it returns quickly to N-S direction. The north pole of this huge magnet must be towards geographic south as to attract South Pole of the suspended magnet.
- Soft iron pieces buried under surface of Earth are found to acquire magnetic properties.
- On mapping magnetic field lines due to bar magnet, we come across neutral points. These points are those where magnetic field of the bar magnet cancel with that of Earth's field. But for the latter, we cannot obtain neutral points.
The exact cause of magnetic field of Earth is not yet known but some important postulates are:
- File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m7c4d4e3d.jpgMagnetic field of Earth may be due to molten charged metallic fluid in core. This rotating fluid results in currents thus magnetising the Earth.
- Since every substance is made up of charged particles, these substances rotating about an axis is equivalent to a circulating current and hence is responsible for the Earth's magnetisation.
- As the earth rotates, strong electric currents are set up due to movement of charged iron (due to showers of cosmic ray). These moving ions magnetise the Earth.
Features of Earth's Magnetic Field
The earth's magnetic field has an axis which is inclined 20o west of the axis of rotation of
earth. The point where this huge earth's magnet cuts the earth's surface are the magnetic poles. A freely suspended magnet has its north pole pointing towards geographic north; we therefore designate the earth's magnetic pole close to geographic north as magnetic south. The same argument follows for the south pole of the freely suspended magnet.
File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m1ef9ebaf.jpgThe magnetic equator divides the earth's surface into two. The field lines enter geographic north and come out of the geographic south.
The strength of the earth's magnetic field is about 10-4 tesla or 1 gauss.
To describe the magnetic field of earth at any place three quantities or elements are required. They are:
- Magnetic declination (q)
- Magnetic inclination (d)
- Horizontal component (BH)
Magnetic declination (q)
Magnetic declination is the angle between magnetic axis and the geographic axis.File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m65aa48a2.jpg
Magnetic dip or Inclination
The angle between the direction of total intensity of Earth's field with the horizontal line in magnetic meridian. It is represented as d.
At poles, the angle of dip = 90o and at the equator, the angle of dip = 0o.
The dip at a place can be determined by an apparatus known as dipcircle as shown below. The needle rotates freely in the vertical plane of scale. The pointed ends move over the graduations on the scale, which are marked 0-0 in the horizontal and 90-90 in the vertical directions.
Horizontal component is the component of the total intensity of Earth's magnetic field in the horizontal direction in magnetic meridian.
Global and Temporal Variation in Earth's Magnetic Field
The dipole pattern of earth's magnetic field is disturbed due to solar winds. Solar winds are a stream of charged particles coming from the sun. These particles ionise the atmosphere above these poles which display a light high up in the atmosphere. This phenomenon occurring in the arctic region is called aurora borealis or northern lights and in south it is called aurora australis.
The earth's magnetic field is found to change with time. The magnetic poles of earth keep shifting their position which is short term change. Detailed charts are maintained and revised periodically. The changes occurring over long term come from the evidence of basalt. The basalt from volcano cools and solidifies and provides the picture of earth's magnetic field. As the basalt can be dated back, a clear picture of the earth's magnetic field has emerged. The currents in the earth's core slow down, stop and pick up in the opposite direction.
Magnetisation and Magnetic Intensity
The ultimate source of magnetism is the magnetic dipole moment, associated with an atom due to orbital motion and intrinsic spin. This suggests that all substances possess magnetic property as energy material consists of atoms having electrons revolving around the nucleus.
Intensity of magnetising field (H)
When a substance is placed in an external magnetic field, the substance experiences a torque due to the field and aligns in the same direction as the field. The magnetisation so produced in the substance is called Induced magnetisation and strength of external field is called intensity of magnetising field (H).
H is measured in Ampere/meter or Joule/Tesla-m3.
Intensity of magnetisation (I)
This gives us the measure of the extent to which substance has been magnetised under the influence of H-field and depends upon the nature of the substance.
where File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 517f1c9.png is the intensity of magnetisation,File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m702f710c.pngnet magnetic moment, v the volume of the material.
The iron bar gets magnetised with north pole at B and south pole at A. The field inside the specimen gets modified. The magnetic induction at a point inside the magnetised specimen is the total number of magnetic field lines crossing a unit area around that point, the area being held perpendicular to the field lines.
The flux density produced by the magnetising field vacuum is proportional to the intensity of field File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html ma7650ad.png.
This indicates how easily the material can be magnetised. It is represented as Xm. File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 3c447df9.png i.e., the ratio of intensity of magnetisation induced in the material to magnetising field (H).
Therefore Xm is a number and has no units.
Magnetic Properties of Material
On the basis of their magnetic properties different materials are classified as:
- Diamagnetic substance
- Paramagnetic substance
- Ferromagnetic substance
File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 6577ae4e.jpgFile:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m5788b4cc.jpgMichael Faraday discovered that a specimen of bismuth was repelled by a strong magnet. Diamagnetism occurs in all materials. These materials are those in which individual atoms do not possess any net magnetic moment. [Their orbital and spin magnetic moment add vectorially to become zero]. The atoms of such material however acquire an induced dipole moments when they are placed in an externalmagnetic field.
The diamagnetic materials are Type 1 superconductors as they exhibit perfect conductivity and perfect diamagnetization when cooled to very low temperature. The superconductor repels a magnet and in turn is repelled. Such perfect diamagnetism in superconductors exhibiting the above phenomena is called Meissner effect.
Some important properties are:
- When suspended in a uniform magnetic field they set their longest axis at right angles to the field as shown
2) In a non-uniform magnetic material, these substances move from stronger parts of the field to the weaker parts. For e.g.,. when diamagnetic liquid is put in a watch glass placed on the two pole pieces of an electromagnet and current is switched on the liquid accumulates on the sides.
[Note on increasing the distance between the pole, the effect is reversed]
3) A diamagnetic liquid in a U shaped tube is depressed, when subjected to a magnetic field.
