Deck 25: Electric Currents and Resistance

Question 1

You are given a copper bar of dimensions 3 cm × 5 cm × 8 cm and asked to attach leads to it in order to make a resistor. If you want to achieve the largest possible resistance, you should attach the leads to the opposite faces that measure

A)3 cm × 5 cm.
B)3 cm × 8 cm.
C)5 cm × 8 cm.
D)Any pair of faces produces the same resistance.

Answer 1

3 cm × 5 cm.

Question 2

A battery creates net electric charge.

Answer 2

False

Question 3

You are given a copper bar of dimensions 3 cm × 5 cm × 8 cm and asked to attach leads to it in order to make a resistor. If you want to achieve the smallest possible resistance, you should attach the leads to the opposite faces that measure

A)3 cm × 5 cm.
B)3 cm × 8 cm.
C)5 cm × 8 cm.
D)Any pair of faces produces the same resistance.

Answer 3

5 cm × 8 cm.

Question 4

A battery is responsible for supplying a constant current to a circuit.

Question 5

Consider two copper wires. One has twice the cross-sectional area of the other. How do the resistances of these two wires compare?

A)Both wires have the same resistance.
B)The thicker wire has half the resistance of the shorter wire.
C)The thicker wire has twice the resistance of the shorter wire.
D)none of the given answers

Question 6

A device that produces electricity by transforming chemical energy into electrical energy is called a

A)generator.
B)transformer.
C)battery.
D)superconductor.
E)none of the given answers

Question 7

A lightbulb cannot absorb charge.

Question 8

The current in a circuit flows in the opposite direction to the direction in which the electrons move.

Question 9

The resistivity of the material of a wire is inversely proportional to the resistance of the wire.

Question 10

Current in a metal wire is inversely proportional to the potential difference applied to its two ends.

Question 11

If a fuse repeatedly blows, it should be replaced by a fuse of one step higher rating.

Question 12

Conventional current is the flow of positive charge from higher to lower potential.

Question 13

Consider two copper wires. One has twice the length of the other. How do the resistances of these two wires compare?

A)Both wires have the same resistance.
B)The longer wire has half the resistance of the shorter wire.
C)The longer wire has twice the resistance of the shorter wire.
D)none of the given answers

Question 14

The longer a given diameter of wire, the higher its resistance.

Question 15

The fatter a given length of wire, the higher its resistance.

Question 16

List two conditions for producing an electric current in a circuit.

Question 17

Describe how a simple electric cell works.

Question 18

A 1200-W toaster draws more current than a 1000-W toaster.

Question 19

The resistance of most metals generally decreases with temperature.

Question 20

Current flows through a resistor

A)with no direction since it is not a vector.
B)from any potential to any different potential.
C)from high potential to low potential.
D)from low potential to high potential.
E)cannot be determined

Question 21

The power rating of a resistor is 0.800 W. The value of the resistor is 400 Ω.
(a) What is the maximum voltage?
(b) What is the maximum current it can draw?

Question 22

The resistivity of most common metals

A)remains constant over wide temperature ranges.
B)increases as the temperature increases.
C)decreases as the temperature increases.
D)varies randomly as the temperature increases.

Question 23

An advantage of a circuit breaker over a fuse is

A)not given.
B)it is bigger.
C)it is smaller.
D)it costs more so it must be better.
E)it is easier to re-enable the circuit.

Question 24

FIGURE 25-2
Fig. 25-2 shows a steady electric current passing through a wire with a narrow region. What happens to the drift velocity of the moving charges as they go from region A to region B and then to region C?

A)The velocity decreases from A to B and increases from B to C.
B)The velocity increases all the time.
C)The velocity remains constant.
D)The velocity decreases all the time.
E)The velocity increases from A to B and decreases from B to C.

Question 25

A 10-A current flows for two minutes.
(a) How much charge has passed through this circuit?
(b) How many electrons have passed any point in the circuit?

Question 26

Outlets on a household circuit are arranged in

A)neither series or parallel.
B)series.
C)parallel.
D)a combination of series and parallel.
E)not given

Question 27

Which of the following cylindrical wires has the largest resistance? All wires are made of the same material.

A)a wire of length L and diameter 2d
B)a wire of length L and diameter d/2
C)a wire of length L/2 and diameter d
D)a wire of length L and diameter d
E)a wire of length L/2 and diameter 2d

Question 28

A 4000-Ω resistor is connected across 220 V. What current will flow?

A)0.055 A
B)1.8 A
C)5.5 A
D)18 A
E)550 A

Question 29

Consider two copper wires. One has twice the length and twice the cross-sectional area of the other. How do the resistances of these two wires compare?

A)Both wires have the same resistance.
B)The longer wire has four times the resistance of the shorter wire.
C)The longer wire has twice the resistance of the shorter wire.
D)The longer wire has eight times the resistance of the shorter wire.
E)none of the given answers

Question 30

What length of copper wire (resistivity 1.68 × 10^-8 Ω∙m) of diameter 0.15 mm is needed for a total resistance of 15 Ω?

A)16 mm
B)16 cm
C)1.6 m
D)16 m
E)160 m

Question 31

An electric device delivers a current of 5.0 A for 10 seconds. How many electrons flow through this device?

A)0.20
B)20
C)2.0
D)3.1 x 10²⁰
E)31 x 10²⁰

Question 32

A charge of 12 C passes through an electroplating apparatus in 2.0 min. What is the average current?

A)0.10 A
B)0.24 A
C)0.60 A
D)1.0 A
E)6.0 A

Question 33

A current that is sinusoidal with respect to time is referred to as

A)a direct current.
B)an alternating current.

Question 34

A light bulb operating at 110 V draws 1.40 A of current. What is its resistance?

