Download Document

Chapter 29
Magnetic Fields
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
Quick Quiz 29.1
Answer: (c). The magnetic force exerted by a magnetic field
on a charge is proportional to the charge’s velocity relative
to the field. If the charge is stationary, as in this situation,
there is no magnetic force.
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
Quick Quiz 29.2
Answer: (b). The maximum value of sin  occurs for  =
90°.
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
Quick Quiz 29.3
Answer: (e). The right-hand rule gives the direction. Be sure
to account for the negative charge on the electron.
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.
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.
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
Quick Quiz 29.5
Answer: (c). Use the right-hand rule to determine the
direction of the magnetic field.
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.
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.
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.
Quick Quiz 29.7
Answer: (e). Because the magnetic field is uniform, there is
zero net force on all three loops.
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
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.
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
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.
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
(c) c, b, a
(b) b, c, a
(d) All their speeds are equal.
Quick Quiz 29.10a
Answer: (d). The velocity selector ensures that all three
types of particles have the same speed.
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.
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.