4) The field lines do not prefer to pass through the specimen, since the ability of a material to permit the passage of magnetic lines of force through it is less.
File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 4e1ff244.jpg5) The permeability of the substance, that is, mr < 1.
6) The substance loses its magnetization as soon as the magnetizing field is removed.
7) Such specimen cannot be easily magnetized and so their susceptibility is negative.
Example: Bismuth, antimony, copper, gold, quartz, mercury, water, alcohol, air, hydrogen etc.
Paramagnetic substance are attracted by a magnet very feebly. In a sample of a paramagnetic material, the atomic dipole moments initially are randomly oriented in space.
When an external field is applied, the dipoles rotate into alignment with field as shown
File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m1b102b1d.jpgThe vector sum of the individual dipole moments is no longer zero.
Some important properties are:
- The paramagnetic substance develops a weak magnetization in the direction of the field.
- When a paramagnetic rod is suspended freely in a uniform magnetic field, it aligns itself in the direction of magnetic field.
- File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 33c219a8.jpgThe lines of force prefer to pass through the material rather than air that is mr > 1 that is their permeability is greater than one.
- As soon as the magnetizing field is removed the paramagnetics lose their magnetization.
- In a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).
- A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.
- The magnetization of paramagnetism decreases with increase in temperature. This is because the thermal motion of the atoms tend to disturb the alignment of the dipoles.
Examples include Aluminum, platinum, chromium, manganese, copper sulphate, oxygen etc.,
Ferromagnetism, like paramagnetism, occurs in materials in which atoms have permanent magnetic dipole moments. The strong interaction between neighboring atomic dipole moments keeps them aligned even when the external magnetic field is removed.
Some important properties are:
- File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m41e0c6c7.jpgThese substances get strongly magnetized in the direction of field.
- The lines of force prefer to pass through the material rather than air that is mr>1 that is their permeability is greater than one.
- File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 33c219a8.jpgIn a non-uniform magnetic, the specimen move from weaker parts of the field to the stronger parts (that is it accumulates in the middle).
- A paramagnetic liquid in U tube placed between two poles of a magnet is elevated.
- File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m1962e359.jpgFor ferromagnetic materials mr is very large and so its susceptibility i.e., Xm is positive.
- Ferromagnetic substances retain their magnetism even after the magnetizing field is removed.
- The effectiveness of coupling between the neighboring atoms that causes ferromagnetism decreases by increasing the temperature of the substance. The temperature at which a ferromagnetic material becomes paramagnetic is called its curie temperature. For example the curie temperature of iron is 1418oF, which means above this temperature, iron is paramagnetic.
File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 7825c906.jpgExamples are Iron, cobalt, nickel and number of alloys.
Consider an iron being magnetized slowly by a changing magnetizing field (H). The intensity of magnetization is found to increase along OA. On decreasing H slowly, I also decreases but does not follow AO. When H = 0, I has a non-zero valve equal to OB. This implies that some magnetism is left in the specimen. This value of I which is non-zero when H = 0 that is OB is called retentivity or residual magnetism.
When the field is applied in the reverse direction, the I decreases along BC till its zero at C. The valve of H which has to be applied to the magnetic material in reverse direction so as to reduce its residual magnetism to zero, is called its coercivity. On increasing H further, I increases along CD till it acquires a saturation at D. On changing the field, I follows a path DEFA. This closed loop is called hysteresis loop and represents cycles of magnetization a specimen has undergone. The hysteresis therefore refers to lagging behind. Here I lags behind H.
The shape and size of hysteresis loop is characteristic of each material, because of their difference in retentivity, coercivity etc.
Materials for making permanent magnet should possess high residual magnetism i.e., when the magnetising field is reduced to zero, the intensity of magnetisation is high. Further, to reduce the residual magnetism to zero, the magnetising field should be applied in the opposite direction. The greater this value, the magnetisation will be a long lasting one. This property of the magnet is called coercivity. Examples of such substance are steel and alnico (alloy of Al, Ni, Co).It is for this reason, that steel in spite of its low residual magnetism has a high coercivity and so is preferred for making permanent magnet
Magnetic susceptibility: his indicates how easily the material can be magnetised. It is represented as Xm.
Magnetic declination (q):Magnetic declination is the angle between magnetic axis and the geographic axis.
Additional Web Resources
Magnetic Effects of Electric Current
Magnetic field around a current carrying conductor
We saw earlier that a moving charge produces a magnetic field around it. It follows, therefore, that a current carrying conductor produces a magnetic field around it that will deflect a compass, an effect demonstrated by Oersted.
It is possible to demonstrate that the deflection will be reversed when the direction of flow of current is reversed. The direction of the field can be found through the right hand thumb rule.
Imagine that you are holding a current carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then, your fingers will wrap around the conductor in the direction of the field lines of the magnetic field. The strength of the fielsd depends on the current flowing through it.
Magnetic field due to a current carrying coil
We have seen how the magnetic field is around a straight current carrying wire. The right hand thumb rule can be used to find the direction of the field around a current carrying loop as well. Each portion of the coil or loop can be treated as a conductor and the field can be found out using the right hand thumb rule. We can see that every section of the wire contributes to the magnetic field lines within the same direction within the loop.
The magnetic field around a coil depends on the number of turns in the loop. If there is a circular coil having “n” turns, the field produced is 'n” times as large as that produced by a single turn. The field produced by each turn has the same direction and the field due to each turn adds up.
Magnetic field due to a current in a solenoid
A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
The magnetic field due to a solenoid resembles the magnetic field produced due to a bar magnet. One end of the solenoid behaves like the north pole and the other end behaves like a south pole and the field is uniform inside.
File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 13941b10.pngThe strong field produced inside a solenoid can be used to magnetise a magnetic material. Such a magnet is called an electromagnet.
