Physics 123

21. Electromagnetic Induction

21.1 Induced EMF (voltage)

21.2 Faraday’s Law - Lenz’s Law

21.3 EMF induced in a moving conductor

21.4 Changing magnetic flux produces E

21.5 Electric Generators

21.7 Transformers

Magnetism produces current

II

B

NS

Magnet thrust into the coil

Faraday’s Law of Induction

Induced emf equals the rate of change of magnetic flux

Voltage = change of flux / time

V = B A / t

Lenz’s Law

II

BN

S

The induced current will oppose the original change in flux

II

B

NS

Lenz’s Law

II

Old wine … New bottle!

II

B increasing B decreasing

EMF induced in a moving conductor

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EMF induced in a moving conductor

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V = B A / t

V = B L d / t

V = B L v

What is the voltage across the wing tips of an airplane?

wingtips = 70 m apart

speed = 1000 km/h and Earth’s B = 5 x 10- 5 T

Voltage = B Lv

V = 5 x 10- 5 x 280 m/s x 70

V = 1.0 volt

Changing magnetic flux produces E

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V = B L v

E L = B L v

E = B v

Electric Generator

V = B L v sin

V = B L d sin

V = B A sin t

V = B A N sin t

Transformer

II

B

II

B

Primary Secondary

Vp = Np B A / t Vs = Ns B A / t

Vs / Vp = Ns / Np

That’s all folks!