Problems
889
The circuit in Figure P28.37 has been connected for a long
time. (a) What is the voltage across the capacitor? (b) If the
battery is disconnected, how long does it take the capacitor
to discharge to one tenth of its initial voltage?
37.
Section 28.5 Electrical Meters
Assume that a galvanometer has an internal resistance of
60.0 ' and requires a current of 0.500 mA to produce full-
scale deflection. What resistance must be connected in
parallel with the galvanometer if the combination is to
serve as an ammeter that has a full-scale deflection for
a current of 0.100 A?
42. A typical galvanometer, which requires a current of
1.50 mA for full-scale deflection and has a resistance of
75.0 ', may be used to measure currents of much greater
values. To enable an operator to measure large currents
without damage to the galvanometer, a relatively small
shunt resistor is wired in parallel with the galvanometer,
as suggested in Figure 28.27. Most of the current then
goes through the shunt resistor. Calculate the value of the
shunt resistor that allows the galvanometer to be used to
measure a current of 1.00 A at full-scale deflection.
(Suggestion: use Kirchhoff’s rules.)
The same galvanometer described in the previous problem
may be used to measure voltages. In this case a large resis-
tor is wired in series with the galvanometer, as suggested in
Figure 28.29. The effect is to limit the current in the
galvanometer when large voltages are applied. Most of the
potential drop occurs across the resistor placed in series.
Calculate the value of the resistor that allows the
galvanometer to measure an applied voltage of 25.0 V at
full-scale deflection.
44.
Meter loading. Work this problem to five-digit precision.
Refer to Figure P28.44. (a) When a 180.00-' resistor is
connected across a battery of emf 6.000 0 V and internal
resistance 20.000 ', what is the current in the resistor?
What is the potential difference across it? (b) Suppose
now an ammeter of resistance 0.500 00 ' and a voltmeter
of resistance 20 000 ' are added to the circuit as shown in
Figure P28.44b. Find the reading of each. (c) What If?
Now one terminal of one wire is moved, as shown in
Figure P28.44c. Find the new meter readings.
45.
Design a multirange ammeter capable of full-scale deflec-
tion for 25.0 mA, 50.0 mA, and 100 mA. Assume the meter
movement is a galvanometer that has a resistance of
25.0 ' and gives a full-scale deflection for 1.00 mA.
46.
Design a multirange voltmeter capable of full-scale
deflection for 20.0 V, 50.0 V, and 100 V. Assume the
meter movement is a galvanometer that has a resistance
of 60.0 ' and gives a full-scale deflection for a current of
1.00 mA.
43.
41.
10.0 V
1.00
Ω
8.00
Ω
2.00
Ω
4.00
Ω
1.00
µ
F
µ
Figure P28.37
(a)
180.00
Ω
20.000
Ω
6.000 0 V
(b)
A
V
(c)
A
V
Figure P28.44
38.
In places such as a hospital operating room and a factory
for electronic circuit boards, electric sparks must be
avoided. A person standing on a grounded floor and
touching nothing else can typically have a body capaci-
tance of 150 pF, in parallel with a foot capacitance of
80.0 pF produced by the dielectric soles of his or her
shoes. The person acquires static electric charge from
interactions with furniture, clothing, equipment, packag-
ing materials, and essentially everything else. The static
charge is conducted to ground through the equivalent
resistance of the two shoe soles in parallel with each other.
A pair of rubber-soled street shoes can present an equiva-
lent resistance of 5 000 M'. A pair of shoes with special
static-dissipative soles can have an equivalent resistance of
1.00 M'. Consider the person’s body and shoes as
forming an RC circuit with the ground. (a) How long does
it take the rubber-soled shoes to reduce a 3 000-V static
charge to 100 V? (b) How long does it take the static-
dissipative shoes to do the same thing?
39.
A 4.00-M' resistor and a 3.00-)F capacitor are connected
in series with a 12.0-V power supply. (a) What is the time
constant for the circuit? (b) Express the current in the
circuit and the charge on the capacitor as functions of
time.
40.
Dielectric materials used in the manufacture of capacitors
are characterized by conductivities that are small but not
zero. Therefore, a charged capacitor slowly loses its charge
by “leaking” across the dielectric. If a capacitor having
capacitance C leaks charge such that the potential differ-
ence has decreased to half its initial (t # 0) value at a time
t, what is the equivalent resistance of the dielectric?