Force on a current carrying conductor in a magnetic field
If a current carrying condcutor can exert a force on a magnet, it must be possible that a magnetic field will exert an equal and opposite force on a conductor. Andre Marie Ampere suggested this would be the case. Experimentally it has been found that this indeed happens and the direction of force exerted on the conductor changes when the magnetic field is reversed. The direction of force exerted on the current carrying conductor can be given by Fleming's Left Hand Rule.
If the index finger indicates the direction of the magnetic field and the middle finger indicates the direction of flow of current, the thumb indicates the direction of force exerted on the conductor.
This force is called the Lorentz Force.
An important application of this effect can be seen in the electric motor. An electric motor is a devide that converts electrical energy to mechanical energy.
It is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming's Left-hand rule and whose magnitude is given by, Force, File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 4f2315e8.gif
Where B is the magnetic field in weber/m2.
I is the current in amperes and
l is the length of the coil in meter
The force, current and the magnetic field are all in mutually perpendicular directions. Thi s is a result of the cross product of the length and magnetic field vectors.
If an Electric current flows through two copper wires that are between the poles of a magnet, an upward force will move one wire up and a downward force will move the other wire down.
Force in DC motor
Magnetic Field in DC Motor
Torque in DC Motor
Current Flow in DC Motor
The loop can be made to spin by fixing a half circle of copper which is known as commutator, to each end of the loop. Current is passed into and out of the loop by brushes that press onto the strips. The brushes do not go round so the wire do not get twisted. This arrangement also makes sure that the current always passes down on the right and back on the left so that the rotation continues. This is how a simple Electric motor is made.
- Right Hand Thumb Rule: The rule that gives the direction of magnetic fiel due to a current carrying conductors
- Fleming's Left Hand Rule: The rule gives the direction of force that would be experienced by a current carrying conductor in a magnetic field
- DC motor: A device that would convert electric energy into mechanical energy
We saw that a current carrying conductor produces a magnetic field around it and that a magnetic will exert a force on a current carrying conductor. Michael Farday and Joseph Henry examined what would happen if a conductor was moving in a magnetic field. They discovered, independently, that electric current can be introduced in a wire by simply moving a magnet in or out of a coiled part of a wire. The mechanical energy of movement of the magnet was enough to induce an electromotive force in the coil when the coils are rotated in a magnetic field.
This has led to the alternate ways of generating current. Till electromagnetic induction was discovered only voltae sources were those of chemical nature such as dry cells. The present large-scale production, distribution is feasible because of this phenomenon of electromagnetic induction.. Electromagnetic induction formed the principle of two important electrical devices namely, electric generator and transformer, which has revolutionized the life styles of mankind.
The induced voltage in a coil is proportional to the product of of the number of loops and the rate at which magnetic field changes within those loops. The key concept here is that of change in the magnetic field, that of magnetic flux.
Magnetic flux can be defined as the number of lines of force passing through a surface normally.
Considering the surface 'Ds' in a magnetic field 'B'.
When a surface is a plane and has total area A then
SI unit of f is weber and magnetic flux is a scalar quantity.
Hence we find that the magnetic flux depends on
(i) the strength of the magnetic field.
(ii) the area of the surface.
(iii) the angle between the magnetic field vector and the area vector.
Increasing the magnetic flux through a surface can be done in 3 ways.
The Experiment of Faraday and Henry
File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 55d6a46f.pngFaraday and Henry performed lots of experiments to learn about the connection between electricity and magnetism. The results of these experiments have led to the life styles of today, who made life easy by using lots of electrical applications.
Some of the experiments are as follows
A solenoid is connected to a sensitive galvanometer. On moving a magnet towards a coil, the galvanometer shows a deflection. When the magnet is reversed, the deflection is seen to be in the opposite direction.
Once the magnet is stopped, there is no deflection in galvanometer. On moving the magnet faster towards the coil, the deflection is longer.
Similar results are obtained when the magnet is kept stationary and the coil is moved. It means that whenever a current was induced in the coil there is a relative motion between the coil and the magnet. The magnitude of the current depended on the strength of the magnet and also on the magnitude of their relative velocity.
Similar results were seen when the magnet is replaced by as coil connected to a battery. Even without physically moving the coils a current was shown in the galvanometer only when the switch is on and when the current is put off i.e., when the current is building up in the coil or when it reduced to zero the galvanometer in the other coil showed a charge.
This current, which is produced in the coil connected to the galvanometer, is called as induced current. The induced currents direction, when the current builds up in the other coil was opposite to that when the current reduced opposite to that when the current reduced. The deflections were momentarily seen only when the switch was opened and closed.
Faraday's laws of electromagnetic induction.
(i) Whenever the magnetic flux linked with a circuit changes, an EMF is induced in the circuit, which lasts as long as the change in magnetic flux associated with the circuit continues.
(ii) The magnitude of the induced EMF is equal to the rate at which the magnetic flux linked with the circuit changes.File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 1ac06770.png
Faraday's laws of electromagnetic induction do not say anything about the direction of the current. The direction is given by Lenz's law.
The motion of the magnet in either direction causes a change in strength of the magnetic field linked with the coil and this causes a current to be induced in the coil. This induced current opposes the change in the magnetic field by producing its own magnetic field.
Whenever an EMF is induced, the induced current is in such a direction so as to oppose the change inducing the current.
The negative sign indicates the opposing nature of the induced EMF.
Methods of producing induced emf
The three methods of producing induced EMF are by:
- Changing the magnitude of magnetic field B
- Changing the area A
- Changing the angle between the direction of B and the normal surface area
Motional e.m.f and Faraday's Law
Suppose a uniform magnetic field B perpendicular to the plane of paper point outward is represented in the region ABCD. A rectangular loop PQRS is pulled such that it moves with a velocity V as shown in the diagram.
This way the area of the loop inside the field changes. This induces an e.m.f in the wire. If in a small time File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m4ce5d5c9.png, the loop moves a distance File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 45ce0699.png, then the decrease in the area of the loop = - lDx.
