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Ch. 19 & 20: Magnets & Electromagnetic Induction Study Guide
Multiple Choice
Identify the choice that best completes the statement or answers the question.
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1. Where is the magnitude of the magnetic field around a permanent magnet greatest?
a. The magnitude is greatest close to the poles.
b. The magnitude is greatest far from the poles.
c. The magnitude is equal at all points on the field.
d. The magnitude is greatest halfway between poles.
2. One useful way to model magnetic field strength is to define a quantity called magnetic flux
. Which of
the following definitions for magnetic flux,
, is correct?
a. the number of field lines that cross a certain area
b.
c. (surface area)  (magnetic field component normal to the plane of surface)
d. all of the above
3. All of the following statements about magnetic field lines around a permanent magnet are true except which
one?
a. Magnetic field lines appear to end at the north pole of a magnet.
b. Magnetic field lines have no beginning or end.
c. Magnetic field lines always form a closed loop.
d. In a permanent magnet, the field lines actually continue within the magnet itself.
4. A microscopic magnetic region composed of a group of atoms whose magnetic fields are aligned in a
common direction is called a(n) ____. In most materials, when these groups are randomly distributed, the
substance will show ____ no magnetism.
a. domain; no
c. cell; unusual
b. pole; some
d. ion; strong
5. In a magnetized substance, the domains
a. are randomly oriented.
c. line up mainly in one direction.
b. cancel each other.
d. can never be reoriented.
6. Which statement describes Earth’s magnetic declination?
a. the angle between Earth’s magnetic field and Earth’s surface
b. Earth’s magnetic field strength at the equator
c. the tendency for Earth’s field to reverse itself
d. the angle between true north and north indicated by a compass
7. A current in a long, straight wire produces a magnetic field. These magnetic field lines
a. radiate 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.
8. A solenoid is in an upright position on a table. A counterclockwise current of electrons—when viewed from
above—causes the solenoid to have a ____ magnetic pole at its bottom end. If a compass is placed at the top
of the solenoid, the north pole of the compass would be ____.
a. north; attracted
c. north; repelled
b. south; attracted
d. south; repelled
9. According to the right-hand rule, if a current-carrying wire is grasped in the right hand with the thumb in the
direction of the current, the four fingers will curl in the direction of
a. the magnetic force,
c. the current’s velocity, .
.
____ 10.
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b. the magnetic field, .
d. the current’s path, .
A 2.0 m wire segment carrying a current of 0.60 A oriented parallel to a uniform magnetic field of 0.50 T
experiences a force of what magnitude?
a. 0.60 N
c. 0.15 N
b. 0.30 N
d. 0.0 N
A current-carrying wire 0.50 m long is positioned perpendicular to a uniform magnetic field. If the current is
10.0 A and there is a resultant force of 3.0 N on the wire due to the interaction of the current and field, what is
the magnetic field strength?
a. 0.60 T
c. 1.8  10 T
b. 15 T
d. 6.7  10 T
Consider two long, straight, parallel wires, each carrying a current I. If the currents move in the same
direction,
a. the two wires will attract each other.
b. the two wires will repel each other.
c. the two wires will exert a torque on each other.
d. neither wire will exert a force on the other.
All of the following are ways to induce an emf in a loop of wire except which one?
a. Move the loop into or out of a magnetic field.
b. Rotate the loop in a magnetic field.
c. Move the loop parallel to a magnetic field.
d. Insert the loop into a changing magnetic field.
A bar magnet falls through a loop of wire with constant velocity, and the north pole enters the loop first. The
induced current will be greatest when the magnet is located so that the loop is
a. near either the north or the south pole.
b. near the north pole only.
c. near the middle of the magnet.
d. With no acceleration, the induced current is zero.
According to Lenz’s law, the magnetic field of an induced current in a conductor will
a. enhance the applied field.
b. heat the conductor.
c. increase the potential difference.
d. oppose a change in the applied magnetic field.
According to Lenz’s law, if the applied magnetic field changes,
a. the induced field attempts to keep the total field strength constant.
b. the induced field attempts to increase the total field strength.
c. the induced field attempts to decrease the total field strength.
d. the induced field attempts to oscillate about an equilibrium value.
____ 17. A coil with 150 turns and a cross-sectional area of
experiences a magnetic field whose strength
increases by +0.65T in 1.80 s. The plane of the coil is perpendicular to the plane of the applied magnetic field.
What is the induced emf in the coil?
a. 54 V
c. 27 V
b. 0.36 V
d. 54 V
____ 18. The operation of an electric motor depends on
a. the Doppler effect.
b. the photoelectric effect.
c. the magnetic force acting on a current-carrying wire in a magnetic field.
d. induced commutators from the rotation of a wire in a magnetic field.
____ 19. Under which condition is the back emf in an electric motor at its maximum value?
a. Motor speed is zero.
c. Voltage is at maximum.
b. Current is at maximum.
d. Motor speed is at maximum.
____ 20. An electric current that changes directions at regular intervals is called
a. directional current.
c. reversible current.
b. alternating current.
d. fluctuating current.
____ 21. Which of the following determines the maximum generated emf in an ac generator?
a. the coil rotation rate or angular frequency
b. the magnetic field strength, B
c. the loop area, A
d. all of the above
Use the following equations to find maximum emf.
.
____ 22. A generator consists of 10.0 turns of wire, and each turn has an area of
. If a maximum emf of
is induced when the coil rotates with a frequency of 60.0 Hz, what is the strength of the magnetic
field?
a. 0.15 T
c. 0.85 T
b. 0.34 T
d. 1.5 T
____ 23. Which conversion process is the basic function of the electric generator?
a. mechanical energy to electrical energy
c. low emf to high emf, or vice versa
b. electrical energy to mechanical energy
d. alternating current to direct current
____ 24. Which conversion process is the basic function of the electric motor?
a. mechanical energy to electrical energy
c. low emf to high emf, or vice versa
b. electrical energy to mechanical energy
d. alternating current to direct current
Short Answer
25. The magnetic field of a bar magnet is shown in the figure above. Is the magnet’s north pole at A or B? _____
26. A negative charge is moving through a magnetic field. The direction of motion and the direction of the force
acting on it at one moment are shown in the figure above. Find the direction of the magnetic field.
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Problem
27. An electron moves north at a velocity of 9.8  10 m/s and has a magnetic force of 5.6  10
on it. If the magnetic field points upward, what is the magnitude of the magnetic field?
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N west exerted
28. A wire 48 m long carries a current of 18 A from west to east. If a magnetic field of
directed
toward the south is acting on the wire, find the direction and magnitude of the magnetic force.
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29. A coil with 275 turns and a cross-sectional area of
experiences a magnetic field whose strength
increases by +0.900 T in 1.25 s. The plane of the coil is perpendicular to the plane of the magnetic field. What
is the induced emf in the coil?
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30. A coil with 650 turns produces an emf of 120 V as it passes through a magnetic field in 2.7 s. The magnetic
field change is
. The plane of the coil moves perpendicularly to the plane of the magnetic field. What
is area of the coil?
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