SECTION 29.4 • Motion of a Charged Particle in a Uniform Magnetic Field
909
Example 29.7 Bending an Electron Beam
In an experiment designed to measure the magnitude of a
uniform magnetic field, electrons are accelerated from rest
through a potential difference of 350 V. The electrons travel
along a curved path because of the magnetic force exerted
on them, and the radius of the path is measured to be 7.5 cm.
(Fig. 29.20 shows such a curved beam of electrons.) If the
magnetic field is perpendicular to the beam,
(A)
what is the magnitude of the field?
Solution Conceptualize the circular motion of the
electrons with the help of Figures 29.18 and 29.20. We
categorize this problem as one involving both uniform
circular motion and a magnetic force. Looking at Equation
29.13, we see that we need the speed v of the electron if we
are to find the magnetic field magnitude, and v is not given.
Consequently, we must find the speed of the electron based
on the potential difference through which it is accelerated.
Therefore, we also categorize this as a problem in conserva-
tion of mechanical energy for an isolated system. To begin
analyzing the problem, we find the electron speed. For the
isolated electron–electric field system, the loss of potential
energy as the electron moves through the 350-V potential
difference appears as an increase in the kinetic energy of
the electron. Because K
i
"
0 and
, we have
Now, using Equation 29.13, we find
(B)
What is the angular speed of the electrons?
Solution Using Equation 29.14, we find that
8.4 % 10
$
4
T
"
B "
m
e
v
e
r
"
(9.11 % 10
$
31
kg)(1.11 % 10
7
m/s)
(1.60 % 10
$
19
C)(0.075 m)
" 1.11 % 10
7
m/s
v "
√
2e
∆V
m
e
"
√
2(1.60 % 10
$
19
C)(350 V)
9.11 % 10
$
31
kg
∆K ' ∆U " 0
9:
1
2
m
e
v
2
'
($e)
∆V " 0
K
f
"
1
2
m
e
v
2
To finalize this problem, note that the angular speed can
be represented as . " (1.5 % 10
8
rad/s)(1 rev/2/ rad) "
2.4 % 10
7
rev/s. The electrons travel around the circle 24
million times per second! This is consistent with the very
high speed that we found in part (A).
What If?
What if a sudden voltage surge causes the accel-
erating voltage to increase to 400 V? How does this affect
the angular speed of the electrons, assuming that the
magnetic field remains constant?
Answer The increase in accelerating voltage 0V will cause
the electrons to enter the magnetic field with a higher speed
v. This will cause them to travel in a circle with a larger
radius r. The angular speed is the ratio of v to r. Both v and
r increase by the same factor, so that the effects cancel and
the angular speed remains the same. Equation 29.14 is an
expression for the cyclotron frequency, which is the same as
the angular speed of the electrons. The cyclotron frequency
depends only on the charge q, the magnetic field B, and the
mass m
e
, none of which have changed. Thus, the voltage
surge has no effect on the angular speed. (However, in real-
ity, the voltage surge may also increase the magnetic field if
the magnetic field is powered by the same source as the
accelerating voltage. In this case, the angular speed will
increase according to Equation 29.14.)
1.5 % 10
8
rad/s
. "
v
r
"
1.11 % 10
7
m/s
0.075 m
"
Example 29.6 A Proton Moving Perpendicular to a Uniform Magnetic Field
A proton is moving in a circular orbit of radius 14 cm in a
uniform 0.35-T magnetic field perpendicular to the velocity
of the proton. Find the linear speed of the proton.
Solution From Equation 29.13, we have
4.7 % 10
6
m/s
"
v "
qBr
m
p
"
(1.60 % 10
$
19
C)(0.35 T)(0.14 m)
1.67 % 10
$
27
kg
What If?
What if an electron, rather than a proton, moves in
a direction perpendicular to the same magnetic field with this
same linear speed? Will the radius of its orbit be different?
Answer An electron has a much smaller mass than a proton,
so the magnetic force should be able to change its velocity
much easier than for the proton. Thus, we should expect the
radius to be smaller. Looking at Equation 29.13, we see that r
is proportional to m with q, B, and v the same for the electron
as for the proton. Consequently, the radius will be smaller by
the same factor as the ratio of masses m
e
/m
p
.
At the Interactive Worked Example link at http://www.pse6.com, you can investigate the relationship between the radius
of the circular path of the electrons and the magnetic field.
Interactive
Figure 29.20 (Example 29.7) The bending of an electron
beam in a magnetic field.
Henry Leap and Jim Lehman