If R is the resistance of the loop,
The direction of the current is given by Fleming's right hand rule. The induced e.m.f. Blv is called motional e.m.f.
The motional e.m.f can be understood by recalling Lorentz force. When the loop moves, the charges inside it moves and so experiences a force = q v b [as the loop is placed in a external magnetic field]. The work done in moving the charge is q v b.l.
Motional e.m.f = B l v
Similarly when a conductor is stationary, the moving magnet or changing magnetic field produces an electric field which forces the charges in the conductors to move thus inducing current in the conductor.
Lenz's Law and Energy Conservation
If the north pole of the magnet is moved towards the coil, the upper face(U.F) of the magnet acquires the north polarity on closing the key between 2 and 3. [This is because the current induced in the coil flows in an anti clockwise and this produces a magnetic field with the upper face acquiring a north polarity]. Therefore, work has to be done against the force of repulsion in bringing the magnet closer to the coil. If the magnet is moved away, south polarity develops on the same face. Therefore, work has to be done against the force of attraction in taking the magnet away from the coil. It is this mechanical work done in moving magnet with respect to the coil that changes into electrical energy producing induced current. Thus, energy is being transformed.
Fleming's Right Hand Rule
The direction of induced current can easily be predicted using Fleming's right hand rule.
If we stretch the first finger, the central finger and the thumb of our right hand in mutually perpendicular directions such that first finger points along the direction of the field and the thumb is along the direction of motion of the conductor, then the central finger would give us the direction of induced current.
Induced currents are produced not only in the wires, but also in the block of metals. If a metallic block is placed in a continuously changing magnetic field, induced currents are set up in the body of the metallic block. In the case of the wires the induced current flows along the direction of the wire. How does it flow in metallic blocks? They flow in a circular path by Lenz's law. These current appear like eddies in a fluid and hence are called as eddy current's.
Unlike the metallic wires where the resistance is less metallic File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m19b8a684.jpgblocks have larger resistance and hence the induced currents lead to large amount of Joule's heat (H = i2k).
If a bar pendulum is suspended between the pole pieces of a magnet: Let us take another identical pendulum and kept in a field free region. If we oscillate both of them with the same force, it is observed that the one within the field damps faster.
The 'bob' of the pendulum consists of a copper plate. The pendulum is made to swing between the pole pieces of the magnet. Its motion is damped due to eddy currents.
Why does it happen? When the pendulum oscillates inside the field it cuts the magnetic lines of force and hence induces a current in the bar, that is, eddy currents. According to Lenz's laws, the eddy currents oppose the motion and hence produce damping.
Is eddy current advantageous or disadvantageous?
Eddy currents produce a large amount of heat, which is undesirable in a number of cases like dynamos, transformers, where the coil is wound on iron core.
How can eddy currents be minimized?
The solid iron core is divided into a number of thin sheets. The sheets are electrically isolated from each other. These sheets are so placed that the path of the induced eddy currents is broken by the insulating material between the sheets. These are called laminated cores. Hence, using laminated cores can minimize the effects of eddy currents
Application of Eddy Currents
When a steady current passes through a moving coil galvanometer, the coil undergoes a torque and does not come to equilibrium position instantly. Hence the coil is wound over a metallic frame so that the eddy currents produced in the frame can damp the oscillation and brings the coil to the equilibrium position instantly.
Induction furnace separates certain metals from their ores. It is done by heating the metal. The type of heating done is called induction heating. This heating can be done using eddy currents. The metal to be heated is placed in a high frequency changing magnetic field. Strong eddy currents produced will give rise to desired heating.
Eddy currents can brake the motion of the train too. A metal drum is attached to the train. To apply brakes a strong magnetic field is applied across the drum. The eddy currents set up in the drum in a direction as to oppose the change in the magnetic flux that is, it opposes the motion of the wheel.
When a current is established in a conductor, a magnetic field is produced in its vicinity. We can visualize this field in terms of magnetic flux. If steady current flows the number of lines of force at a given place would remain the same. But if the current changes with time, the flux associated with the loop changes and hence an e.m.f is induced in the loop. This phenomenon is called as self induction.
Self-induction is the property of a coil, which enables the induction of an EMF in it when the current in the coil changes.
Consider a coil carrying a current I having N turns and lets the magnetic flux f be linked with the coil. If the current in the coil is changed, the flux link also changes. Thus, according to Faraday's law of electromagnetic induction, induces an EMF on to itself. This EMF is called self-induced EMF and this phenomenon is called self-induction.
It is found that the flux linkage is proportional to the current through it.
i.e., f a I or f = LI
Here, L is constant and is called self-inductance of the coil or coefficient of self-induction.
S.I., the unit of self-inductance, is Henry
i.e., 1 Henry = 1 Weber turns / Ampereor 1 Henry = 1 Volt / ampere/second
Self-inductance of a coil is 1 Henry when a current changes at the rate of 1 amp/sec through the coil induces EMF of 1 volt in the coil.
'Mutual Inductance '
We know that if a current builds up or varies in a coil, the flux change leads to induced e.m.f in the same coil. This can happen event mutually between two interacting coils are close together, and if current is passed in one of them, it sets up a magnetic flux surrounding itself. When the second coil is near the first coil, the changes in the magnetic flux of the first coil produces similar changes in the second. Thereby, producing induced e.m.f in the second coil. To distinguish it from self-induction, it is called as mutual induction.
It is the property of two circuits (or coils) by virtue of which each oppose any change in the magnitude of the current flowing through the other circuit by producing an induced EMF in it.
Consider two coils P and S placed near coil P connected to a battery and key and is called the primary coil. Coil S is connected to a sensitive galvanometer and is called the secondary coil.
When the key K is closed, the flux linked with the coil in the primary circuit changes. This induces an EMF in the secondary coil indicated by the deflection in the galvanometer.
