W11D2
Concept Questions Review
Concept Questions with
Answers
8.01
W11D2
2
W07D1
Magnetic Dipoles, Force and
Torque on a Dipole, Experiment 2
Concept Questions with
Answers
W07D1 Magnetic Dipoles, Torque and Force on a
Dipole, Experiment 2: Magnetic Dipole in a Helmholtz
Coil
Reading Course Notes: Sections 8.4,8.11.6, 9.5 9.9
8.01
W7D1
4
Concept Question: Magnetic Field Lines
The picture shows the field lines outside a
permanent magnet The field lines inside the
magnet point:
1.
2.
3.
4.
5.
6.
Concept Q. Answer: Magnetic Field Lines
Answer: 1. They point up inside the magnet
Up
Down
Left to right
Right to left
The field inside is zero
I don’t know
5
Magnetic field lines are continuous.
E field lines begin and end on charges.
There are no magnetic charges (monopoles)
so B field lines never begin or end
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1
Concept Question: Parallel Wires
Concept Q. Answer: Parallel Wires
Consider two parallel current carrying
wires. With the currents running in the
opposite direction, the wires are
1.
2.
3.
4.
5.
Answer: 1. The wires are repelled
I1 creates a magnetic field into the
page at wire 2. That makes a force
on wire 2 to the right.
attracted (opposites attract?)
repelled (opposites repel?)
pushed another direction
not pushed – no net force
I don’t know
I2 creates a magnetic field into the
page at wire 1. That makes a force
on wire 1 to the left.
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Concept Question: Dipole in Uniform
Magnetic Field
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Concept Q. Answer: Dipole in Field
From rest, the coil in a uniform magnetic field above will:
1.
2.
3.
4.
5.
6.
7.
8.
rotate wise, not move
rotate counterclockwise, not move
move to the right, not rotate
move to the left, not rotate
move in another direction, without rotating
both move and rotate
neither rotate nor move
I don’t know
9
Concept Question: Dipole in Field
Answer: 1. Coil will rotate clockwise (not move)
No net force so no center of mass motion. BUT
Magnetic dipoles rotate to align with external
field (think compass)
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Concept Q. Answer: Dipole in Field
The current carrying coil above will feel a net force
1.
2.
3.
4.
upwards
downwards
of zero
I don’t know
Answer: 2. Feels downward force. The forces shown
produce a net downward force
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2
Concept Question: Dipole in Helmholtz
A dipole pointing along
the positive x-direction
and located at the center
of a Helmholtz coil will
feel:
1.
2.
3.
4.
a force but not a torque.
a torque but not a force.
both a torque and a force.
neither force nor torque.
Concept Question: Dipole in AntiHelmholtz Coil
A dipole pointing along
the positive z-direction
and located at the center
of an anti- Helmholtz coil
will feel:
1.
2.
3.
4.
a force but not a torque.
a torque but not a force.
both a torque and a force.
neither force nor torque.
Concept Questions with
Answers
8.01
W9D1
Concept Q. Answer: Dipole in Helmholtz
Answer: 2. a torque but not
a force. The Helmholtz coil
makes a UNIFORM FIELD.
Dipole feels only torque
(need gradient for force).
Concept Q. Answer: Dipole in AntiHelmholtz Coil
Answer: 1. A force
because there is a nongradient of the magnetic
field but no torque because
the magnetic field at the
center is zero.
W09D1:
Sources of Magnetic Fields:
Ampere’s Law
Today’s Reading Assignment Course Notes: Sections 10.1-10.6
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3
Concept Question: Line Integral
C.Q. Answer: Line Integral
The integral expression
1.
is equal to the magnetic work done around a closed
path.
2.
is an infinite sum of the product of the tangent
component of the magnetic field along a small
element of the closed path with a small element of
the path up to a choice of plus or minus sign.
3.
is always zero.
4.
is equal to the magnetic potential energy between
two points.
5.
None of the above.
2. A line integral by definition is the sum
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Concept Question: Ampere’s Law
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C.Q. Answer: Ampere’s Law
Integrating B around the loop shown gives us:
1.  a positive number
2.  a negative number
3.  zero
We need to make a choice of integration direction
(circulation) for the line integral. The small line
element
is tangent to the line and points in the
direction of circulation. The dot product therefore is
the product of the tangent component of the
magnetic field in the direction of the line element.
So the answer depends on which way we circulate
around the path.
