 Electromagnetic Induction

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Electromagnetic Induction
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 • Changing magnetic field produces electric field

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

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

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

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

2) Motional emf b) Application

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

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

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

3) Magnetic flux, F Define Flux:
- Proportional to the lines of force through a surface B Normal f

b) Flux and motional emf

4) Lenz’s law Direction of induced current produces a magnetic field that opposes the change in flux flux into loop increases because of v: DF/Dt > 0 induced current produces flux out of loop (opposite direction) Define external flux as positive. Then

flux into loop 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

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

7) Induction and sound Microphone

Guitar pickup

Magnetic tape