ch 19-20 practice - Geneva Area City Schools

AP Test ch 19-20
Multiple Choice
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1. An electron moves through a region of crossed electric and magnetic fields. The electric field E = 2 000 V/m
and is directed straight down. The magnetic field B = 0.80 T and is directed to the left. For what velocity v of
the electron into the paper will the electric force exactly cancel the magnetic force?
a. 2 500 m/s
b. 4 000 m/s
c. 5 000 m/s
d. 8 000 m/s
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2. A wire is lying horizontally in the north-south direction and the horizontal magnetic field is toward the east.
Some positive charges in the wire move north and an equal number of negative charges move south. The
direction of the force on the wire will be:
a. east.
b. down, into the page.
c. up, out of the page.
d. There is no magnetic force.
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3. A circular loop carrying a current of 1.0 A is oriented in a magnetic field of 0.35 T. The loop has an area of
0.24 m 2 and is mounted on an axis, perpendicular to the magnetic field, which allows the loop to rotate. If the
plane of the loop is oriented parallel to the field, what torque is created by the interaction of the loop current
and the field?
a. 5.8 N m
b. 0.68 N m
c. 0.084 N m
d. 0.017 N m
____ 4. A proton moving with a speed of 3.0 10 5 m/s perpendicular to a uniform magnetic field of 0.20 T will
follow which of the paths described below? (q p = 1.6 10 19 C and m p = 1.67 10 27 kg)
a. a straight line path
b. a circular path of 1.6 cm radius
c. a circular path of 3.1 cm radius
d. a circular path of 0.78 cm radius
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5. A proton, which moves perpendicular to a magnetic field of 1.2 T in a circular path of radius 0.080 m, has
what speed? (q p = 1.6 10 19 C and m p = 1.67 10 27 kg)

a. 3.4 10 6 m/s
b. 4.6 10 6 m/s
c. 9.6 10 6 m/s
d. 9.2 10 6 m/s
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6. A proton with initial kinetic energy E is moving in circular motion in a uniform magnetic field. When it has
completed one eighth of a revolution, what is its kinetic energy?
a. 1.4 E
b. 0.71 E
c. E
d. The value is not given.
7. A 100-m-long wire carrying a current of 4.0 A will be accompanied by a magnetic field of what strength at a
distance of 0.050 m from the wire? (magnetic permeability in empty space µ 0 = 4 10 7 T m/A)
a. 4.0 10 5 T
b. 2.0 10 5 T
c. 1.6 10 5 T
d. zero
8. A current in a long, straight wire produces a magnetic field. The magnetic field lines:
a. go out from the wire to infinity.
b. come in from infinity to the wire.
c. form circles that pass through the wire.
d. form circles that go around the wire.
9. Two long parallel wires 40 cm apart are carrying currents of 10 A and 20 A in the opposite direction. What is
the magnitude of the magnetic field halfway between the wires?
a. 1.0 10 -5 T
b. 2.0 10 -5 T
c. 3.0 10 -5 T
d. 4.0 10 -5 T
____ 10. Two parallel conductors are carrying currents in the same direction. The currents are non-zero and not
necessarily equal. The magnitude of the magnetic field midway between them is 40 T. If one of the currents
then has its direction reversed, what is the resulting magnitude of the magnetic field midway between them?
a. a value greater than 40 T
b. 40 T
c. a value less than 40 T
d. It could be any value.
____ 11. Two parallel wires are separated by 0.25 m. Wire A carries 5.0 A and Wire B carries 10 A, both currents in
the same direction. The force on 0.80 m of Wire A is:
a. 3.2 10 -5 N.
b. 2.6 10 -5 N.
c. 1.6 10 -5 N.
d. less than 1.0 10 -5 N.
____ 12. Solenoid #1 has a length L, cross-sectional area A, and N turns. Solenoid #2 has a length 2L, cross-sectional
area 2A, and 2N turns. Which solenoid has the greater magnetic field at its center when equal currents are
going through them?
a. #1

. #2
c. Both have the same magnetic field.
d. Since it depends on the values of A and L, none of the above are correct.
____ 13. A 10-turn square coil of area 0.036 m 2 and a 20-turn circular coil are both placed perpendicular to the same
changing magnetic field. The voltage induced in each of the coils is the same. What is the area of the
circular coil?
a. 0.072 m 2
b. 0.60 m 2
c. 0.018 m 2
d. 0.036 m 2
____ 14. A coil is placed in a changing magnetic field and an emf is induced. What happens to the induced emf if the
rate of change of magnetic field quadruples?
a. There is no change.
b. The emf doubles.
c. The emf quadruples.
d. The emf increases by a factor of 16.
____ 15. A straight wire of length is oriented east-west and is in a magnetic field B pointing north. The wire is
moving downward at a constant speed v. If the resistance of the rod is R, what is the current through the rod?
a. R/B v
b. B v/R
c. B 2 2 v 2 /R 2
d. not given
____ 16. The operation of a tape player to play music depends on which of the following?
a. the Doppler effect
b. the photoelectric effect
c. the force acting on a current-carrying wire in a magnetic field
d. induced current from the motion of a magnet past a wire
____ 17. If a bar magnet is falling through a loop of wire, the induced current in the loop of wire sets up a field which
exerts a force on the magnet. This force between the magnet and the loop will be attractive when:
a. the magnet enters the loop.
b. the magnet is halfway through.
c. the magnet is leaving the loop.
d. never.
____ 18. The operation of an electric motor depends on which of the following effects?
a. the Doppler effect
b. the photoelectric effect
c. the force acting on a current-carrying wire in a magnetic field
d. current from the motion of a wire in a magnetic field
____ 19. The back emf in an electric motor is its maximum value under which condition?
a. motor speed is zero
b. current is a maximum
c. voltage is a maximum
d. motor speed is a maximum
____ 20. The current in a coil with a self-inductance of 1.5 mH increases from 0 to 1.0 A in a tenth of a second. What
is the induced emf in the coil?

a. 15 mV
b. 30 mV
c. 0.10 V
d. 0.30 V
____ 21. By what factor is the self-inductance of an air solenoid changed if only its number of coil turns, N, is tripled?
a. 1/3
b. 3
c. 6
d. 9
____ 22. Two loops of wire are arranged so that a changing current in one, the primary, will induce a current in the
other, the secondary. The secondary loop has twice as many turns as the primary loop. As long as the current
in the primary is steady at 3 A, the current in the secondary will be:
a. 3 A.
b. 6 A.
c. 1.5 A.
d. zero.
____ 23. Two loops of wire are arranged so that a changing current in one, the primary, will induce a current in the
other, the secondary. The secondary loop has twice as many turns as the primary loop. The current in the
primary at this moment is 3 A and increasing. The current in the secondary must be:
a. 3 A.
b. 6 A.
c. zero.
d. There is insufficient information to work this problem.
____ 24. A series circuit contains a 12-V battery, a 2.0- resistor, and a 3.0-mH inductor. If the switch to the battery is
closed at t = 0, find the time required for the current in the circuit to reach 63% of its final value.
a. 1.5 ms
b. 3.0 ms
c. 4.0 ms
d. 5.0 ms
____ 25. One time constant after an RL circuit has its switch closed, how does the current I in it compare to the
maximum current I max that occurs for this circuit?
a. I > I max /2
b. I = I max /2
c. I < I max /2
d. Without knowing R and L, this cannot be determined.