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## Electromagnetic Induction

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### ... Faraday s Law For any changing flux in any loop, the emf induced in the loop is (for N turns) a) ... Back emf of an electric motor 7) ... – PowerPoint PPT presentation

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Title: Electromagnetic Induction

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
1. The principle

B-field exerts force on moving charges
6
gt free charges separate
separated charges produce electric field and a
corresponding potential difference
7
E - field exerts force on charges
8
Charges stop moving when the forces balance
Induced emf predicted from statics
9
• b) Application

10
• c) Induced emf from conservation of energy

11
Power to push rod at speed v
12
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
(No Transcript)
16
• b) Flux and motional emf

17
4) Lenzs 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 gt 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
DF/Dt lt 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
• 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
20
• a) Conservation of energy

Lenzs 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
zero
Example Find direction of the current in each
loop
zero
zero
23
• 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
24
• c) Eddy currents

Power loss
hence, laminated cores in transformers
25
Induction stove
26
6) Electric Generator
27
• a) The principle

28
(No Transcript)
29
• b) Conservation of energy and countertorque

If I 0, coil turns without resistance
If I gt 0, force on wires resists spinning
30
• c) Back emf of an electric motor

Rotating coil in a motor acts like a generator
According to Lenzs law, the induced emf opposes
external current
higher speed produces higher back emf, and
lower current
max speed --gt minimum current (minimum power)
more power required initially to accelerate
motor
stopped motor draws maximum current
31
7) Induction and sound
• Microphone

32
• Guitar pickup

33
• Magnetic tape