Transcript Lecture_13

Chapter 29
Electromagnetic Induction
and Faraday’s Law
Copyright © 2009 Pearson Education, Inc.
Units of Chapter 29
• Induced EMF
• Faraday’s Law of Induction; Lenz’s Law
• EMF Induced in a Moving Conductor
• Electric Generators
• Back EMF and Counter Torque; Eddy
Currents
Copyright © 2009 Pearson Education, Inc.
Units of Chapter 29
• Transformers and Transmission of
Power
• A Changing Magnetic Flux Produces an
Electric Field
• Applications of Induction: Sound
Systems, Computer Memory,
Seismograph, GFCI
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29-1 Induced EMF
Almost 200 years ago, Faraday looked for
evidence that a magnetic field would induce
an electric current with this apparatus:
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29-1 Induced EMF
He found no evidence when the current was
steady, but did see a current induced when the
switch was turned on or off.
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29-1 Induced EMF
Therefore, a changing magnetic field induces
an emf.
Faraday’s experiment used a magnetic field
that was changing because the current
producing it was changing; the previous
graphic shows a magnetic field that is
changing because the magnet is moving.
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29-2 Faraday’s Law of Induction
The induced emf in a wire loop is proportional
to the rate of change of magnetic flux through
the loop.
Magnetic flux:
Unit of magnetic flux: weber, Wb:
1 Wb = 1 T·m2.
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29-2 Faraday’s Law of Induction
This drawing shows the variables in the flux
equation:
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29-2 Faraday’s Law of Induction
The magnetic flux is analogous to the electric
flux – it is proportional to the total number of
magnetic field lines passing through the loop.
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29-2 Faraday’s Law of Induction
Conceptual Example 29-1: Determining flux.
A square loop of wire encloses area A1. A uniform
magnetic field B perpendicular to the loop
extends over the area A2. What is the magnetic
flux through the loop A1?
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29-2 Faraday’s Law of Induction
Faraday’s law of induction: the emf induced in
a circuit is equal to the rate of change of
magnetic flux through the circuit:
dB
d
d
 

