Power

7. Faraday’s Law

Electromagnetism

1

7. FARADAY’S LAW

7.1 Faraday’s Law of Induction

7.2 Motional emf

7.3 Lenz’s Law

7.4 Induced emf and Electric Fields

7.6 Generators and motors

7.5 Eddy Currents

Electromagnetism

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7.1 Faraday’s Law of Induction

• Consider

a

bar

magnet places near a

wire loop connected

to a voltmeter. The

voltmeter deflects

when the magnet is

moved towards or

away from the coil.

However, when the

magnet is stationary,

there

is

no

deflection.

Electromagnetism

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7.1 Faraday’s Law of Induction

• There

is

an

induced

electromotive force (emf)

when there is a relative

motion between the magnet

and the coil.

• The relative motion gives rise

to a changing magnetic field

in the loop, which leads to the

induced emf. This is known

as electromagnetic induction.

• When

the

magnet

is

stationary, there is no change

in the magnetic field through

the loop with time, hence

there is no induced emf.

Electromagnetism

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7.1 Faraday’s Law of Induction

• An emf is induced in a circuit

when the magnetic field

through the circuit changes

with time.

• The effect is described by the

Faraday’s law of induction:

the emf induced in a circuit is

directly proportional to the

time rate of change of the

magnetic flux through the

circuit.

d  B  dA

dB





dt

dt

Electromagnetism

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7.1 Faraday’s Law of Induction

• Induced emf , ε, is in the unit of volt (V).

• An emf can be induced in the circuit in several ways:

a) Change |B | with time.

b) Change the area A enclosed by the loop with time.

c) Change the angle θ between the vectors B and A with time.

d) Any combination of the above.

• No emf is produced when dΦB/dt = 0.

• If the circuit consists of N identical loops, the total induced emf

is given by

d B

  N

dt

Electromagnetism

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7.1 Faraday’s Law of Induction

• Example: A coil consists of 200 turns of wire. Each turn is a

square of side 18 cm, and a uniform magnetic field directed

perpendicular to the plane of the coil is turned on. If the field...