Answer: 3. Total enclosed current is zero, so
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Concept Question: Ampere’s Law
22
C.Q. Answer: Ampere’s Law
Integrating B around the loop in the clockwise
direction shown gives us:
1.  a positive number
2.  a negative number
3.  zero
Answer: 2.
Net enclosed current is out of the page, so field is
counter-clockwise (opposite to circulation direction)
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W09D2:
Faraday’s Law
Concept Questions with
Answers
8.01
W9D2
Today’s Reading Assignment Course Notes: Sections 9.3-9.4, 9.6
26
Concept Question: Loop in Uniform Field
Concept Q. Ans.: Loop in Uniform Field
Answer: 1. The motion changes the magnetic flux through the
loop. The magnetic flux is decreasing in time as more of the
loop enters a region of zero magnetic field. According to
Faraday’s Law there is an induced current through the loop.
While a rectangular wire loop is
pulled upward though a uniform
magnetic field B field penetrating its
bottom half, as shown, there is
1.  a current in the loop.
2.  no current in the loop.
3.  I do not understand the concepts of current and
magnetic field.
4.  I understand the concepts of current and magnetic field
but am not sure of the answer.
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Concept Q.: Loop in Uniform Field
28
Concept Q. Ans.: Loop in Uniform Field
Answer: 2. The motion does not change the magnetic flux
through the loop. The magnetic flux is constant in time.
According to Faraday’s Law there is no induced current
through the loop.
While a rectangular wire loop is
pulled sideways though a uniform
magnetic field B field penetrating its
bottom half, as shown, there is
1.  a current in the loop.
2.  no current in the loop.
3.  I do not understand the concepts of current and
magnetic field.
4.  I understand the concepts of current and magnetic field
but am not sure of the answer.
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5
Concept Question: Loop
Concept Question Answer: Loop
Answer: 1. Induced current
is clockwise
The magnetic field through
a wire loop is pointed
upwards and increasing
with time. The induced
current in the coil is
This produces an “induced”
B field pointing down over
the area of the loop.
The “induced” B field
opposes the increasing flux
through the loop – Lenz’s
Law
1.  Clockwise as seen from the top
2.  Counterclockwise
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Concept Question: Moving Loop
Concept Q. Answer: Moving Loop
Answer: 1. Induced current is
clockwise
A circuit in the form of a
rectangular piece of wire is
pulled away from a long wire
carrying current I in the
direction shown in the sketch.
The induced current in the
rectangular circuit is
B due to I is into page; the flux
through the circuit due to that field
decreases as the circuit moves
away. So the induced current is
clockwise (to make a B into the
page)
1.  Clockwise
2.  Counterclockwise
3.  Neither, the current is zero
33
Concept Question: Faraday’s Law:
Loop
Current clockwise; force up
Current counterclockwise; force up
Current clockwise; force down
Current counterclockwise; force down
Note: Iind dl x B force is left on the left segment and
right on the right, but the force on the left is bigger.
So the net force on the rectangular circuit is to the
left, again trying to keep the flux from decreasing by
slowing the circuit’s motion
34
Concept Question Answer:
Faraday’s Law: Loop
Answer: 3. Current is clockwise; force is down
A coil moves up from
underneath a magnet
with its north pole
pointing upward. The
current in the coil and
the force on the coil:
1.
2.
3.
4.
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The clockwise current creates
a self-field downward, trying
to offset the increase of
magnetic flux through the coil
as it moves upward into
stronger fields (Lenz’s Law).
The I dl x B force on the coil is a force which is trying
to keep the flux through the coil from increasing by
slowing it down (Lenz’s Law again).
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W10D1:
Inductance and Magnetic
Field Energy
Concept Questions with
Answers
8.01
W10D1
Today’s Reading Assignment W10D1 Inductance & Magnetic
Energy
38
Concept Question: Solenoid
Concept Q. Ans.: Solenoid
A very long solenoid consisting of N turns has
radius R and length d, (d>>R). Suppose the
number of turns is halved keeping all the other
parameters fixed. The self inductance
1. remains the same.
2. doubles.
3. is halved.
4. is four times as large.
5. is four times as small.
6. None of the above.
39
Solution 5. The self-induction of the
solenoid is equal to the total flux
through the object which is the product
of the number of turns time the flux
through each turn. The flux through
each turn is proportional to the
magnitude of magnetic field which is
proportional to the number of turns per
unit length or hence proportional to the
number of turns. Hence the selfinduction of the solenoid is proportional
to the square of the number of turns.
If the number of turns is halved keeping
all the other parameters fixed then he
self inductance is four times as small.
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