1. Chapter 22 Electromagnetic Induction

2. 1) Induced emf and induced current Changing B-field induces current

3. Changing coil area or orientation induces current

4. Changing the number of lines of force through loop induces current Induced current indicates induced emf and induced electric field Changing magnetic field produces electric field

5. 2) Motional emf a)The principle B-field exerts force on moving charges + v FBFB - v FBFB

6. ==> free charges separate E E separated charges produce electric field and a corresponding potential difference

7. E E E - field exerts force on charges + FEFE - FEFE

8. E E Charges stop moving when the forces balance: Induced emf predicted from statics so,

9. b) Application

10. c) Induced emf from conservation of energy F A = F B FBFB

11. FBFB Power to push rod at speed v: Using,

12. F A = F B FBFB Power to push rod at speed v: Electrical power consumed: So,

13. 3) Magnetic flux,  a)Define Flux: - Proportional to the lines of force through a surface

14. Flux depends on angle

16. b) Flux and motional emf t

17. 4) Lenz’s law Direction of induced current produces a magnetic field that opposes the change in flux -flux through loop (in) increases because of v:  t > 0 - induced current produces flux out of diagram (opposite direction) -Define flux from external field as positive. Then

18. -flux through loop (in) decreases because of v:  t < 0 - induced current produces flux into loop (same direction to oppose the decrease) -Define external flux as positive. Then induced flux is also positive, so again v

19. 5) Faraday’s Law For any changing flux in any loop, the emf induced in the loop is (for N turns) where  is the flux through one turn

20. a) Conservation of energy Lenz’s law is statement of energy conservation Flux increases Induced field repels magnet; work required to produce current

21. Flux decreases Induced field attracts magnet; work required to produce current

22. Example: Find direction of the current in each loop zero ccw cw

23. b) Induced polarity Inside the loop, current is forced from negative to positive (by work that produces the change in flux) I (like inside a battery) In the external circuit, current flows from positive to negative

24. c) Eddy currents Power loss hence, laminated cores in transformers

25. Induction stove

26. 6) Electric Generator

27. a) The principle

29. b) Conservation of energy and countertorque If I = 0, coil turns without resistance If I > 0, force on wires resists spinning Larger load requires more fuel

30. c) Back emf of an electric motor Rotating coil in a motor acts like a generator According to Lenz’s law, the induced emf opposes external current more power required initially to accelerate motor higher speed produces higher back emf, and lower current max speed --> minimum current (minimum power) stopped motor draws maximum current

31. 7) Induction and sound Microphone

32. Guitar pickup

33. Magnetic tape