When the key K is opened, an EMF is induced in the secondary coil, but in a direction opposite to that induced during the make, i.e., current in S always oppose any change in current in P.
Note:The mutual inductance of two coils depends on the geometry of the two coils, distance between the coils and orientation of the two coils.
The following diagrams indicate the maximum coupling between the two coils.
(i) Coupling between the coils is maximum.
(ii) Coupling is less than in case (a)
(iii) Coupling is minimum
Mutual inductance of two long solenoids
Mutual Inductance of Two Long Solenoid
Consider a solenoid P within the core having N1 turns. Another solenoid S having N2 turns is wound over the solenoid P. Let 'l' be the length of each solenoid and let them have nearly the same area of cross-sections A.
The magnetic field B1 at any point inside P due to current I1 is
The flux f linked with each turn of S
= B1 x area of each turn
= B1 A
Total magnetic flux linked with S
Now f2= M12 I1
Note:If the area of cross section was different from the area of cross section of the inner solenoid, the smaller one is to be considered.
Do we use dry cells for operating electrical appliance? It is not impossible to tap continuous supply of energy from electrochemical cells. Electrical circuits in homes, factories and offices receive such energy form local power companies. In most countries the energy is supplied via oscillating e.m.fs and currents. These oscillating currents are called as alternating currents, shortly as a.c.
Circuits involving alternating currents are used in electric power distribution systems, in radio, TV and other communication devices and in a wide variety of electric motors. The designation 'alternating' mean current changes direction and value periodically with time.
Can you guess the frequency with which their direction is going to alter? In India, the frequency of the alternating current supplied to homes is 50Hz. What does this mean? The current flows along the length of the wire in one direction and changes to the opposite direction, and this happens at the rate of 50 times in one second. That means every 1/100 seconds, there is a change of direction. It is an amazing fact that the charge carriers get this signal of direction change is propagated at the speed of light.
What difference does it make if direct current flows or an alternating current flows in a conductor? As far as the heating effects are concerned such as light bulbs and heaters the direction of current is not important and the electrons transfer the energy to the device via collisions with atoms in the device.
It does make a big difference when the magnetic effects electric current are concerned. As the currents alternate, the magnetic field surrounding the conductor also oscillates. This makes possible the Faraday's laws of induction. Moreover, alternating current is readily adaptable to rotating machinery such as generator and motors.
One of the methods of producing a sinusoid EMF is to rotate the coil in uniform magnetic filed. Graphically, such a varying EMF or current is represented as follows.
Note that Eo and Io represent the peak or maximum values of EMF at a particular time.
Therefore induced EMF in a coil varies in magnitude and direction periodically. Such an EMF is called alternating EMF. The corresponding current is called alternating current (AC). The AC or EMF first rises to a maximum +Eo or (+Io) in one direction and falls to zero, the direction then quickly reverses so that the EMF and current rise to maximum value Eo or (-Io) in the reverse direction and again falls to zero. This completes one cycle of AC voltage, the instantaneous value of EMF is therefore, E = Eo sinwt and current is given by I = Io sinwt where Eo, Io are the peak values of the EMF and current and wt are the phase angles.
Average value of alternating e.m.f
Average value of the alternating e.m.f over a half cycle is that steady e.m.f which will send the same amount of charge in a circuit in a time of half cycle as is sent by the given alternating e.m.f in the same circuit in the same time.
Following the above definition, it can be proved that the average value of the alternating value of alternating e.m.f for positive half cycles is 0.637 time the peak value of the alternating current.
Why do we talk about half cycle? What if the whole cycle is taken into account? Due to positive half cycle it is 63.7% o and then due to negative half cycle it should be -63.7 %o and hence for the whole cycle the average e.m.f vanishes.
Then how to go about full cycles? We define a new term called 'root mean square value' of e.m.f (or) current.
The whole process repeats once again. The energy of system oscillates between capacitor and the inductor.
AC Generator or Dynamo
An 'AC generator' or 'dynamo' is a machine which produces AC from mechanical energy. Actually, it is an alternator which converts one form of energy into another.
AC Dynamo is based on the phenomenon of electromagnetic induction. That is, when the relative orientation between the coil and the magnetic field changes, the flux linked with the coil changes and this induces a current in the coil.
As the armature coil rotates, the angle Q changes continuously. Therefore, the flux linked with the coil changes.
= NBA cos q
= NBA cos wt
where q is the flux linked with the coil, N is the number of turns in the coil, A is the area enclosed by each three of the coil and B is the strength of the magnetic field.
= - NBA (-sin wt )w
E = + NBA w sin wt
e = eo sinwt. This is the EMF Supplied by the A.C. generator
ABCD is the armature coil consisting of a large number of turns of the insulated copper wire wound over a laminated soft iron core I. The coil can be rotated about the central axis.
N and S are the pole pieces of a strong electromagnet in which the armature coil is rotated.
R1 and R2 are two hollow metallic rings to which both ends of the armature coil are connected. These rings rotate with the rotation of the coil.
Brushes B1 and B2 are two flexible metal plates or carbon rods. These brushes are used to pass current from the coil to the external load resistance.
To start with, suppose the plane of the coil is perpendicular to the plane of the paper in which the magnetic field is applied, with AB at the front and CD at the back, the flux linked with the coil is maximum in this position. As the coil rotates clockwise, AB moves inwards and CD moves outwards. According to Fleming's right hand rule, the current induced in AB is from A to B, and in CD, from C to D. In the external circuit, current flows from B2 to B1. After half of the rotation of the coil, AB is at the back and CD is at the front. Therefore, AB starts moving outwards and CD inwards. The current induced in AB is from B to A, and in CD, from D to C. The current flows from B1 to B2 through the external circuit. We therefore see that the induced current in the external circuit changes direction after every half rotation of the coil, and hence is alternating in nature.
