Chapter 34. Electromagnetic Induction
Electromagnetic induction
is the scientific principle
that underlies many
modern technologies, from
the generation of
electricity to
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electricity to
communications and data
storage.
Chapter Goal: To
understand and apply
electromagnetic induction.
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Chapter 34. Electromagnetic Induction
Topics:
• Induced Currents
• Motional emf
• Magnetic Flux
• Lenz’s Law
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• Lenz’s Law
• Faraday’s Law
• Induced Fields
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Chapter 34. Reading Quizzes
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Chapter 34. Reading Quizzes
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Electromagnetic induction was
discovered by
A. Faraday.
B. Henry.
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B. Henry.
C. Maxwell.
D. Both Faraday and Henry.
E. All three.
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Electromagnetic induction was
discovered by
A. Faraday.
B. Henry.
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B. Henry.
C. Maxwell.
D. Both Faraday and Henry.
E. All three.
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The direction that an induced
current flows in a circuit is
given by
A. Faraday’s law.
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A. Faraday’s law.
B. Lenz’s law.
C. Henry’s law.
D. Hertz’s law.
E. Maxwell’s law.
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The direction that an induced
current flows in a circuit is
given by
A. Faraday’s law.
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A. Faraday’s law.
B. Lenz’s law.
C. Henry’s law.
D. Hertz’s law.
E. Maxwell’s law.
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Chapter 34. Basic Content and Examples
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Chapter 34. Basic Content and Examples
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Faraday’s Discovery
Faraday found that there is a current in a coil
of wire if and only if the magnetic field passing
through the coil is changing. This is an informal
statement of Faraday’s law.
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statement of Faraday’s law.
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Induced Emf and Induced Current
There are a number of ways a magnetic field can be used to generate an electric current.
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It is the changing field that produces the current.
Induced Emf and Induced Current
An emf can be induced by changing thearea of a coil in a constant magnetic field
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In each example, both an emf and a current are induced because the coil is part of a complete circuit. If the circuit were open, therewould be no induced current, but there would be an induced emf.
The phenomena of producing an induced emf with the aid of amagnetic field is called electromagnetic induction.
Motional Emf
THE EMF INDUCED IN A MOVING CONDUCTOR
Each charge within the conductoris moving and experiences a magnetic force
qvBF =
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The separated charges on theends of the conductor give riseto an induced emf, called amotional emf.
Motional Emf
vBL=E
Motional emf when v, B,
and L are mutually
perpendicular
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vBL=E
Motional emf
The motional emf of a conductor of length l
moving with velocity v perpendicular to a
magnetic field B is
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EXAMPLE 34.1 Measuring the earth’s magnetic
field
QUESTION:
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EXAMPLE 34.1 Measuring the earth’s magnetic
field
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EXAMPLE 34.1 Measuring the earth’s magnetic
field
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Magnetic flux can be
defined in terms of an area vector
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Magnetic Flux
The magnetic flux measures the amount of
magnetic field passing through a loop of area A
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magnetic field passing through a loop of area A
if the loop is tilted at an angle θ from the field,
B. As a dot-product, the equation becomes:
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Lenz’s Law
There is an induced current in a closed,
conducting loop if and only if the magnetic flux
through the loop is changing. The direction of
the induced current is such that the induced
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the induced current is such that the induced
magnetic field opposes the change in the flux.
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Lenz’s Law
The Emf Produced by a Moving Copper Ring.
There is a constant magnetic field directed into the page in the shaded region. The fieldis zero outside the shaded region. A copperring slides through the region.
For each of the five positions, determine whetheran induced current exists and, if so, find itsdirection.
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direction.
Tactics: Using Lenz’s Law
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Faraday’s Law
An emf is induced in a conducting loop if the
magnetic flux through the loop changes. The
magnitude of the emf is
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and the direction of the emf is such as to drive
an induced current in the direction
given by Lenz’s law.26
Problem-Solving Strategy: Electromagnetic
Induction
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Electromagnetic Waves
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Applications of Electromagnetic Induction to the Reproduction of Sound
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Transformers
A transformer is a device for increasing or decreasing an ac voltage.
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tN
∆
∆Φ−= ssE
tN
∆
∆Φ−= ppE
p
s
p
s
N
N=
E
E
Transformers
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s
p
s
p
p
s
N
N
V
V
I
I==
A transformer that steps up the voltage simultaneously steps
down the current, and a transformer that steps down the voltage
steps up the current.
Chapter 34. Summary Slides
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Chapter 34. Summary Slides
33
General Principles
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General Principles
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General Principles
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Important Concepts
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Important Concepts
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Chapter 34. Questions
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Chapter 34. Questions
39
Is there an induced current in this circuit? If
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Is there an induced current in this circuit? If
so, what is its direction?
A. No
B. Yes, clockwise
C. Yes, counterclockwise
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Is there an induced current in this circuit? If
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A. No
B. Yes, clockwise
C. Yes, counterclockwise
Is there an induced current in this circuit? If
so, what is its direction?
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A square loop of copper
wire is pulled through a
region of magnetic field.
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A. Fb = Fd > Fa = Fc
B. Fc > Fb = Fd > Fa
C. Fc > Fd > Fb > Fa
D. Fd > Fb > Fa = Fc
E. Fd > Fc > Fb > Fa
region of magnetic field.
Rank in order, from
strongest to weakest, the
pulling forces Fa, Fb, Fc
and Fd that must be
applied to keep the loop
moving at constant speed.
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A square loop of copper
wire is pulled through a
region of magnetic field.
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A. Fb = Fd > Fa = Fc
B. Fc > Fb = Fd > Fa
C. Fc > Fd > Fb > Fa
D. Fd > Fb > Fa = Fc
E. Fd > Fc > Fb > Fa
region of magnetic field.
Rank in order, from
strongest to weakest, the
pulling forces Fa, Fb, Fc
and Fd that must be
applied to keep the loop
moving at constant speed.
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A current-carrying wire is pulled away from a
conducting loop in the direction shown. As the
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conducting loop in the direction shown. As the
wire is moving, is there a cw current around the
loop, a ccw current or no current?
A. There is no current around the loop.
B. There is a clockwise current around the loop.
C. There is a counterclockwise current around the loop.
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A current-carrying wire is pulled away from a
conducting loop in the direction shown. As the
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A. There is no current around the loop.
B. There is a clockwise current around the loop.
C. There is a counterclockwise current around the loop.
conducting loop in the direction shown. As the
wire is moving, is there a cw current around the
loop, a ccw current or no current?
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