ConcepTest

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ConcepTest 17 A uniform magnetic field B
points into the page as shown. A short straight
wire (length L) is moved to the right with speed
v. Consider the following directions. Hold up
as many cards as you need to specify the
direction.
l
v
What is the magnitude of the electric field in
the wire, in steady state?
What is the magnitude of the potential difference
in the wire?
a) What direction is the magnetic force acting
on the electrons in the wire? b) What direction
is the electric field acting in the wire?
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ConcepTest 18 A uniform magnetic field B
points into the page as shown. A piece of
conducting wire is bent into a sideways U
shape. Another piece of short straight wire
can slide along the rails of the U shape, and
is moved to the right speed with speed v.
What is the direction of the induced current in
the loop?
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v
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Example Applying Faradays Law for Changing
Area A uniform magnetic field points into the
page as shown. A piece of short straight wire
can slide along the rails of the U shape, and
is moved to the right speed with speed v.
What is the magnitude of the induced e in this
situation?
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l
v
x
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Example Applying Faradays Law for Changing
Angle A circular conducting loop of area 0.1
m2 has 5 turns and is in a uniform magnetic field
of 0.02 T that points in the positive x
direction. What is the magnetic flux through
the loop at the instant pictured? What
happens as the loop rotates about the z axis?
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Example Applying Faradays Law for Changing
Magnetic Field A uniform magnetic field B is
produced in a solenoid of cross sectional area A,
as shown. A loop is concentric with the axis of
the solenoid, and has radius R. The current in
the solenoid varies with time, so the magnetic
field in the solenoid also varies with time, such
that B(t) B0t. What is the magnitude of the
induced e in the loop? What is the magnitude of
the induced electric field? If the loop were
conducting, what direction would the induced
current flow?
R
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Lenzs Law for determining direction of induced
electric field, current
Magnetic field increases?
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Example Applying Lenzs Law Fight the Flux
Change! A solenoid is used to create a uniform
magnetic field that points into the page. A wire
loop of area A is in the magnetic field, as
shown. For each of the following cases, choose
one of the following for the direction of the
induced current in the loop

• The loop is moved to the left, staying in
• the B field.
• The loop is crushed to a smaller area.
• The loop is rotated so that the top of the loop
  comes out of the page and the bottom of the loop
  goes into the page.
• The magnetic field is increased.
• e) The magnetic field direction is reversed.

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ConcepTest 19 A long straight wire carries
current to the right, as shown. A square loop of
wire (with a resistor) is initially stationary
below the straight wire. Consider the following
directions

• If the current in the straight wire is
  decreasing, what is the direction of the induced
  current in the resistor?
• c) What direction will the loop move? (Ignore
  rotations)

a) What is the direction of the magnetic field
due to the straight wire at the location of the
wire loop?
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ConcepTest 20 A bar magnet is released above
a circular loop of wire as shown, and falls
through the loop. The loop is held
fixed. Consider the following directions, as
viewed when looking at the loop from above
N S
N S
N
b) What is the direction of the induced current
in the wire loop, while the bar magnet is falling
away from the loop from below?
a) What is the direction of the induced current
in the wire loop, while the bar magnet is falling
towards the loop from above?
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Try it Yourself A uniform magnetic field
points into the page as shown. A disk made out
of a conducting material that lies in the plane
of the page is rotated clockwise. Is
any e induced in the disk? Explain.
Try it Yourself A long straight wire carries
current I out of the page as shown, and is
surrounded by a circular wire loop of radius a.
The current in the straight wire is
decreasing. Is any current induced in the
circular loop? If so, in what direction?
Explain.
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