APHY20104/21/23 1

21.1 Induced EMF

A changing magnetic field will induce an emf which produces a current.

APHY20104/21/23 2

21.2 Faraday’s Law of Induction and Lenz’s Law

Magnetic FluxФB = BA cosθ

Induction

tN B

APHY20104/21/23 3

21.2 Faraday’s Law of Induction and Lenz’s Law

The induced current moves in a direction so its magnetic field opposes the original change in flux

Note there are two magnetic fields: one that induces the current in the coil and the one that is produced by the induced current

APHY20104/21/23 4

21.3 Induced EMF in a Moving Conductor

If B, l and v are perpendicularto each other, then

Example

BlvtB

APHY20104/21/23 5

21.5 Electric Generators

Mechanical energy to Electrical energy

DC and AC Commutators and slip rings

If the coil in an AC generator rotates

with angular speed ω then

ε = NBAω sin(ωt) ω = 2πf

APHY20104/21/23 6

21.5 Electric Generators

Examples

APHY20104/21/23 7

21.6 Back EMF, Counter Torque and Eddy Currents

As the coil rotates, an induced emf is produced that reduces the current in the coil.

Faster rotation → larger back emf Video Larger current in device → larger counter

torque

Armatures are laminated to reduce the heating due to eddy currents.