Transcript Slide 1

Physics for Scientists and Engineers, 6e
Chapter 29 – Magnetic Fields
The north-pole end of a bar magnet is held near a
positively charged piece of plastic. The plastic is
1
1.
attracted
2.
repelled
3.
unaffected by the magnet
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3
4
5
33%
1
33%
2
33%
3
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.
A charged particle moves with velocity v in a
magnetic field B. The magnetic force on the particle
is a maximum when v is
1
1.
parallel to B
2.
perpendicular to B
3.
zero
2
3
4
5
33%
1
33%
2
33%
3
The maximum value of sin θ occurs for θ = 90°.
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
1
1.
toward the top of the page
2.
toward the bottom of the
page
3.
toward the left edge of the
page
4.
toward the right edge of
the page
5.
upward out of the page
6.
downward into the page
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3
4
5
17% 17% 17% 17% 17% 17%
1
2
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6
The right-hand rule gives the direction. Be sure
to account for the negative charge on the
electron.
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?
1
1.
b, c, d
2.
a, c, b
3.
d, c, b
4.
c, a, b
5.
No force is exerted on
any of the wires.
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3
4
20% 20% 20% 20% 20%
5
1
2
3
4
5
(a), (b) = (c), (d). The magnitude of the force
depends on the value of sin θ. 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 (2) and (3) 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.
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
1. in the plane of the page
and toward the left edge
25% 25% 25% 25%
1
2.
in the plane of the page
and toward the bottom
edge
3.
upward out of the page
4.
downward into the page
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5
1
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4
Use the right-hand rule to determine the
direction of the magnetic field.
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.)
1
1.
a, b, c
2.
b, c, a
3.
c, b, a
4.
a, c, b.
5.
All loops experience
zero torque.
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20% 20% 20% 20% 20%
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5
Because all loops enclose the same area and
carry the same current, the magnitude of μ is
the same for all. For part (c) in the image, μ
points upward and is perpendicular to the
magnetic field and τ = μB, the maximum torque
possible. For the loop in (a), μ points along the
direction of B and the torque is zero. For (b), the
torque is intermediate between zero and the
maximum value.
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.)
1
1.
a, b, c
2.
b, c, a
3.
c, b, a
4.
b, a, c
5.
All loops
experience zero
net force.
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3
4
20% 20% 20% 20% 20%
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1
2
3
4
5
Because the magnetic field is uniform, there is
zero net force on all three loops.
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
1
1.
smaller
2.
larger
3.
equal in size
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3
4
33%
5
1
33%
2
33%
3
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.
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
1
1.
smaller
2.
larger
3.
equal in size
2
3
4
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33%
1
33%
2
33%
3
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.
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.
1
1.
a, b, c
2.
b, c, a
3.
c, b, a
4.
All their speeds
are equal.
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3
4
The velocity selector ensures that all three
types of particles have the same speed.
Rank the particles that arrive at a, b, and c by m/q ratio.
1
1.
a, b, c
2.
b, c, a
3.
c, b, a
4.
All their m/q ratios
are equal.
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3
4
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1
2
3
4
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.