Transcript Chapter 20 - apphysicswarren

Lecture Outline
Chapter 20
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
20.1 Induced emf: Faraday’s Law
and Lenz’s Law
• Emf – what is it???
• It is capable of producing what???
20.1 Induced emf: Faraday’s Law
and Lenz’s Law
We observe that, when a magnet is moved
near a conducting loop, a current is induced.
When the motion stops, the current stops.
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20.1 Induced emf: Faraday’s Law
and Lenz’s Law
On the other hand, when a loop moves
parallel to a magnetic field, no current is
induced.
© 2010 Pearson Education, Inc.
20.1 Induced emf: Faraday’s Law
and Lenz’s Law
Changing current in one
loop can induce a current
in a second loop. (3rd way)
a.) When the switch is
closing the buildup of
current produces a
changing magnetic field in
the other loop (inducing
current)
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20.1 Induced emf: Faraday’s Law
and Lenz’s Law
We conclude that current is induced only
when the magnetic field through the loop
changes. This is called…
An induced emf is produced in a loop or complete
circuit whenever the number of magnetic field lines
passing through the plane of the loop or circuit
changes.
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20.1 Induced emf: Faraday’s Law
and Lenz’s Law
In order to measure the change in the magnetic
field through a loop, we define the magnetic
flux:
SI unit of magnetic flux: the weber, Wb
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20.1 Induced emf: Faraday’s Law
and Lenz’s Law
Faraday’s law for the induced emf:
The minus sign indicates the direction of
the induced emf, which is given by Lenz’s
law.
In words: this is the amount magnetic flux
changes in a certain amount of time.
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20.1 Induced emf: Faraday’s Law
and Lenz’s Law
Lenz’s law:
An induced emf in a wire loop or coil has a direction
such that the current it creates produces its own
magnetic field that opposes the change in magnetic
flux through that loop or coil.
So if the magnetic field is increasing, the
induced current will produce a field in the
opposite direction, tending to decrease the
field.
Aka…if flux is positive, current is negative.
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20.1 Induced emf: Faraday’s Law
and Lenz’s Law
The direction of the induced current is given by
a right-hand rule.
With the thumb of the right hand pointing in the
direction of the induced field, the fingers curl in the
direction of the induced current.
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20.1 Induced emf: Faraday’s Law
and Lenz’s Law
• A) The south end of a bar magnet
is pulled far away from a small wire
coil.
• B) Looking from behind the coil
toward the south end of the
magnet what is the direction of the
induced current: clockwise,
counterclockwise, or no induced
current?
• C) Suppose the magnetic field over
the area of the coil is initially
constant at 40 mT, the coil’s radius
is 2.0 mm, and there are 100 loops
in the coil. Determine the induced
emf in the coil if the magnet is
removed in 0.750s.
20.1 Induced emf: Faraday’s Law
and Lenz’s Law
• In rural areas where electric power
lines carry electricity to big cities, it
is possible to generate small
electric currents by means of
induction in a conducting loop. The
overhead power lines carry
alternating currents that
periodically reverse direction 60
times per second. How would you
orient the plane of the loop to
maximize the induced current if the
power lines run north to south:
• Parallel to Earth’s surface
• Perpendicular to Earth’s surface
• Perpendicular to Earth’s surface in
the east-to-west direction
20.1 Induced emf: Faraday’s Law
and Lenz’s Law
• Suppose an electromagnet
exposes a speaker to a maximum
magnetic field of 1.00 mT that
reverses direction every 1/120s.
Assume the speaker’s coil consists
of 100 circular loops (each with
radius of 3.00 cm) and has a total
resistance of 1.00 ohm. According
to the manufacturer of the speaker,
the average current in the coil
should not exceed 25.0 mA.
• A) Calculate the magnitude of the
average induced emf in the coil
during the time period.
• B) Is the induced current likely to
damage the speaker coil?
20.4 Electromagnetic Waves
James Clerk Maxwell showed how the electric
and magnetic fields could be viewed as a single
electromagnetic field, with the following
properties:
A time-varying magnetic field produces a time-varying
electric field.
A time-varying electric field produces a time-varying
magnetic field.
For the first time, both fields thought of as ONE!
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20.4 Electromagnetic Waves
An accelerating charge produces an
electromagnetic wave. The electric and
magnetic fields are perpendicular to each other
and to the direction of propagation of the wave.
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20.4 Electromagnetic Waves
All electromagnetic waves travel at the
same speed in vacuum:
In a vacuum, all electromagnetic waves, regardless
of frequency or wavelength, travel at the same
speed, c = 3.00 × 108 m/s.
This finite speed of electromagnetic waves
leads to delays in transmitting signals over
long distances, such as to spacecraft.
All self-propagating; act as transverse
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20.4 Electromagnetic Waves
An electromagnetic wave
transmits energy; its electric
and magnetic fields are
capable of accelerating
charged particles. It will exert
a force on any surface it
intercepts; this phenomenon
is called radiation pressure. It
is negligible in everyday
experience, but could be
used to power “solar sails”
for interplanetary travel.
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20.4 Electromagnetic Waves
Electromagnetic waves can have any
frequency. Different frequencies have been
given different labels.
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20.4 Electromagnetic Waves
• Frequency and
wavelength are
inversely related.
• 2 formulas! SUPER
SIMPLE!
• Categories of
spectrum:
–
–
–
–
–
–
–
–
Power Waves
Radio Waves
Microwaves
Infrared Waves
Visible Light Waves
UV Rays
X-Rays
Gamma Rays
20.4 Electromagnetic Waves
• The first successful Mars landings were the Viking
probes in 1976. They sent radio and TV signals back to
Earth. How much longer would it have taken for a signal
to reach us when Mars was farthest from the Earth than
when it was closest to us? The average distance of Mars
and Earth and the Earth from the Sun are 229 million km
and 150 million km respectively. Assume that both
planets have circular orbits, and use the average
distances as the radii of the circles.