Last updated: September 26, 2008
PHY 2054 Discussion – Fall ‘08
Practice Exam Problems (Chapter 17)
1.
In a certain material there is a current of 16 A flowing through a surface to the right, and there is an
equal amount of positive and negative charge passing through the surface producing the current. How
much negative charge passes through the surface? (Electric Current)
8 C/s toward the left
2.
c. 16 C/s toward the right
a. 8 C/s toward the right
b.
d. 16 C/s toward the left
If a metallic wire of cross sectional area 3.0×10-6 m² carries a current of 6.0 A and has a mobile
charge (free electron) density of 4.24×1028 carriers/m3, what is the average drift velocity of the
c. 1.5×10-4 m/s
d.
The unit of electric resistance, the ohm, is equivalent to which of the following? (Resistance)
a.
mobile charge carriers? (Drift Speed)
a. 3.4×103 m/s
b. 1.7×103 m/s
2.9×10-4 m/s
3.
V/A
4.
b. V m
c. A/s
d. A/m
A flashlight bulb operating at a voltage of 4.5 V has a resistance of 8.0 Ω. How many electrons pass
through the bulb filament per second? (Ohm’s Law)
3.5×10
5.
a. 3.7×1016
b. 1.8×1021
c. 9.4×1017
d.
18
Two cylindrical resistors are made of the same material and have the same resistance. The resistors,
R1 and R2, have different radii, r1 and r2, and different length L1 and L2 Which of the following
relative values for radii and lengths would result in equal resistances? (Resistivity)
L1 = 2L2
6.
b. 2r1 = r2 and L1 = 2L2 c. r1 = r2 and 4L1 =
d. 2r1 = r2 and 4L1 = L2
How long is a wire made from 100 cm3 of copper if its resistance is 8.5 Ω? The resistivity of copper
is 1.7×10-8 Ωm. (Construction of Resistors)
7.
L2
a. r1 = r2 and
a. 7.1 m b. 1.7×102 m c. 2.2×102 m d. 3.0×103 m
A metal wire has a resistance of 10.00 Ω at a temperature of 20 ˚C. If the same wire has a resistance
of 10.55 Ω at 90 ˚C, what is the resistance when its temperature is -20 ˚C? (Temperature Variation of
Resistance) a. 0.70 Ω
8.
b. 9.69 Ω
c. 10.31 Ω
d. 13.8 Ω
A tungsten wire is used to determine the melting point of indium. The resistance of the tungsten wire
is 3.000 Ω at 20 ˚C and increases to 4.850 Ω as the indium starts to melt. αtungsten = 4.50×10-3 (˚C)-1.
What is the melting temperature of indium? (Temperature Measurement)
a. 132 ˚C
b. 157 ˚C
c.
351 ˚C d. 731 ˚C
9.
When a superconductor’s temperature drops below the critical temperature, its resistance:
(Superconductors)
c. drops to zero.
a. equals that of a semiconductor of equal dimensions.
b. increases by two.
d. reduces to one half.
10. A steam turbine at an electric power plant delivers 4 500 kW of power to an electrical generator
which converts 95 % of this mechanical energy into electrical energy. What is the current delivered
by the generator if it delivers at 3 600 V? (Power & Current)
1.2×103 A
a. 0.66×103 A
b. 1.0×103 A
c.
d. 5.9×103 A
11. Suppose that a voltage surge produces 140 V for a moment in a 120-V line. What will temporarily be
the output of a 100-W light bulb assuming its resistance does not change? (Power & Voltage)
109 W
b. 118 W
a.
c. 127 W d. 136 W
12. A copper cable (ρ = 1.7×10-8 Ωm) needs to carry a current of 200 A with a power loss of only 3.0
W/m. What is the required radius of the copper cable? (Power & Resistivity)
a. 0.21 cm b. 0.85
cm c. 3.2 cm d. 4.0 cm
13. The heating coil of a hot water heater has a resistance of 20 Ω and operates at 210 V. How long a
time is required to raise the temperature of 200 kg of water from 15˚C to 80 ˚C? The specific heat for
water = 103 cal/kg and 1.0 cal = 4.186 J. (Electrical Heating)
Answers: 1-b
2-d
3-a
4-d
5-d
6-c
7-b
8-b
9-c
10-c
a. 1.7 h
11-d
b. 3.8 h
12-b
c. 5.1 h
13-d
d. 6.9 h