Download The loop has a counterclockwise current.

Chapter 25
Electromagnetic
Induction and
Electromagnetic
Waves
© 2010 Pearson Education, Inc.
PowerPoint® Lectures for
College Physics: A Strategic Approach, Second Edition
25 Electromagnetic Induction and
Electromagnetic Waves
© 2010 Pearson Education, Inc.
Slide 25-2
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Slide 25-3
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Slide 25-4
Electromagnetic Induction
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Slide 25-11
Motional emf
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Slide 25-12
Induced Current in a Circuit
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Slide 25-13
Magnetic Flux
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Slide 25-14
Magnetic Flux video that helped Steve R. of
PH204 summer session II 2012
 Ok wow, I'm super special and I completly
forgot to add the link for the video lol. Here it
is:
http://www.youtube.com/watch?v=pB7oZNBIqq
c
© 2010 Pearson Education, Inc.
Checking Understanding
A loop of wire of area A is tipped at an angle q to a uniform
magnetic field B. The maximum flux occurs for an angle q = 0°.
What angle q will give a flux that is ½ of this maximum value?
A.
B.
C.
D.
  30
  45
  60
  90
© 2010 Pearson Education, Inc.
Slide 25-15
Answer
A loop of wire of area A is tipped at an angle q to a uniform
magnetic field B. The maximum flux occurs for an angle q = 0°.
What angle q will give a flux that is ½ of this maximum value?
A.
B.
C.
D.
  30
  45
  60
  90
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Slide 25-16
Lenz’s Law
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Slide 25-17
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Slide 25-18
Checking Understanding
A long conductor carrying a current runs next to a loop of wire. The
current in the wire varies as in the graph. Which segment of the
graph corresponds to the largest induced current in the loop?
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Slide 25-19
Answer
A long conductor carrying a current runs next to a loop of wire. The
current in the wire varies as in the graph. Which segment of the
graph corresponds to the largest induced current in the loop?
E
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Slide 25-20
Checking Understanding
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is increasing, what can we say
about the current in the loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-21
Answer
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is increasing, what can we say
about the current in the loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-22
Checking Understanding
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is decreasing, what can we say
about the current in the loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-23
Answer
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is decreasing, what can we say
about the current in the loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-24
Checking Understanding
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is constant, what can we say about
the current in the loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-25
Answer
A magnetic field goes through a loop of wire, as below. If the
magnitude of the magnetic field is constant, what can we say about
the current in the loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-26
Checking Understanding
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Just before the
switch is closed, what can we say about the current in the lower
loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-27
Answer
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Just before the
switch is closed, what can we say about the current in the lower
loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-28
Checking Understanding
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Immediately after
the switch is closed, what can we say about the current in the
lower loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-29
Answer
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Immediately after
the switch is closed, what can we say about the current in the
lower loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-30
Checking Understanding
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Long after the
switch is closed, what can we say about the current in the lower
loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-31
Answer
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Immediately after
the switch is closed, what can we say about the current in the
lower loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-32
Checking Understanding
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Immediately after
the switch is reopened, what can we say about the current in the
lower loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
© 2010 Pearson Education, Inc.
Slide 25-33
Answer
A battery, a loop of wire, and a switch make a circuit below. A
second loop of wire sits directly below the first. Immediately after
the switch is reopened, what can we say about the current in the
lower loop?
A. The loop has a clockwise current.
B. The loop has a counterclockwise current.
C. The loop has no current.
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Slide 25-34
Eddy Currents
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Slide 25-35
Faraday’s Law
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Slide 25-36
Faraday’s Law of Induction
 Wikipedia Farady’s Law of Induction
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Example Problems
The figure shows a 10-cm-diameter loop in three different magnetic
fields. The loop’s resistance is 0.1 Ω. For each situation,
determine the magnitude and direction of the induced current.
A coil used to produce changing magnetic fields in a TMS
(transcranial magnetic field stimulation) device produces a
magnetic field that increases from 0 T to 2.5 T in a time of
200 s. Suppose this field extends throughout the entire head.
