1. Transforming energy with magnetism
Magnetic Induction
Transforming energy with magnetism

2. What’s the Point?
This is how electric current is harnessed to do mechanical work.
It’s another way to think about the Lorentz force. ☺

3. Magnetic Flux Field through a surface FB = ∫B·dA
Effectively the number of field lines enclosed by a loop

4. Flux change creates emf
Faraday’s Law
The rate of magnetic flux change through a loop equals the emf induced around the loop.
e = − dF dt

5. Group Work a b c d e
At which position(s) does the most magnetic flux pass through the loop?
At which position(s) does the magnetic flux through the loop change the fastest?

6. Group Work a b c d e
What is the current direction at these points?

7. Faraday’s Law Greatest flux F when A || Bc d e
Greatest flux F when A || B
Fastest change in flux DF/Dt when A  B

8. Induced current direction
Lenz’s Law
When a changing magnetic flux induces a current, the induced current’s magnetic field opposes the flux change.

9. Lenz’s Law Example From a to b: flux decreases; DF into face of loop ←
Induced current a b c d e
From a to b:
flux decreases; DF into face of loop ←
Loop current increases
Induced field increases out of face of loop

10. AP Physics L15_Faraday
Board Work
What is the Lorentz force on each side?
Which direction is the flux change?
What current would oppose it? B
If the loop inside this magnetic field expands, in what direction will the induced current around it be?

11. Group Work
Which direction is the flux change?
What current would oppose it?
What is the Lorentz force on each side? B
If the magnetic field intensifies, in what direction will the induced current be?

12. Dynamo
changing field e
opposing field B
needed I

13. How a Microphone Works microphone geophone
Flux inside loop becomes greater

14. Wave-Powered Generator
Copyright__. Originally appeared in Smithsonian magazine.

15. How a Loudspeaker Works
induced motion of magnet
current-generated field attracts or repels magnet
current