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Chapter 22

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Chapter 22. Electromagnetic Induction. 1) Induced emf and induced current. Changing B-field induces current. Changing coil area or orientation induces current. Changing the number of lines of force through loop induces current. Induced current indicates induced emf and induced electric field.

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Chapter 22

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  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. FB v + v - FB 2) Motional emf • The principle • B-field exerts force on moving charges

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

  7. E E FE + - FE • E - field exerts force on charges

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

  9. b) Application

  10. FB FA= FB c) Induced emf from conservation of energy

  11. Using, FB FA= FB Power to push rod at speed v:

  12. So, FB FA= FB Power to push rod at speed v: Electrical power consumed:

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

  14. Flux depends on angle

  15. t b) Flux and motional emf

  16. 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: DF/Dt > 0 • induced current produces flux out of diagram (opposite direction) • Define flux from external field as positive. Then

  17. flux through loop (in) decreases because of v: DF/Dt < 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

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

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

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

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

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

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

  24. Induction stove

  25. 6) Electric Generator

  26. a) The principle

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

  28. 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 • higher speed produces higher back emf, and lower current • max speed --> minimum current (minimum power) • more power required initially to accelerate motor • stopped motor draws maximum current

  29. 7) Induction and sound • Microphone

  30. Guitar pickup

  31. Magnetic tape

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