Problems
921
of the field. Calculate the radius of the path of the ion in
the field.
31.
Review Problem. One electron collides elastically with a
second electron initially at rest. After the collision, the
radii of their trajectories are 1.00 cm and 2.40 cm. The tra-
jectories are perpendicular to a uniform magnetic field of
magnitude 0.044 0 T. Determine the energy (in keV) of
the incident electron.
32.
A proton moving in a circular path perpendicular to a
constant magnetic field takes 1.00 +s to complete one
revolution. Determine the magnitude of the magnetic
field.
A proton (charge ' e, mass m
p
), a deuteron (charge ' e,
mass 2m
p
), and an alpha particle (charge ' 2e, mass 4m
p
)
are accelerated through a common potential difference
0
V. Each of the particles enters a uniform magnetic field
B, with its velocity in a direction perpendicular to B. The
proton moves in a circular path of radius r
p
. Determine
the radii of the circular orbits for the deuteron, r
d
, and
the alpha particle, r
1
, in terms of r
p
.
34.
Review Problem. An electron moves in a circular path
perpendicular to a constant magnetic field of magnitude
1.00 mT. The angular momentum of the electron about
the center of the circle is 4.00 % 10
$
25
J # s. Determine
(a) the radius of the circular path and (b) the speed of the
electron.
35. Calculate the cyclotron frequency of a proton in a mag-
netic field of magnitude 5.20 T.
36.
A singly charged ion of mass m is accelerated from rest by a
potential difference 0V. It is then deflected by a uniform
magnetic field (perpendicular to the ion’s velocity) into a
semicircle of radius R. Now a doubly charged ion of mass
m( is accelerated through the same potential difference
and deflected by the same magnetic field into a semicircle
of radius R( " 2R. What is the ratio of the masses of the
ions?
A cosmic-ray proton in interstellar space has an energy of
10.0 MeV and executes a circular orbit having a radius
equal to that of Mercury’s orbit around the Sun (5.80 %
10
10
m). What is the magnetic field in that region of
space?
38.
Figure 29.21 shows a charged particle traveling in a
nonuniform magnetic field forming a magnetic bottle.
(a) Explain why the positively charged particle in the
figure must be moving clockwise. The particle travels
along a helix whose radius decreases and whose pitch
decreases as the particle moves into a stronger magnetic
field. If the particle is moving to the right along the x axis,
its velocity in this direction will be reduced to zero and it
will be reflected from the right-hand side of the bottle,
acting as a “magnetic mirror.” The particle ends up
bouncing back and forth between the ends of the bottle.
(b) Explain qualitatively why the axial velocity is reduced
to zero as the particle moves into the region of strong
magnetic field at the end of the bottle. (c) Explain why the
tangential velocity increases as the particle approaches the
end of the bottle. (d) Explain why the orbiting particle has
a magnetic dipole moment. (e) Sketch the magnetic
moment and use the result of Problem 17 to explain again
37.
33.
how the nonuniform magnetic field exerts a force on the
orbiting particle along the x axis.
39.
A singly charged positive ion moving at 4.60 % 10
5
m/s
leaves a circular track of radius 7.94 mm along a direction
perpendicular to the 1.80-T magnetic field of a bubble
chamber. Compute the mass (in atomic mass units) of this
ion, and, from that value, identify it.
Section 29.5 Applications Involving Charged Particles
Moving in a Magnetic Field
40.
A velocity selector consists of electric and magnetic fields
described by the expressions
E " E kˆ and B " Bˆj, with
B " 15.0 mT. Find the value of E such that a 750-eV
electron moving along the positive x axis is undeflected.
41.
Singly charged uranium-238 ions are accelerated through
a potential difference of 2.00 kV and enter a uniform
magnetic field of 1.20 T directed perpendicular to their
velocities. (a) Determine the radius of their circular path.
(b) Repeat for uranium-235 ions. What If? How does the
ratio of these path radii depend on the accelerating
voltage and on the magnitude of the magnetic field?
42.
Consider the mass spectrometer shown schematically in
Figure 29.24. The magnitude of the electric field between
the plates of the velocity selector is 2 500 V/m, and
the magnetic field in both the velocity selector and the
deflection chamber has a magnitude of 0.035 0 T.
Calculate the radius of the path for a singly charged ion
having a mass m " 2.18 % 10
$
26
kg.
A cyclotron designed to accelerate protons has a magnetic
field of magnitude 0.450 T over a region of radius 1.20 m.
What are (a) the cyclotron frequency and (b) the maxi-
mum speed acquired by the protons?
44.
What is the required radius of a cyclotron designed to
accelerate protons to energies of 34.0 MeV using a
magnetic field of 5.20 T?
45.
A cyclotron designed to accelerate protons has an outer
radius of 0.350 m. The protons are emitted nearly at rest
from a source at the center and are accelerated through
600 V each time they cross the gap between the dees. The
dees are between the poles of an electromagnet where the
field is 0.800 T. (a) Find the cyclotron frequency. (b) Find
the speed at which protons exit the cyclotron and
(c) their maximum kinetic energy. (d) How many revolu-
tions does a proton make in the cyclotron? (e) For what
time interval does one proton accelerate?
46.
At the Fermilab accelerator in Batavia, Illinois, protons
having momentum 4.80 % 10
$
16
kg # m/s are held in a cir-
cular orbit of radius 1.00 km by an upward magnetic field.
What is the magnitude of this field?
The picture tube in a television uses magnetic deflec-
tion coils rather than electric deflection plates. Suppose an
electron beam is accelerated through a 50.0-kV potential
difference and then through a region of uniform magnetic
field 1.00 cm wide. The screen is located 10.0 cm from the
center of the coils and is 50.0 cm wide. When the field is
turned off, the electron beam hits the center of the screen.
What field magnitude is necessary to deflect the beam to
the side of the screen? Ignore relativistic corrections.
47.
43.