Problems
825
sis of an infinite array, determine the equivalent capaci-
tance C between terminals X and Y of the infinite set of
capacitors represented in Figure P26.30. Each capacitor
has capacitance C
0
. Suggestion: Imagine that the ladder is
cut at the line AB, and note that the equivalent capaci-
tance of the infinite section to the right of AB is also C.
person. A particular device can be destroyed by a
discharge releasing an energy of 250 %J. To what voltage
on the body does this correspond?
36.
A uniform electric field E # 3 000 V/m exists within a
certain region. What volume of space contains an energy
equal to 1.00 * 10
$
7
J? Express your answer in cubic
meters and in liters.
A parallel-plate capacitor has a charge Q and plates of
area A. What force acts on one plate to attract it toward
the other plate? Because the electric field between the
plates is E # Q /A)
0
, you might think that the force is
F # QE # Q
2
/A)
0
. This is wrong, because the field E in-
cludes contributions from both plates, and the field cre-
ated by the positive plate cannot exert any force on the
positive plate. Show that the force exerted on each plate is
actually F # Q
2
/2)
0
A. (Suggestion: Let C # )
0
A/x for an
arbitrary plate separation x; then require that the work
done in separating the two charged plates be W #
)F dx.)
The force exerted by one charged plate on another is
sometimes used in a machine shop to hold a workpiece
stationary.
38.
The circuit in Figure P26.38 consists of two identical paral-
lel metal plates connected by identical metal springs to a
100-V battery. With the switch open, the plates are
uncharged, are separated by a distance d # 8.00 mm, and
have a capacitance C # 2.00 %F. When the switch is closed,
the distance between the plates decreases by a factor of
0.500. (a) How much charge collects on each plate and
(b) what is the spring constant for each spring? (Sugges-
tion: Use the result of Problem 37.)
37.
C
0
C
0
C
0
X
Y
A
B
C
0
Figure P26.30
+
–
k
k
d
∆V
S
Figure P26.38
Section 26.4 Energy Stored in a Charged Capacitor
31. (a) A 3.00-%F capacitor is connected to a 12.0-V battery.
How much energy is stored in the capacitor? (b) If the
capacitor had been connected to a 6.00-V battery, how
much energy would have been stored?
32. The immediate cause of many deaths is ventricular fibrilla-
tion, uncoordinated quivering of the heart as opposed to
proper beating. An electric shock to the chest can cause
momentary paralysis of the heart muscle, after which the
heart will sometimes start organized beating again. A defib-
rillator (Fig. 26.14) is a device that applies a strong electric
shock to the chest over a time interval of a few milliseconds.
The device contains a capacitor of several microfarads,
charged to several thousand volts. Electrodes called pad-
dles, about 8 cm across and coated with conducting paste,
are held against the chest on both sides of the heart. Their
handles are insulated to prevent injury to the operator, who
calls, “Clear!’’ and pushes a button on one paddle to dis-
charge the capacitor through the patient’s chest. Assume
that an energy of 300 J is to be delivered from a 30.0-%F ca-
pacitor. To what potential difference must it be charged?
33.
Two capacitors, C
1
#
25.0 %F and C
2
#
5.00 %F, are con-
nected in parallel and charged with a 100-V power supply.
(a) Draw a circuit diagram and calculate the total energy
stored in the two capacitors. (b) What If? What potential
difference would be required across the same two capaci-
tors connected in series in order that the combination
stores the same amount of energy as in (a)? Draw a circuit
diagram of this circuit.
34. A parallel-plate capacitor is charged and then discon-
nected from a battery. By what fraction does the stored
energy change (increase or decrease) when the plate sepa-
ration is doubled?
35. As a person moves about in a dry environment, electric
charge accumulates on his body. Once it is at high voltage,
either positive or negative, the body can discharge via
sometimes noticeable sparks and shocks. Consider a
human body well separated from ground, with the typical
capacitance 150 pF. (a) What charge on the body will
produce a potential of 10.0 kV? (b) Sensitive electronic
devices can be destroyed by electrostatic discharge from a
39.
Review problem. A certain storm cloud has a potential of
1.00 * 10
8
V relative to a tree. If, during a lightning storm,
50.0 C of charge is transferred through this potential dif-
ference and 1.00% of the energy is absorbed by the tree,
how much sap in the tree can be boiled away? Model the
sap as water initially at 30.0°C. Water has a specific heat of
4 186 J/kg°C, a boiling point of 100°C, and a latent heat of
vaporization of 2.26 * 10
6
J/kg.
40.
Two identical parallel-plate capacitors, each with capaci-
tance C, are charged to potential difference !V and con-
nected in parallel. Then the plate separation in one of the
capacitors is doubled. (a) Find the total energy of the
system of two capacitors before the plate separation is
doubled. (b) Find the potential difference across each
capacitor after the plate separation is doubled. (c) Find the