Transcript Giancoli, PHYSICS,6/E

Chapter 21
Electromagnetic Induction
Faraday’s Law
AC Circuits
© 2006, B.J. Lieb
Some figures electronically reproduced by permission of
Pearson Education, Inc., Upper Saddle River, New Jersey
Giancoli, PHYSICS,6/E © 2004.
Ch 21
1
Induction
•Discovered in 1820 by Michael Faraday and Joseph Henry
•Magnetic field causes a current, but only when the
magnetic flux is changing.
Ch 21
2
Magnetic Flux
•Necessary to explain induction
•In the figure below A is the area of a surface-usually the
area inside of a coil of wire.
 B  B A  BA cos 
•In a properly drawn magnetic field, flux is proportional to the
total number of lines passing through a point.
Ch 21
3
Faraday’s Law of induction

 B
 N
t
• the flux B can change because
B is changing
A is changing or moving
A is rotating
Ch 21
4
Lenz’s Law
An induced emf always gives rise to a current whose magnetic field
opposes the original change in flux. In the figure below:
(a) Original state is no flux, so current flows in clockwise
direction which would give a downward magnetic flux
(b) Original state is upward flux so current flows
counterclockwise to maintain that state.
(c) No flux change, so no current.
Ch 21
5
Example
In the figures below, a circular loop of wire and a straight wire carrying a
current lie on the surface of a table. The straight wire is shown as an arrow in
the current direction. Determine the direction of the current induced in the
loop of wire due to the changing current in the straight wire for each figure.
I decreasing
I increasing
I constant
Ch 21
I increasing
6
Example 21-2 The loop shown is a 10-turn coil of wire of radius 12 cm and is in
a magnetic field of strength 0.15 T. Calculate the average EMF induced if the coil
rotates 900 about an axis perpendicular to the field in 0.20 seconds.
 B  B A cos
Initially   0
 Bi  B A cos 0  B A
Final Position   90
 Bf  B A cos 90  0
  B 


t


   N 
A   R2
  (0.12 m) 2
 0.045 m2
Ch 21
 0  B A
BA
  N
t
 t 
   N 
10 (0.15 T ) (0.045 m 2 )

0.20 s
  0.34 V
7
Example 21-1

  B
 B A

t
t

 B l x
 B l v
t
We can calculate the force from the magnetic
force on the 0.35 m wire
F  I l B sin   I l B
  Bl v
 (0.45 T ) (0.35 m) (3.4 m s)
  0.54 V
 IR
I
Ch 21
0.54 V

 2 .3 A

0.23 
R
F IlB
 (2.3 A) (0.35 m) (0.45 T )
 N 
  0.36 N
 0.36 A m 
A
m


current tries to maintain

a downward flux so it is
clockwise

8
Electric Generators
A coil of wire rotating in a magnetic field experiences
an sinusoidal EMF:

 2 NBlv sin 
This is the basis of electric generator.
Ch 21
9
Transformers
Changing current in the primary creates a changing flux in
the secondary coil
Vs  N s
 B
t
VP  N P
 B
t
If the flux change is equal
Vs
N
 s
Vp
Np
In an ideal transformer: Power Out = Power In
Ch 21
V p I p  Vs I s
10
Example 21-3
VS N S

VP N P
 1240 
N 
120 V   45 V
VS   S  VP  
 330 
 NP 
POWER
IN  POWER OUT
I P VP  I S Vs
V 
 451 V 
 15 A  56 A
I P   S  I S  
 120 V 
 VP 
Ch 21
11
Power Grid
Power is transmitted at the highest possible voltage
in order to minimize losses.
Ch 21
12