Estimate the size of the brain and calculate the induced emf in
a loop around the outside of the brain.
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Slide 25-37
Induced Fields
A changing magnetic field
induces an electric field.
A changing electric field induces
a magnetic field too.
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Slide 25-38
Electromagnetic Waves
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Slide 25-39
Checking Understanding
A plane electromagnetic wave has electric and magnetic fields at
all points in the plane as noted below. With the fields oriented as
shown, the wave is moving
A.
B.
C.
D.
E.
into the plane of the paper.
out of the plane of the paper.
to the left.
to the right.
toward the top of the paper.
© 2010 Pearson Education, Inc.
Slide 25-40
Answer
A plane electromagnetic wave has electric and magnetic fields at
all points in the plane as noted below. With the fields oriented as
shown, the wave is moving
A.
B.
C.
D.
E.
into the plane of the paper.
out of the plane of the paper.
to the left.
to the right.
toward the top of the paper.
© 2010 Pearson Education, Inc.
Slide 25-41
Intensity of an Electromagnetic Wave
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Slide 25-42
Example Problems: Electromagnetic Waves
Carry Energy
Inside the cavity of a microwave oven, the 2.4 GHz
electromagnetic waves have an intensity of 5.0 kW/m2. What is
the strength of the electric field? The magnetic field?
A digital cell phone emits a 1.9 GHz electromagnetic wave with
total power 0.60 W. At a cell phone tower 2.0 km away, what is the
intensity of the wave? (Assume that the wave spreads out
uniformly in all directions.) What are the electric and magnetic
field strengths at this distance?
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Slide 25-43
Polarization
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Slide 25-44
Example Problem
Light passed through a polarizing filter has an intensity of 2.0 W/m2.
How should a second polarizing filter be arranged to decrease the
intensity to 1.0 W/m2?
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Slide 25-45
The Electromagnetic Spectrum
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Slide 25-46
The Photon Model of Electromagnetic Waves
Ephoton = hf
h = 6.63 ´10 J · s
-34
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Slide 25-47
Example Problems
A gamma ray has a frequency of 2.4  1020 Hz. What is the
energy of an individual photon?
A typical digital cell phone emits radio waves with a frequency of
1.9 GHz. What is the wavelength, and what is the energy of
individual photons? If the phone emits 0.60 W, how many photons
are emitted each second?
© 2010 Pearson Education, Inc.
Slide 25-48
Example Problem: The Microwave Oven
Inside the cavity of a microwave oven, the 2.4 GHz
electromagnetic waves have an intensity of 5.0 kW/m2.
A.
B.
What is the strength of the electric field?
The magnetic field?
© 2010 Pearson Education, Inc.
Slide 25-49
Thermal Emission Spectrum
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Slide 25-50
Understanding Global Warming with Wien’s
Law and the Stephan-Boltzmann Law and
spectal absorption in the atmosphere.
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Understanding Global Warming with Wien’s
Law and the Stephan-Boltzmann Law and
spectal absorption in the atmosphere.
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Hunting with Thermal Radiation
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Slide 25-51
Seeing the Universe in a Different Light
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Slide 25-52
Summary
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Slide 25-53
Summary
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Slide 25-54
Additional Questions
A bar magnet sits inside a coil of wire that is connected to a
meter. The bar magnet is at rest in the coil. What can we say
about the current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
© 2010 Pearson Education, Inc.
Slide 25-55
Answer
A bar magnet sits inside a coil of wire that is connected to a
meter. The bar magnet is at rest in the coil. What can we say
about the current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
© 2010 Pearson Education, Inc.
Slide 25-56
Additional Questions
A bar magnet sits inside a coil of wire that is connected to a
meter. The bar magnet is pulled out of the coil. What can we say
about the current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
© 2010 Pearson Education, Inc.
Slide 25-57
Answer
A bar magnet sits inside a coil of wire that is connected to a
meter. The bar magnet is pulled out of the coil. What can we say
about the current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
© 2010 Pearson Education, Inc.
