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Ch 21 1
Chapter 21
Electromagnetic InductionFaraday’s Law
AC Circuits
© 2006, B.J. LiebSome figures electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey
Giancoli, PHYSICS,6/E © 2004.
Ch 21 2
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 3
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.
cosBAABB
•In a properly drawn magnetic field, flux is proportional to the total number of lines passing through a point.
Ch 21 4
Faraday’s Law of induction
• the flux B can change because
B is changing
A is changing or moving
A is rotating
tN B
Ch 21 5
Lenz’s LawAn 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 6
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 increasing
I decreasing
I constant I increasing
Ch 21 7
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.
cosABB
0Initially
ABABBi 0cos
90PositionFinal
090cos ABBf
t
N B
t
ABN
0t
ABN
s
mT
20.0
)045.0()15.0(10 2
V34.0
2RA 2)12.0( m
2045.0 m
Ch 21 8
Example 21-1
tB
t
AB
t
xlB
vlB
vlB)4.3()35.0()45.0( smmT
V54.0
RI
RI
23.0
54.0 V A3.2
clockwise
isitsofluxdownwarda
maintaintotriescurrent
wirem0.35theonforce
magneticthefromforcethecalculatecanWe
sinBlIF BlI
BlIF )45.0()35.0()3.2( TmA
NmA
NmA 36.036.0
Ch 21 9
Electric Generators
A coil of wire rotating in a magnetic field experiences an sinusoidal EMF:
sinNBlv2This is the basis of electric generator.
Ch 21 10
Transformers
Changing current in the primary creates a changing flux in the secondary coil
tNV B
ss
t
NV BPP
If the flux change is equal
p
s
p
s
N
N
V
V
In an ideal transformer: Power Out = Power In
sspp IVIV
Ch 21 11
Example 21-3
P
S
P
S
N
N
V
V
PP
SS V
N
NV
V120
330
1240
V45
OUTPOWERINPOWER
sSPP VIVI
SP
SP I
V
VI
A
V
V15
120
451
A56
Ch 21 12
Power Grid
Power is transmitted at the highest possible voltage in order to minimize losses.