Download SP212E.1121 JVanhoy Test 2 – Magnetic Fields 27 Mar 03 You may

SP212E.1121
JVanhoy
27 Mar 03
Test 2 – Magnetic Fields
You may use an equation sheet for this test. Although most questions are multiple choice, you
should write down any work that you do while arriving at the answer as partial credit may be
possible on a few questions.
1. An electron moves in the negative x direction, through a uniform magnetic field in the
negative y direction. The magnetic force on the electron is:
A)
B)
C)
D)
E)
in the negative x direction
in the positive y direction
in the negative y direction
in the positive z direction
in the negative z direction
2. At any point the magnetic field lines are in the direction of:
A) the magnetic force on a moving positive charge
B) the magnetic force on a moving negative charge
C) the velocity of a moving positive charge
D) the velocity of a moving negative charge
E) none of the above
3. A magnetic field CANNOT:
A) exert a force on a charge
B) accelerate a charge
C) change the momentum of a charge
D) change the kinetic energy of a charge
E) exist
4. At one instant an electron (charge = –1.6 × 10–19 C) is moving in the xy plane, the
components of its velocity being vx = 5 × 105 m/s and vy = 3 × 105 m/s. A magnetic field
of 0.8 T is in the positive z direction. At that instant the magnitude of the magnetic force
on the electron is:
A) 0
B) 3.8 × 10–14 N
C) 5.1 × 10–14 N
D) 6.4 × 10–14 N
E) 7.5 × 10–14 N
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SP212E.1121
JVanhoy
27 Mar 03
5.
Test 2 – Magnetic Fields
A uniform magnetic field is directed into the page. A charged particle, moving in the
plane of the page, follows a clockwise spiral of decreasing radius as shown. A
reasonable explanation is:
A)
B)
C)
D)
E)
the charge is positive and slowing down
the charge is negative and slowing down
the charge is positive and speeding up
the charge is negative and speeding up
none of the above
6. An electron is launched with velocity v in a uniform magnetic field B. The angle
between v and B is between 0 and 90o. As a result, the electron follows a helix, its
velocity vector v returning to its initial value in a time interval of:
A) 2m/eB
B) 2mv/eB
C) 2mv sin/eB
D) 2mv cos/eB
E) none of these
7. The current is from left to right in the conductor shown. The magnetic field is into the
page and point S is at a higher potential than point T. The charge carriers are:
A)
B)
C)
D)
E)
positive
negative
neutral
absent
moving near the speed of light
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SP212E.1121
JVanhoy
27 Mar 03
Test 2 – Magnetic Fields
8. The diagram shows a straight wire carrying a flow of electrons into the page. The wire is
between the poles of a permanent magnet. The direction of the magnetic force exerted on
the wire is:
A)
B)
C)
D)
E)




into the page
9. The diagrams show five possible orientations of a magnetic dipole  in a uniform
magnetic field B. For which of these does the magnetic torque on the dipole have the
greatest magnitude?
(circle the figure)
10. The diagrams show five possible orientations of a magnetic dipole  in a uniform
magnetic field B. For which of these is the potential energy the greatest?
(circle the figure)
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SP212E.1121
JVanhoy
27 Mar 03
Test 2 – Magnetic Fields
11. Electrons are going around a circle in a counterclockwise direction as shown. At the
center of the circle they produce a magnetic field that is:
A)
B)
C)
D)
E)
into the page
out of the page
to the left
to the right
zero
12. Which graph correctly gives the magnitude of the magnetic field outside an infinitely
long straight current-carrying wire as a function of the distance r from the wire?
(circle the figure)
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SP212E.1121
JVanhoy
27 Mar 03
Test 2 – Magnetic Fields
13. Four long straight wires carry equal currents into the page as shown. The magnetic force
exerted on wire F is:
A)
B)
C)
D)
E)
north
east
south
west
zero
14. Magnetic field lines inside the solenoid shown are:
A)
B)
C)
D)
E)
clockwise circles as one looks down the axis from the top of the page
counterclockwise circles as one looks down the axis from the top of the page
toward the top of the page
toward the bottom of the page
in no direction since B = 0
15. Faraday's law states that an induced emf is proportional to:
A) the rate of change of the magnetic field
B) the rate of change of the electric field
C) the rate of change of the magnetic flux
D) the rate of change of the electric flux
E) zero
16. A vertical bar magnet is dropped through the center of a horizontal loop of wire, with its
north pole leading. At the instant when the midpoint of the magnet is in the plane of the
loop, the induced current at point P, viewed from above, is:
A) maximum and clockwise
B) maximum and counterclockwise
C) not maximum but clockwise
D) not maximum but counterclockwise
E) essentially zero
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SP212E.1121
JVanhoy
27 Mar 03
Test 2 – Magnetic Fields
17. The circuit shown is in a uniform magnetic field that is into the page and is decreasing in
magnitude at the rate 150 T/s. The current in the circuit (in amperes) is:
A)
B)
C)
D)
E)
0.18
0.22
0.40
0.62
none of these
18. The diagrams show three circuits with identical batteries, identical inductors, and
identical resistors. Rank them according to the current through the battery just after the
switch is closed, from least to greatest.
A)
B)
C)
D)
E)
3, 2, 1
2 and 3 ties, then 1
1, 3, 2
1, 2, 3
3, 1, 2
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SP212E.1121
JVanhoy
27 Mar 03
Test 2 – Magnetic Fields
19. A single loop of wire with a radius of 7.5 cm rotates about a diameter in a uniform
magnetic field of 1.6 T. To produce a maximum emf of 1.0 V, it should rotate at:
A) 0
B) 2.7 rad/s
C) 5.6 rad/s
D) 35 rad/s
E) 71 rad/s
20. A 6.0 mH inductor is in a series circuit with a resistor and an ideal battery. At the instant
the current in the circuit is 5.0 A the energy stored in the inductor is:
A) 0
B) 7.5 × 10–2 J
C) 15 × 10–2 J
D) 30 × 10–2 J
E) unknown since the rate of change of the current is not given
21. In the circuit shown, switch S is first pushed up to charge the capacitor. When S is then
pushed down, the current in the circuit will oscillate at a frequency of:
A)
B)
C)
D)
E)
318 Hz
0.01 Hz
12.500 Hz
2000 Hz
depends on V0
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SP212E.1121
JVanhoy
27 Mar 03
Test 2 – Magnetic Fields
22. For the situations described in the figures below, indicate the direction of the induced
current through the resistor. If there is no induced current, state so.
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SP212E.1121
JVanhoy
27 Mar 03
Test 2 – Magnetic Fields
23.
The figure below shows a cross section of a long coaxial cable. The cable has an inner
conductor of radius "a" and a thin outer shield of radius "b". Equal but opposite currents I exist
in the two conductors.
b
a
Starting from Ampere's Law, derive expressions for B(r) in the ranges:
a)
r<a
b)
a<r<b
c)
r>b
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SP212E.1121
JVanhoy
27 Mar 03
Test 2 – Magnetic Fields
24. Starting from the Biot-Savart Law, derive an expression for the magnitude of the B-field at
the center of a circular loop of wire of radius R carrying a current I by following the
procedure:
a) Write down the Biot-Savart Law
b) Draw a sketch of the loop and annotate it with each variable/vector in the Biot-Savart
Law. (note that 4 and o are not considered variables, but all other symbols are).
c) Work out your expression for the B-field below.
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