Slide 25-58
Additional Questions
A bar magnet sits inside a coil of wire that is connected to a
meter. The bar magnet is completely out of the coil and at rest.
What can we say about the current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
© 2010 Pearson Education, Inc.
Slide 25-59
Answer
A bar magnet sits inside a coil of wire that is connected to a
meter. The bar magnet is completely out of the coil and at rest.
What can we say about the current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
© 2010 Pearson Education, Inc.
Slide 25-60
Additional Questions
A bar magnet sits inside a coil of wire that is connected to a
meter. The bar magnet is reinserted into the coil. What can we
say about the current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
© 2010 Pearson Education, Inc.
Slide 25-61
Answer
A bar magnet sits inside a coil of wire that is connected to a
meter. The bar magnet is reinserted into the coil. What can we
say about the current in the meter?
A. The current goes from right to left.
B. The current goes from left to right.
C. There is no current in the meter.
© 2010 Pearson Education, Inc.
Slide 25-62
Additional Questions
A typical analog cell phone has a frequency of 850 MHz, a digital
phone a frequency of 1950 MHz. Compared to the signal from an
analog cell phone, the digital signal has
A.
B.
C.
D.
longer wavelength and lower photon energy.
longer wavelength and higher photon energy.
shorter wavelength and lower photon energy.
shorter wavelength and higher photon energy.
© 2010 Pearson Education, Inc.
Slide 25-63
Answer
A typical analog cell phone has a frequency of 850 MHz, a digital
phone a frequency of 1950 MHz. Compared to the signal from an
analog cell phone, the digital signal has
A.
B.
C.
D.
longer wavelength and lower photon energy.
longer wavelength and higher photon energy.
shorter wavelength and lower photon energy.
shorter wavelength and higher photon energy.
© 2010 Pearson Education, Inc.
Slide 25-64
Additional Questions
A radio tower emits two 50 W signals, one an AM signal at a
frequency of 850 kHz, one an FM signal at a frequency of 85 MHz.
Which signal has more photons per second?
A. The AM signal has more photons per second.
B. The FM signal has more photons per second.
C. Both signals have the same photons per second.
© 2010 Pearson Education, Inc.
Slide 25-65
Answer
A radio tower emits two 50 W signals, one an AM signal at a
frequency of 850 kHz, one an FM signal at a frequency of 85 MHz.
Which signal has more photons per second?
A. The AM signal has more photons per second.
B. The FM signal has more photons per second.
C. Both signals have the same photons per second.
© 2010 Pearson Education, Inc.
Slide 25-66
Additional Example Problems
Two metal loops face each other. The upper loop is suspended by
plastic springs and can move up or down. The lower loop is fixed
in place and is attached to a battery and a switch. Immediately
after the switch is closed,
A.
B.
Is there a force on the upper loop? If so, in which
direction will it move? Explain your reasoning.
Is there a torque on the upper loop? If so, which way will
it rotate? Explain your reasoning.
© 2010 Pearson Education, Inc.
Slide 25-67
Additional Example Problems
The outer coil of wire is 10 cm long, 2 cm in diameter, wrapped
tightly with one layer of 0.5-mm-diameter wire, and has a total
resistance of 1.0 Ω. It is attached to a battery, as shown, that
steadily decreases in voltage from 12 V to 0 V in 0.5 s, then
remains at 0 V for t > 0.5 s. The inner coil of wire is 1 cm long,
1 cm in diameter, has 10 turns of wire, and has a total resistance
of 0.01 Ω. It is connected, as shown, to a current meter.
A.
B.
As the voltage to the outer coil begins to decrease, in
which direction (left-to-right or right-to-left) does current
flow through the meter? Explain.
Draw a graph showing the current in the inner coil as a
function of time for 0 ≤ t ≤ 1 s. Include a numerical scale
on the vertical axis.
© 2010 Pearson Education, Inc.
Slide 25-68