Problems
737
under the influence of the forces exerted by the three
fixed charges. Find a value for s for which Q is in equilib-
rium. You will need to solve a transcendental equation.
Two small spheres of mass m are suspended from strings of
length ! that are connected at a common point. One
sphere has charge Q ; the other has charge 2Q. The strings
make angles +
1
and +
2
with the vertical. (a) How are +
1
and
+
2
related? (b) Assume +
1
and +
2
are small. Show that the
distance r between the spheres is given by
66.
Review problem. Four identical particles, each having
charge # q, are fixed at the corners of a square of side L. A
fifth point charge " Q lies a distance z along the line per-
pendicular to the plane of the square and passing through
the center of the square (Fig. P23.66). (a) Show that the
force exerted by the other four charges on " Q is
Note that this force is directed toward the center of the
square whether z is positive (" Q above the square) or neg-
ative (" Q below the square). (b) If z is small compared
with L, the above expression reduces to F
≈ "(constant)zkˆ.
Why does this imply that the motion of the charge " Q is
simple harmonic, and what is the period of this motion if
the mass of " Q is m?
F ! "
4k
e
q
Qz
[z
2
#
(L
2
/2)]
3/2
kˆ
r
"
&
4k
e
Q
2
!
mg
'
1/3
65.
69.
Eight point charges, each of magnitude q, are located on
the corners of a cube of edge s, as shown in Figure P23.69.
(a) Determine the x, y, and z components of the resultant
force exerted by the other charges on the charge located
at point A. (b) What are the magnitude and direction of
this resultant force?
R
R
m
R
m
Figure P23.68
Figure P23.69 Problems 69 and 70.
Point
A
x
y
z
q
q
q
q
q
q
q
q
s
s
s
L
L
+q
+q
z
–Q
z
+q
+q
Figure P23.66
67.
Review problem. A 1.00-g cork ball with charge 2.00 *C is
suspended vertically on a 0.500-m-long light string in the
presence of a uniform, downward-directed electric field of
magnitude E ! 1.00 & 10
5
N/C. If the ball is displaced
slightly from the vertical, it oscillates like a simple pendu-
lum. (a) Determine the period of this oscillation.
(b) Should gravity be included in the calculation for part
(a)? Explain.
68.
Two identical beads each have a mass m and charge q.
When placed in a hemispherical bowl of radius R with fric-
tionless, nonconducting walls, the beads move, and at
equilibrium they are a distance R apart (Fig. P23.68). De-
termine the charge on each bead.
70.
Consider the charge distribution shown in Figure P23.69.
(a) Show that the magnitude of the electric field at the
center of any face of the cube has a value of 2.18k
e
q/s
2
.
(b) What is the direction of the electric field at the center
of the top face of the cube?
Review problem. A negatively charged particle " q is
placed at the center of a uniformly charged ring, where
the ring has a total positive charge Q as shown in Example
23.8. The particle, confined to move along the x axis, is
displaced a small distance x along the axis (where x 55 a)
and released. Show that the particle oscillates in simple
harmonic motion with a frequency given by
72.
A line of charge with uniform density 35.0 nC/m lies
along the line y ! " 15.0 cm, between the points with co-
ordinates x ! 0 and x ! 40.0 cm. Find the electric field it
creates at the origin.
73.
Review problem. An electric dipole in a uniform electric
field is displaced slightly from its equilibrium position, as
shown in Figure P23.73, where + is small. The separation
of the charges is 2a, and the moment of inertia of the
dipole is I. Assuming the dipole is released from this
f !
1
2(
&
k
e
q
Q
ma
3
'
1/2
71.