For a given power requirement, one has the choice of the relative values of Irms and Erms. That is, for the product to be a constant, we can choose a relatively large current I and a relatively small potential difference v or just the reverse. In an electric power distribution system, it is desirable - both for reasons of safety and the efficient design of equipment; to have relatively low voltage at both the generating end and receiving end. But for transmission of electrical energy from the generating plant to the consumer, we want the lowest practical current so as to minimize the I2R energy dissipation in transmission line. This mismatch between the requirements for transmission and consumption calls for a device which raises or lowers the potential difference in a circuit, keeping the product IrmsErms essentially constant. This device is a transformer whose operations are based on Faraday's law of induction.
An electrical device is used to change the AC voltage. A transformer which increases the AC voltage is called a 'step up transformer' and a transformer which decreases the AC voltage is called a 'step down transformer'.
A transformer is based on the principle of mutual induction.
It consists of a soft iron core made of laminated sheets well insulated from each other. The coils P1 P2 and S1 S2 are wounded on the same core.
The coil P1 P2 is a primary coil connected to AC source and S1 S2 is a secondary coil connected across a load resistance R.
As the current in the primary varies, the flux linked with P1 P2 and hence S1 S2, changes.
If Np is the number of turns in P1 P2, and Ep is the alternating EMF fed to P1 P2 at instant t under ideal conditions;
Self induced EMF in P1 P2 at instant t = EMF fed to P1 P2 at this instant.
Assuming there is no flux leakage, the rate of change of flux through each turn of S1 S2 is df/dt,
For Step Up Transformers
Es > Ep
i.e., K >1 Ns > Np
For a Step Down Transformers
Es < Ep
i.e., K <1 ns < Np
If we assume there is no loss of power,
Out put power = Input power
EsIs = EpIp
Energy Losses in a Transformer
(i) Copper loss is the energy lost due to heating of copper coils of transformers.
(ii) Iron loss due formation of induced current in the iron line resulting in lot of heat.
(iii) Leakage of magnetic flux. All flux linked with primary may not be linked with secondary.
(iv) Magnetostriction i.e., humming noise of a transformer.
Activity 1: Repelling Strings
Principle: Electric charge, Static Electricity
- Tie about 8 to 10 nylon strings to a rod.
- Rub the rod with fur or wool, and you remove electrons from the fur and deposit them on the strings.
- The strings will fly apart since they are all charged, and like charges repel.
1. Why do the strings move apart?
Activity 2: Charge of the light balloons
Principle: Electric charge, Static Electricity
- Rub two balloons through your hair and you transfer some electrons to them.
- Suspend them by strings to show that they repel.
- You can illustrate polarization by showing that a charged balloon will attract an uncharged balloon, but once they touch and transfer charge, they repel.
- You can deflect a stream of water with a charged balloon because of the polarization of the water molecules.
- You can also "levitate" light strings and joke about snake charming.
- Sticking them to walls and ceilings is also fun.
Activity 3: Rub the tube
Principle: Electric charge, Static Electricity
- Rub a fluorescent tube with wool or fur and it will glow.
- Electrons are transferred to the glass from the fur, and some electrons dislodge and fly away from the other deposited electrons and excite atoms in the gas inside the tube.
- As the atoms de-excite, they emit ultraviolet radiation which is absorbed by the phosphor coating on the inside of the tube, which causes the phosphor, and the tube, to glow.
1. Why do the balloons move apart?
2. Why uncharged balloon gets attracted towards the charged balloon?
3. Why stream of water gets deflected away when balloon is brought near?
Activity 3: Electroscope
Principle: Electric charge, Moving electrons
- You can construct a simple charge detector with a glass jar, aluminum foil, and some stiff wire.
- Choose a quart glass jar with straight sides and a plastic lid.
- Bend a small (~ 2 cm) sideways hook into a 25 cm piece of stiff wire (a stripped piece of coat hanger will work).
- Stick the unbent end of the wire through a small hole drilled in the plastic lid, and fix the wire in a position so that the hook is in the middle of the jar. A small glob of clay will work just fine for this.
- Cut the unbent end of the wire so that only a few inches of wire sticks up out of the lid. Once you have the wire where you want it, fix it in place and seal the hole with wax dripped from a candle.
- Hang two thin (~ 3 to 5 mm) aluminum foil strips from the hook so that they touch.
- Heat the jar so that it is dry and warm inside, then quickly seal the jar.
- Top the protruding wire with a ball of crumpled aluminum foil, and you are done!
- Bring something charged close to the ball, and the aluminum strips repel each other.
- This is because the wire and foil polarize when something charged is brought near.
- The opposite charge is attracted to the ball on top, and the like charge is repelled down into the strips, which then repel each other.
- Remove the charged object, and the strips return to normal. If you touch the charged object to the ball, you transfer charge, and the strips will remain deflected. Knowing the charge of one object, you can determine the charge of other objects with this device.
1. How can we test the presence of the charges in a body?
2.Can we transfer charges from one object to another object?
Activity 4: Conductors
Principle: Electric charge, Moving electrons
- Make two 1 cm wide, 10 cm long aluminum foil wires by folding strips of foil 3 or 4 times.
- Tape one end of each to opposite ends of a D battery.
- Connect the other end of one wire to the side of a flashlight bulb with a clothespin.
- Tap the bottom of the bulb on the other wire and the bulb will light.
- You can have your students check various objects to see if they conduct by placing the objects between the loose wire and the flashlight bulb.
- Why do some materials allow electric current to pass through them?
- What makes them to conduct electricity?
Activity 5: Conductivity
Locate the PHET “Conductivity” Simulation (either on a classroom computer or at [] )
- Check that the battery voltage menu is set to 0
- Under the materials menu, select metal. What, if anything, happens?
- Now, set the battery voltage to 0.5. What, if anything, happens? Illustrate with a diagram.
