1. APHY201 1/4/2016 1 21.1 Induced EMF   A changing magnetic field will induce an emf which produces a current.

2. APHY201 1/4/2016 2 21.2 Faraday’s Law of Induction and Lenz’s Law   Magnetic Flux Ф B = BA cosθ   Induction

3. APHY201 1/4/2016 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

4. APHY201 1/4/2016 4 21.3 Induced EMF in a Moving Conductor   If B, l and v are perpendicular to each other, then Example

5. APHY201 1/4/2016 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

6. APHY201 1/4/2016 6 21.5 Electric Generators Examples

7. APHY201 1/4/2016 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.