A  B    A B cos 
dt
dt
dt



or
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
29-2 Faraday’s Law of Induction
Example 29-2: A loop of wire in a magnetic
field.
A square loop of wire of side l = 5.0 cm is in a
uniform magnetic field B = 0.16 T. What is the
magnetic flux in the loop (a) when B is
perpendicular to the face of the loop and (b)
when B is at an angle of 30° to the area A of the
loop? (c) What is the magnitude of the average
current in the loop if it has a resistance of
0.012 Ω and it is rotated from position (b) to
position (a) in 0.14 s?
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ConcepTest 29.2a Moving Bar Magnet I
If a north pole moves toward the
1) clockwise
loop from above the page, in what
2) counterclockwise
direction is the induced current?
3) no induced current
ConcepTest 29.2a Moving Bar Magnet I
If a north pole moves toward the
1) clockwise
loop from above the page, in what
2) counterclockwise
direction is the induced current?
3) no induced current
The magnetic field of the moving bar
magnet is pointing into the page and
getting larger as the magnet moves
closer to the loop. Thus the induced
magnetic field has to point out of the
page. A counterclockwise induced
current will give just such an induced
magnetic field.
Follow-up: What happens if the magnet is stationary but the loop moves?
ConcepTest 29.2b Moving Bar Magnet II
If a north pole moves toward
1) clockwise
the loop in the plane of the
2) counterclockwise
page, in what direction is the
3) no induced current
induced current?
ConcepTest 29.2b Moving Bar Magnet II
If a north pole moves toward
1) clockwise
the loop in the plane of the
2) counterclockwise
page, in what direction is the
3) no induced current
induced current?
Since the magnet is moving parallel
to the loop, there is no magnetic
flux through the loop. Thus the
induced current is zero.
29-2 Faraday’s Law of Induction;
Lenz’s Law
The minus sign gives the direction of the
induced emf:
A current produced by an induced emf moves in a
direction so that the magnetic field it produces tends to
restore the changed field.
or:
An induced emf is always in a direction that opposes
the original change in flux that caused it.
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29-2 Faraday’s Law of Induction;
Lenz’s Law
Magnetic flux will change if the area of the
loop changes.
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29-2 Faraday’s Law of Induction;
Lenz’s Law
Magnetic flux will change if the angle between
the loop and the field changes.
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29-2 Faraday’s Law of Induction;
Lenz’s Law
Conceptual Example 29-3: Induction stove.
In an induction stove, an ac current exists in
a coil that is the “burner” (a burner that
never gets hot). Why will it heat a metal pan
but not a glass container?
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29-2 Faraday’s Law of Induction;
Lenz’s Law
Problem Solving: Lenz’s Law
1. Determine whether the magnetic flux is increasing,
decreasing, or unchanged.
2. The magnetic field due to the induced current
points in the opposite direction to the original field
if the flux is increasing; in the same direction if it is
decreasing; and is zero if the flux is not changing.
3. Use the right-hand rule to determine the direction
of the current.
4. Remember that the external field and the field due
to the induced current are separate.
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29-2 Faraday’s Law of Induction;
Lenz’s Law
Conceptual Example 29-4: Practice with
Lenz’s law.
In which direction is the current induced in
the circular loop for each situation?
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29-2 Faraday’s Law of Induction;
Lenz’s Law
Example 29-5: Pulling a coil from
a magnetic field.
A 100-loop square coil of wire, with side
l = 5.00 cm and total resistance 100 Ω, is
positioned perpendicular to a uniform
0.600-T magnetic field. It is quickly
pulled from the field at constant speed
(moving perpendicular to B
B) to a region
where B drops abruptly to zero. At t = 0,
the right edge of the coil is at the edge
of the field. It takes 0.100 s for the whole
coil to reach the field-free region. Find
(a) the rate of change in flux through the
coil, and (b) the emf and current
induced. (c) How much energy is
dissipated in the coil? (d) What was the
average force required (Fext)?
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29-3 EMF Induced in a Moving
Conductor
This image shows another way the magnetic
flux can change:
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29-3 EMF Induced in a Moving
Conductor
The induced current is in a direction that tends
to slow the moving bar – it will take an external
force to keep it moving.
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29-3 EMF Induced in a Moving
Conductor
The induced emf has magnitude
This equation is valid as long as B, l, and
v are mutually perpendicular (if not, it is
true for their perpendicular components).
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29-3 EMF Induced in a Moving
Conductor
Example 29-8: Force on the rod.
To make the rod move to the right at speed v, you
need to apply an external force on the rod to the
right. (a) Explain and determine the magnitude of
the required force. (b) What external power is
needed to move the rod?
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ConcepTest 29.8a Loop and Wire I
A wire loop is being pulled away
from a current-carrying wire.
What is the direction of the
induced current in the loop?
I
1) clockwise
2) counterclockwise
3) no induced current
ConcepTest 29.8a Loop and Wire I
A wire loop is being pulled away
from a current-carrying wire.
What is the direction of the
induced current in the loop?
The magnetic flux is into the page on the
right side of the wire and decreasing due
to the fact that the loop is being pulled
away. By Lenz’s law, the induced B field
will oppose this decrease. Thus, the new
B field points into the page, which
requires an induced clockwise current to
produce such a B field.
1) clockwise
2) counterclockwise
3) no induced current
I
ConcepTest 29.8b Loop and Wire II
What is the induced current if
1) clockwise
the wire loop moves in the
2) counterclockwise
direction of the yellow arrow?
3) no induced current
I
ConcepTest 29.8b Loop and Wire II
What is the induced current if
1) clockwise
the wire loop moves in the
2) counterclockwise
direction of the yellow arrow?
3) no induced current
The magnetic flux through the loop
is not changing as it moves parallel
to the wire. Therefore, there is no
induced current.
I
29-4 Electric Generators
A generator is the opposite of a motor – it
transforms mechanical energy into electrical
energy. This is an ac generator:
The axle is rotated by an
external force such as
falling water or steam.
The brushes are in
constant electrical
contact with the slip
rings.
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29-4 Electric Generators
If the loop is rotating with constant angular
velocity ω, the induced emf is sinusoidal:
For a coil of N loops,
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29-4 Electric Generators
Example 29-9: An ac generator.
The armature of a 60-Hz ac
generator rotates in a 0.15-T
magnetic field. If the area of the coil
is 2.0 x 10-2 m2, how many loops
must the coil contain if the peak
output is to be V0 = 170 V?
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29-4 Electric Generators
A dc generator is similar, except that it
has a split-ring commutator instead of slip
rings.
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29-5 Back EMF and Counter Torque;
Eddy Currents
An electric motor turns because there is a
torque on it due to the current. We would
expect the motor to accelerate unless there is
some sort of drag torque.
That drag torque exists, and is due to the
induced emf, called a back emf.
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29-5 Back EMF and Counter Torque;
Eddy Currents
A similar effect occurs in a generator – if it is
connected to a circuit, current will flow in it,
and will produce a counter torque. This
means the external applied torque must
increase to keep the generator turning.
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29-5 Back EMF and Counter Torque;
Eddy Currents
Induced currents can flow
in bulk material as well as
through wires. These are
called eddy currents, and
can dramatically slow a
conductor moving into or
out of a magnetic field.
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29-6 Transformers and Transmission
of Power
A transformer consists of two coils, either
interwoven or linked by an iron core. A
changing emf in one induces an emf in the
other.
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29-6 Transformers and Transmission
of Power
I P  BP   P  N P BP A
BP  I P   P  N P   I P  A
B
BS  BP !
  S  N S BS A  N S B P A  N S   I P  A
dI
dP
 N P A P
dt
dt
dS
dI
 N S A P
VS  
dt
dt
N
V
 S  S
VP N P
VP  
Conservation of Energy:
I PVP  I SVS

