CHAPTER
21
MAGNETIC FORCES AND
MAGNETIC FIELDS
ANSWERS TO FOCUS ON CONCEPTS QUESTIONS
____________________________________________________________________________________________
1.
(d)
RightHand Rule No. 1 gives the direction of the magnetic force as

x
for both drawings
A and B. In drawing C, the velocity is parallel to the magnetic field, so the magnetic force is
zero.
2.
(b)
Using RightHand Rule No. 1 (see Section 21.2), we find that the direction of the
magnetic force on a positively charged particle is to the west. Reversing this direction
because the particle is a negative electron, we see that the magnetic force acting on it points
to the east.
3.
(a)
Using RightHand Rule No. 1 (see Section 21.2), we find that the direction of the
magnetic force on a positively charged particle is straight down toward the bottom of the
screen.
4.
B
= 1.1
×
10

1
T, south
5.
(c)
The electric force points out of the screen, in the direction of the electric field. An
application of RightHand Rule No. 1 shows that the magnetic force also points out of the
screen, parallel to the electric force. When two forces have the same direction, the
magnitude of their sum has the largest possible value.
6.
(e)
In this situation, the centripetal force,
F
c
=
mv
2
/
r
(Equation 5.3), is provided by the
magnetic force,
F
=
qvB
sin 90.0
°
(Equation 21.1), so
mv
2
/
r
=
qvB
sin 90.0
°
. Thus,
(
29
/
q
mv
rB
=
, and the charge magnitude
q
is inversely proportional to the radius
r
. Since the
radius of curve 1 is smaller than that of curve 2, and the radius of curve 2 is smaller than
that of curve 3, we conclude that
q
1
is larger than
q
2
, which is larger than
q
3
.
7.
(a)
The magnetic force that acts on the electron in regions 1 and 2 is always perpendicular
to its path, so the force does no work. According to the workenergy theorem, Equation 6.3,
the kinetic energy, and hence speed, of the electron does not change when no work is done.
8.
(d)
According to Equation 21.2, the radius
r
of the circular path is given by
(
29
/
r
mv
qB
=
.
Since
v
,
q
, and
B
are the same for the proton and the electron, the moremassive proton
travels on the circle with the greater radius. The centripetal force
F
c
acting on the proton
must point toward the center of the circle. In this case, the centripetal force is provided by
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111
MAGNETIC FORCES AND MAGNETIC FIELDS
the magnetic force
F
. According to RightHand Rule No. 1, the direction of
F
is related to
the velocity
v
and the magnetic field
B
. An application of this rule shows that the proton
must travel counterclockwise around the circle in order that the magnetic force point toward
the center of the circle.
9.
r
proton
/
r
electron
= 1835
10. (c)
When, for example, a particle moves perpendicular to a magnetic field, the field exerts a
force that causes the particle to move on a circular path. Any object moving on a circular
path experiences a centripetal acceleration.
11.
F
= 3.0 N, along the

y
axis
12. (e)
The magnetic field is directed from the north pole to the south pole (Section 21.1).
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 Fall '11
 rollino
 Physics, Electron, Proton, Force, Magnetic Force, Magnetic Field

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