If we can get magnetism out of electricity, why can’t we get electricity from magnetism?

The answer………………..

Electromagnetic induction

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This is how first experiment by Faraday was done

He only got a deflection of the galvanometer when the switch is opened or closed

Steady current does not make induced emf.

Transformers

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This effect can be quantified by Faraday’s Law

Experimental Observation of Induction

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Electromagnetic InductionFaraday discovered that a changing magnetic flux

leads to a voltage in a wire loop Induced voltage (emf) causes a current to flow !!

Symmetry: electricity magnetism

electric current magnetic field

magnetic field electric current

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What does Faraday’s law say?Faraday’s law says that

a) an emf is induced in a loop when it moves through an electric field

b) the induced emf produces a current whose magnetic field opposes the original change

c) the induced emf is proportional to the rate of change of magnetic flux

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Faraday’s Law of Induction

tN B

induced emf

number of loops

rate of changeof flux with time

The faster the change, the larger the induced emf

The induced emf is a voltage

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TYPES OF INDUCED EMFStatically induced emf

Conductor remains stationary and flux linked with it is changed

(the current which creates the flux changes i.e increases or decreases)

TYPESSelf induced Mutually induced

Dynamically induced emfField is stationary and conductors cut across itEither the coil or the magnet moves.

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Can we get emf induced in

a motionless circuit?An induced emf produced in a

motionless circuit is due to1) a static (steady) magnetic field2) a changing magnetic field3) a strong magnetic field4) the Earth’s magnetic field5) a zero magnetic field

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Induction in Stationary Circuit

Switch closed (or opened)Current induced in coil B

Steady state current in coil ANo current induced in coil B

A

B

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How does a magnetic field change?

The field can itself be changing in nature

Either the magnet itself should move or the conductor should move with respect to each other

Hence there should be a relative motion between magnet and the conductor

tN B

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Electric GeneratorsRotate a loop of wire in a uniform magnetic field:

changing changing flux induced emfB = B A cos = B A cos(t)

Rotation: = t

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Faraday’s Law

How to change the flux? Recall that flux is:

Changing B or A or will change the flux.

B B A cos

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Faraday’s Law of Induction

tN B

induced emf

number of loops

rate of changeof flux with time

Minus sign from Lenz’s Law: Induced current produces a magnetic field

which opposes the original change in flux

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Comment on Lenz’s LawWhy does the induced current oppose the change

in flux?

Consider the alternative If the induced current reinforced the change, then the

change would get bigger, which would then induce a larger current, and then the change would get even bigger, and so on . . .

This leads to a clear violation of conservation of energy!!

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Bar magnet moves through coil Current induced in coil v

S N

Reverse pole Induced current changes sign vN S

Coil moves past fixed bar magnet

Current induced in coil

S N

Bar magnet stationary inside coil

No current induced in coil

N S

Direction of Induced Current

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ConcepTest: Lenz’s Law If a N pole moves towards the loop from above the

page, in what direction is the induced current? (a) clockwise (b) counter-clockwise (c) no induced current

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SELF INDUCTANCE AND MUTUAL INDUCTANCE

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Self - InductanceConsider a single isolated coil:

Current (red) starts to flow clockwise due to the batteryBut the buildup of current leads to changing flux in loop Induced emf (green) opposes the change

d dIN Ldt dt

L is the self-inductance

units = “Henry (H)”=N2/R

induced emf

This is a self-induced emf (also called “back” emf)

PROPERTY OF A COIL DUE TO WHICH PROPERTY OF A COIL DUE TO WHICH IT IT OPPOSES OPPOSES THE THE CHANGE CHANGE OF OF

CURRENTCURRENT OR OR FLUXFLUX THROUGH IT THROUGH IT

SELF INDUCTANCESELF INDUCTANCE

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Mutual InductanceConsider two neighboring coils:

if current changes in coil #1, an emf is induced in coil #2

BB I1 rewrite as:

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dNdt

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dIMdt

M is the “mutual inductance”units = Henry (H)

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MUTUAL INDUCTANCEPrinciple of operation of Transformer

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