- The little spheres rotating around the ring represent electrons in a wire. Look at the battery. What terminal (positive or negative) is supplying the electrons? (hint: look for the side of the battery that has a “button”. That would be the positive terminal. The opposite side is the negative terminal).
- The battery and the wires form an electric circuit, that is, a complete path from the power source, through a wire and back to the same power source.. If an electric circuit is broken in any spot, the flow of electrons will stop.
- Adjust the battery voltage higher and describe the effect on electron movement in the wire.
- Adjust the battery voltage lower and describe the effect on electron movement in the wire.
- With the battery voltage at 0.5, Shine the light. What, if anything, happens?
- Set the battery voltage to zero
- Complete the following statement. Metals are conductors because they will allow a current of electrons to -------------
- Check that the battery voltage menu is set to 0
- Under the materials menu, select plastic. What, if anything, happens?
- Now, set the battery voltage to 0.5. What, if anything, happens? Illustrate with a diagram.
- Adjust the battery voltage higher and describe the effect on electron movement in the wire.
- Adjust the battery voltage lower and describe the effect on electron movement in the wire.
- With the battery voltage at 0.5, Shine the light. What, if anything, happens?
- Set the battery voltage to zero
- Complete the following statement. Plastics are non-conductors because ---------
Activity 6: A current is a magnet; a magnet is a current
Principle: Moving electrons. magnetism Prοcedure:
- Poke a hole in the middle of a piece of poster board and run a straight 60 cm large gauge solid wire (not twisted) through the hole.
- Support the wire and poster board so that the board is horizontal and at the mid point of the wire, with the wire perpendicular to the board.Place several small compasses on the poster board in a circle about the wire.
- Connect the wire to a 12 V lantern battery with allegator clip wires, and watch the magnets! Moving charges create a magnetic field in the form of circular loops perpendicular to the direction of their motion.
- For large classes, it might be more convenient to set the wire horizontal pointed towards the class, and trace the magnetic field loops with a dip compass.
Activity 7: Electromagnet
Principle: Moving electrons. Magnetism Procedure:
- Magnetic fields can be much stronger in materials than they are in air.
- Wrap an aluminum foil wire several times about a nail, and connect the wire to a D battery, and you have an electromagnet!
- With the wire looped, the "loops" of magnetic field produced by the moving charges all add up in the center of the wire loop, creating a much stronger field than what a single wire could produce.
Activity 8: Make a magnet, break a magnet
Principle: Moving electrons. Magnetism
- Magnetize a hacksaw blade by rubbing it in one direction with a strong permanent magnet using firm slow strokes, with the orientation of the permanent magnet the same at all times. Twenty to thirty strokes should suffice.
- Use iron or metal filings to show the location of the magnetic poles at the ends of the blade. Also demonstrate that the filings do not stick to the blade in the middle.
- Break the hacksaw blade in half, and now you have two magnets, each with a pole at each end!
Activity 9: Totally tubular magnets
- Magnetic fields affect moving charges, but not stationary ones.
- Similarly, a moving magnet (or a simply a changing magnetic field) can affect a stationary charge.
- Only relative motion is important (this is what got Einstein going).
- In other words, a changing magnetic field creates an electric field, and a changing electric field creates a magnetic field.
- As a general rule, the electric field created by a changing magnetic field will be oriented so that it could cause nearby charges to move in a manner that would create a second magnetic field directed to oppose the change in the original magnetic field.
- Electric currents created from this effect are called "eddy currents" due the circular motion of the charges.
- In other words, a moving magnet will create a "virtual magnet" if it moves near a conductor.
- The virtual magnet will be oriented to slow the moving magnet down.
- To illustrate this, drop a strong magnet down a copper tube, and show that it takes a lot longer to drop out the end than anything else of similar size and weight.
- You can also move a strong magnet past a non-magnetic conductor (copper is best) and feel the resistance.
Activity 10 – PhET Magnetism
Principle: To study magnetism, polarity
- Go to []
- Click on electricity and magnetism sims.
- Select the simulation “Magnets and Electromagnets.” It is at this link
- Move the compass slowly along a semicircular path above the bar magnet until you’ve put it on the opposite side of the bar magnet. Describe what happens to the compass needle.
- What do you suppose the compass needles drawn all over the screen tell you?
- How is the strength of the force/torque on the compass needle indicated?
- What are the similarities between the compass needle (magnetism) and a test charge (electricity)?
- Move the compass along a semicircular path below the bar magnet until you’ve put it on the opposite side of the bar magnet. Describe what happens to the compass needle.
- How many complete rotations does the compass needle make when the compass is moved once around the bar magnet?
- Click “flip polarity” and repeat the steps above after you’ve let the compass stabilize.
- Click on the electromagnet tab. Place the compass on the left side of the coil so that the compass center lies along the axis of the coil. (The y-component of the magnetic field is zero along the axis of the coil.)
- Move the compass along a semicircular path above the coil until you’ve put it on the opposite side of the coil. Describe what happens to the compass needle.
- Move the compass along a semicircular path below the coil until you’ve put it on the opposite side of the coil. Describe what happens to the compass needle.
- How many complete rotations does the compass needle make when the compass is moved once around the coil?
- Use the voltage slider to change the direction of the current and repeat the steps above for the coil after you’ve let the compass stabilize.
- Based on your observations, summarize the similarities between the bar magnet and the coil.
- What happens to the current in the coil when you set the voltage of the battery to zero?
- What happens to the magnetic field around the coil when you set the voltage of the battery to zero?
- Play with the voltage slider and describe what happens to the current in the coil and the magnetic field around the coil.
- What is your guess as to the relationship between the current in the coil and the magnetic field?
Part II – Graphing relationships - Field Strength vs. Position
- Using the Electromagnet simulation, click on “Show Field Meter.”
- Set the battery voltage to 10V where the positive is on the right of the battery.
