Magnetic Fields

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Chapter 29

• Magnetic Fields

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Quick Quiz 29.1
The north-pole end of a bar magnet is held near a
positively charged piece of plastic. The plastic
is (a) attracted (b) repelled (c) unaffected by
the magnet
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Quick Quiz 29.1
Answer (c). The magnetic force exerted by a
magnetic field on a charge is proportional to the
charges velocity relative to the field. If the
charge is stationary, as in this situation, there
is no magnetic force.
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Quick Quiz 29.2
A charged particle moves with velocity v in a
magnetic field B. The magnetic force on the
particle is a maximum when v is (a) parallel to
B (b) perpendicular to B (c) zero
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Quick Quiz 29.2
Answer (b). The maximum value of sin ? occurs
for ? 90.
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Quick Quiz 29.3
An electron moves in the plane of this paper
toward the top of the page. A magnetic field is
also in the plane of the page and directed toward
the right. The direction of the magnetic force on
the electron is (a) toward the top of the
page (b) toward the bottom of the page (c) toward
the left edge of the page (d) toward the right
edge of the page (e) upward out of the page (f)
downward into the page
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Quick Quiz 29.3
Answer (e). The right-hand rule gives the
direction. Be sure to account for the negative
charge on the electron.
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Quick Quiz 29.4
The four wires shown below all carry the same
current from point A to point B through the same
magnetic field. In all four parts of the figure,
the points A and B are 10 cm apart. Which of the
following ranks wires according to the magnitude
of the magnetic force exerted on them, from
greatest to least? (a) b, c, d (b) a, c, b (c)
d, c, b (d) c, a, b (e) No force is exerted on
any of the wires.
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Quick Quiz 29.4
Answer (a). (a), (b) (c), (d). The magnitude
of the force depends on the value of sin ?. The
maximum force occurs when the wire is
perpendicular to the field (a), and there is zero
force when the wire is parallel (d). Choices (b)
and (c) represent the same force because Case 1
tells us that a straight wire between A and B
will have the same force on it as the curved
wire.
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Quick Quiz 29.5
A wire carries current in the plane of this paper
toward the top of the page. The wire experiences
a magnetic force toward the right edge of the
page. The direction of the magnetic field causing
this force is (a) in the plane of the page and
toward the left edge (b) in the plane of the page
and toward the bottom edge (c) upward out of the
page (d) downward into the page
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Quick Quiz 29.5
Answer (c). Use the right-hand rule to determine
the direction of the magnetic field.
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Quick Quiz 29.6
Rank the magnitudes of the torques acting on the
rectangular loops shown in the figure below, from
highest to lowest. (All the loops are identical
and carry the same current.) (a) a, b, c (b) b,
c, a (c) c, b, a (d) a, c, b. (e) All loops
experience zero torque.
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Quick Quiz 29.6
Answer (c). Because all loops enclose the same
area and carry the same current, the magnitude of
? is the same for all. For part (c) in the image,
? points upward and is perpendicular to the
magnetic field and ? ?B, the maximum torque
possible. For the loop in (a), ? points along the
direction of B and the torque is zero. For (b),
the torque is intermediate between zero and the
maximum value.
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Quick Quiz 29.7
Rank the magnitudes of the net forces acting on
the rectangular loops shown in this figure, from
highest to lowest. (All the loops are identical
and carry the same current.) (a) a, b, c (b) b,
c, a (c) c, b, a (d) b, a, c (e) All loops
experience zero net force.
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Quick Quiz 29.7
Answer (e). Because the magnetic field is
uniform, there is zero net force on all three
loops.
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Quick Quiz 29.8
A charged particle is moving perpendicular to a
magnetic field in a circle with a radius r. An
identical particle enters the field, with v
perpendicular to B, but with a higher speed v
than the first particle. Compared to the radius
of the circle for the first particle, the radius
of the circle for the second particle is (a)
smaller (b) larger (c) equal in size
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Quick Quiz 29.8
Answer (b). The magnetic force on the particle
increases in proportion to v, but the centripetal
acceleration increases according to the square of
v. The result is a larger radius, as we can see
from Equation 29.13.
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Quick Quiz 29.9
A charged particle is moving perpendicular to a
magnetic field in a circle with a radius r. The
magnitude of the magnetic field is increased.
Compared to the initial radius of the circular
path, the radius of the new path is (a) smaller
(b) larger (c) equal in size
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Quick Quiz 29.9
Answer (a). The magnetic force on the particle
increases in proportion to B. The result is a
smaller radius, as we can see from Equation
29.13.
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Quick Quiz 29.10a
Three types of particles enter a mass
spectrometer like the one shown in your book as
Figure 29.24. The figure below shows where the
particles strike the detector array. Rank the
particles that arrive at a, b, and c by
speed. (a) a, b, c (b) b, c, a (c) c, b, a
(d) All their speeds are equal.
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Quick Quiz 29.10a
Answer (d). The velocity selector ensures that
all three types of particles have the same speed.
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Quick Quiz 29.10b
Rank the particles that arrive at a, b, and c by
m/q ratio. (a) a, b, c (b) b, c, a (c) c, b, a
(d) All their m/q ratios are equal.
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Quick Quiz 29.10b
Answer (c). We cannot determine individual
masses or charges, but we can rank the particles
by m/q ratio. Equation 29.18 indicates that those
particles traveling through the circle of
greatest radius have the greatest m/q ratio.