Transcript Notes & ConcepTests

ConcepTest #17:
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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.
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Consider the following directions. Hold
up as many cards as you need to specify
the direction.
1. Up
4. Left
2. Down
5. Into the page
3. Right
6. Out of the page
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ℓ
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What is the magnitude of the
electric field in the wire, in
steady state?
(back of card) No Direction

 
 F  0  Fe  Fm  0
  

 
qE  qv  B  0  E  ( v  B)

|E|  vB sin   vB
a) What direction is the magnetic force
acting on the electrons in the wire?
What is the magnitude of the
potential difference in the wire?
b) What direction is the electric field
acting in the wire?
 
|V |  | E  d |
 | Ed cos |  vB
ConcepTest #18:
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A uniform magnetic field B points into the
page as shown. A piece of conducting wire
is bent into a sideways “U” shape.
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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.
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What is the direction of the induced
current in the loop?
1. Clockwise
2. Counterclockwise
3. No current
4. Not enough info
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Example: Applying Faraday’s 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?
ℓ
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Example: Applying Faraday’s Law for
Changing Angle
y
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?
60o
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60o
What happens as the loop rotates about the
z axis?
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Example: Applying Faraday’s 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
Lenz’s Law for determining direction of induced electric field, current
Magnetic field increases?
Example: Applying Lenz’s 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:
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a) The loop is moved to the left, staying in
the B field.
b) The loop is crushed to a smaller area.
1. Clockwise
2. Counterclockwise
c) 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.
3. No current
d) The magnetic field is increased.
4. Not enough info
e) The magnetic field direction is
reversed.
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:
1. Up
4. Left
2. Down
5. Into the page
3. Right
6. Out of the page
b) If the current in the straight
wire is decreasing, what is the
direction of the induced
current in the resistor?
(back of card) No Direction
a) What is the direction of the magnetic
field due to the straight wire at the
location of the wire loop?
c) What direction will the loop
move? (Ignore rotations)
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.
N
S
Consider the following directions, as viewed
when looking at the loop from above:
N
1. Clockwise
S
2. Counterclockwise
3. No current
N
4. Not enough info
a) What is the direction of the
induced current in the wire loop,
while the bar magnet is falling
towards the loop from above?
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?
Try it Yourself:
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.
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 current induced in the circular
loop? If so, in what direction? Explain.
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Is any e induced in the disk? Explain.