- Along the axis of the coil and at the center of each compass needle starting 5 to the left of the coil, record the value of B. Move one compass needle to the right and record the value of B. Repeat until you’ve completed the table below. NOTE: Be sure to take all of your values along the axis of the coil. You’ll know you’re on the axis because the y component of the magnetic field is zero along the axis.
Compass position (arbitrary units)
Magnetic Field Strength (fill in units)
- What happens to the value of magnetic field strength inside the coil?
- Graph the compass position on the horizontal axis and magnetic field magnitude on the y axis. Print your graph. Make sure to label the axes and title the graph.
- Is your graph symmetric?
- Using your graph, what is the relationship between magnetic field strength and position? (Use the fit feature of graphical analysis to help you.)
Part III – Using the simulation to design an experiment.
Field Strength vs. Number of Coils
- Design an experiment to test how field strength varies with the number of coils.
- Collect data in a table and graph your results.
- Field Strength vs. Current
- Design an experiment to test how field strength varies with the Current. (Recall that voltage is directly proportional to current….Ohm’s Law.)
- Collect data in a table and graph your results.
- Test your predictions from part III using the electromagnet built in class and the Logger Pro sensor.
- Were your predictions correct? Explain.
Activity 11: Simple Battery
Principle: Moving electrons, chemical reaction
- Clean or brighten an iron washer and a copper penny. Soak a 1 inch square piece of heavy blotter paper or folded paper towel in vinegar.
- Press the soaked paper between the washer and the penny to form a simple battery.
- Measure the current with wires connected to the pocket current meter, with one wire pressed against the washer and the other pressed to the penny.
- This battery will not be strong enough to light a flashlight bulb.
- The vinegar induces a chemical reaction between the copper and the iron.
- Charged "ions" will flow through the vinegar from the copper to the iron.
- The reaction continues as long as one piece of metal can get rid of its excess electrons through the wires to the other piece of metal to balance the natural flow of charge through the blotter paper.
Activity 12: Electromagnetism
Hint: Download the file using the ‘save’ option then run the ‘.jar’ file using Java.
2. Complete the following tasks to help you investigate Faraday’s Electromagnet Lab. These tasks will help you conduct appropriate experiments to answer the lab questions.
We will be using the Bar Magnet and Electromagnet tabs for this activity and the other tabs later in the unit. Click on the Bar Magnet tab. File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 6bfd456c.png
- Click on File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m7cdbf5e.png. Explain the two changes this causes in the simulation.
- MFile:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 45126618.pngove compass to various locations around the bar magnet. Explain what orientation the needle takes with respect to the bar magnet.
- SFile:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m4de6ccf7.pngelect ‘Show Field Meter’ File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html 5242b6c6.png. The image below will appear. The meter can be moved to various locations and indicates the magnetic field strength at the crosshairs. Label: Total magnetic field, y-component of the magnetic field, x-component of the magnetic field, angle and units in the following diagram.
- You should be able to determine the direction of the magnetic field vector using the meter.
- Select File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html ma1f6eac.png. Observe the orientation of the small compass needles.
- Click on the Electromagnet tab. File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m276d0de7.png.
- What is behaving like a magnet : The battery or The coils of current carrying wire ? File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m6b8ab73d.png
- Using the slider on the battery, change the voltage of the battery from 10V to 0V.
Then from 0V to ‘-’ 10V. File:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m42f9b776.png
Record the changes you observe in the direction of the compass needle.
Select AC as your current source.
Observe and record the changes in the compass needle.
3. Design and execute an experiment using the simulation that will allow you to understand the direction and strength of the magnetic field around
(a) a bar magnet
(b) an electromagnet
You do not need to submit the procedure of your experiment, only your results.
- Using diagrams and written explanation, explain the magnetic field direction and strength around a bar magnet, and an electromagnet.
- Explain the similarities and differences of a bar magnet and an electromagnet.
- Identify the characteristics of electromagnets that are variable (can be changed) and what effects each variable has on the magnetic field’s strength and direction.
Activity 13: Faraday's Electromagnetic Lab – AC/DC Current and Electromagnetism
Procedure – do the following activity using this web site
- GFile:Electromagnetism Resource Material Subject Teacher Forum September 2011 html m519d161e.gifetting started. Open the website listed above and on the top of the screen select the tab marked electromagnet.
- Make observations & draw conclusions. Change the current source back and forth from DC to AC looking for how the electrons move in the wire. AC current is distinguished from DC current by the motion of the current. In this applet the current is represented by the balls moving in the wire. Based on your observations write a general rule for how current moves in AC verses how current moves in DC.
- Make observations & draw conclusions. Set up the applet so it is using a DC current and place a compass near the electromagnet. Your screen should look something like what you see to the right, on Screen 1. Using the slider on the battery, observe how changing the voltage changes the current flow and what happens to the compass needle. Write down your observations regarding the voltage, the current flow and the change in the compass. What does changing the current flow do to the magnetic field?
- Make observations & draw conclusions. Insert a field meter into your screen. Your screen should now look something like what you see to the right, Screen 2. move the battery slider back and forth and observe what happens to the strength of the magnetic field, the top number on the field meter. Write a general rule for how the voltage affects the magnetic field’s strength.
- Make observations & draw conclusions. Using the same setup as you used in step 4 change the number of loops and observe how this affects field strength. Write a general rule for how the number of loops affects the magnetic fields strength.
- Make observations & draw conclusions. Using the same setup as you used in step 4 move the filed meter from place to place and observe how the field strength changes. Write a general rule for how changing the distance from the magnet affects the magnetic fields strength.
- Make observations & draw conclusions. Use the same setup as you used in step 4 but change the source of current to AC. Your screen should look something like what you see to the right, Screen 3. Observe how the AC changes the compass and the magnetic field strength. Write down your observations regarding the change in the strength and direction of the magnetic field. Describe a way to get a DC supplied electromagnet to change the direction of the magnetic field, like the AC does.