VS I P

VP I S
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
B
B

B
29-6 Transformers and Transmission
of Power
Example 29-12: Cell phone charger.
The charger for a cell phone contains a
transformer that reduces 120-V ac to 5.0-V ac
to charge the 3.7-V battery. (It also contains
diodes to change the 5.0-V ac to 5.0-V dc.)
Suppose the secondary coil contains 30 turns
and the charger supplies 700 mA. Calculate
(a) the number of turns in the primary coil, (b)
the current in the primary, and (c) the power
transformed.
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29-6 Transformers and Transmission
of Power
Transformers work only if the current is
changing; this is one reason why electricity
is transmitted as ac.
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29-6 Transformers and Transmission
of Power
Example 29-13: Transmission lines.
An average of 120 kW of electric power is sent
to a small town from a power plant 10 km away.
The transmission lines have a total resistance
of 0.40 Ω. Calculate the power loss if the power
is transmitted at (a) 240 V and (b) 24,000 V.
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ConcepTest 29.12b Transformers II
1) 1/4 A
Given that the intermediate
2) 1/2 A
current is 1 A, what is the
3) 1 A
current through the
4) 2 A
lightbulb?
5) 5 A
1 A
120 V
240 V
120 V
ConcepTest 29.12b Transformers II
1) 1/4 A
Given that the intermediate
current is 1 A, what is the
current through the
lightbulb?
2) 1/2 A
3) 1 A
4) 2 A
5) 5 A
Power in = Power out
240 V  1 A = 120 V  ???
1 A
The unknown current is 2 A.
120 V
240 V
120 V
ConcepTest 29.12c Transformers III
A 6 V battery is connected to
one side of a transformer.
Compared to the voltage drop
1) greater than 6 V
2) 6 V
across coil A, the voltage
3) less than 6 V
across coil B is:
4) zero
A
6V
B
ConcepTest 29.12c Transformers III
A 6 V battery is connected to
1) greater than 6 V
one side of a transformer.
2) 6 V
Compared to the voltage drop
across coil A, the voltage
3) less than 6 V
across coil B is:
4) zero
The voltage across B is zero.
Only a changing magnetic flux
induces an emf. Batteries can
provide only dc current.
A
6V
B