A)12.7 Ω
B)78.6 Ω
C)109 Ω
D)117 Ω
E)154 Ω

Question 35

Materials in which the resistivity becomes essentially zero at very low temperatures are referred to as

A)conductors.
B)insulators.
C)semiconductors.
D)superconductors.

Question 36

The resistivity of the material of a wire is 1.76 × 10^-8 Ωm. If the diameter of the wire is 2 × 10^-3 m and its length is 2 m, what is its resistance?

A)112 Ω
B)11.2 Ω
C)1.12 Ω
D)0.112 Ω
E)0.0112 Ω

Question 37

At very low temperatures the resistivity of some materials becomes zero as indicated by high precision measurements. This phenomenon is known as

A)supercapacitance.
B)superconductivity.
C)BSCCO.
D)superresistivity.
E)supraresistivity.

Question 38

What happens to the resistivity of a typical semiconductor as its temperature increases?

A)The resistivity increases for a while and then decreases.
B)The answer depends on the semiconductor.
C)The resistivity decreases.
D)The resistivity increases.
E)The resistivity remains the same.

Question 39

A wire of resistivity ρ must be replaced in a circuit by a wire four times as long. If, however, the total resistance is to remain as before, the diameter of the new wire must

A)remain unchanged.
B)be two times smaller.
C)be four times smaller.
D)be two times larger.
E)be four times larger.

Question 40

FIGURE 25-1
Fig. 25-1 shows an electric charge of unknown polarity moving inside a conducting wire. If the electric field in the conductor is oriented as shown, what is the sign of the charge?

A)The charge should be equal to zero.
B)The charge could be either positive or negative.
C)positive
D)negative
E)More information is needed to answer the question.

Question 41

A 200-W light bulb is connected across 110 V. What current will flow through this bulb?

A)0.90 A
B)0.36 A
C)0 A
D)0.60 A
E)1.8 A

Question 42

A 2.0 mm diameter wire of length 20 m has a resistance of 0.25 Ω. What is the resistivity of the wire?

A)5 × 10^-7 Ωm
B)3.9 × 10^-8 Ωm
C)4 × 10^-7 Ωm
D)16 × 10^-8 Ωm
E)0.25 Ω

Question 43

If an all electric house uses 2400 kWh in a month, what will the amount of the bill for electricity be at a rate of 9.5 cents per kWh?

A)$23
B)$2280
C)$228
D)$25,300
E)$11.50

Question 44

A 110-V hair dryer is rated at 1200 W. What current will it draw?

A)0.090 A
B)1.0 A
C)5.0 A
D)11 A
E)12 A

Question 45

A Nichrome wire is used as a heating element in a toaster. From the moment the toaster is first turned on to the moment the wire reaches it maximum temperature, the current in the wire drops by 20% from its initial value. What is the temperature change in the wire? The temperature coefficient for Nichrome is α = 0.0004 (°C)^-1.

A)200°C
B)400°C
C)500°C
D)100°C
E)300°C

Question 46

A 24-kW electric furnace is connected to a 240-V line. What is the resistance of the furnace?

A)1000 Ω
B)10 Ω
C)2.4 Ω
D)0.42 Ω
E)100 Ω

Question 47

When a 60-W lightbulb is operating on a 120-V household line, what current does it draw?

A)0.5 A
B)120 J/C
C)2.0 A
D)1.414 A
E)40 J/C

Question 48

A 100-W driveway light bulb is on 10 hours per day. Assuming the power company charges 10 cents for each kilowatt-hour of electricity used, estimate the annual cost to operate the bulb.

A)$3.65
B)$7.30
C)$21.40
D)$36.50
E)$73.00

Question 49

A light bulb operating at a voltage of 120 V has a resistance of 200 Ω. What is the power?

A)100 W
B)60 W
C)72 W
D)14 x 10^-3 W
E)7.2 W

Question 50

A 100-W light bulb is connected to a 110-V source. What is the resistance of this bulb?

A)100 Ω
B)8.00 x 10^-3 Ω
C)6.00 x 10^-3 Ω
D)121 Ω
E)242 Ω

Question 51

How much energy does a 100-W light bulb use in 8.0 hours?

A)0.0080 kWh
B)0.80 kWh
C)13 kWh
D)130 kWh
E)800 kWh

Question 52

A platinum wire is used to determine the melting point of indium. The resistance of the platinum wire is 2.000 Ω at 20°C and increases to 3.072 Ω as indium starts to melt. What is the melting point of indium? (The temperature coefficient of resistivity for platinum is 3.9 × 10^-3/C°.)

A)117°C
B)137°C
C)157°C
D)235°C
E)351°C

Question 53

A 1500-W heater is connected to a 120-V line for 2.0 hours. How much heat energy is produced?

A)1.5 kJ
B)3.0 kJ
C)0.18 MJ
D)11 MJ
E)18 MJ

Question 54

Calculate the current through a 10.0-m long 22 gauge (the radius is 0.321 mm) nichrome wire if it is connected to a 12.0-V battery. (The resistivity of nichrome is 100 × 10^-8 Ω∙m.)

A)17.5 A
B)30.9 A
C)61.8 A
D)0.388 A
E)0.776 A

Question 55

The resistivity of a 1.0 m long wire is 1.72 × 10^-8 Ωm and its cross sectional area is 2.0 × 10^-6 m². If the wire carries a current of 0.20 A, what is the voltage across the wire?

A)17 mV
B)0.90 mV
C)1.7 mV
D)10 mV
E)90 mV

Question 56

A battery is rated at 12 V and 160 A-h. How much energy does the battery store?

A)1.9 kJ
B)3.2 kJ
C)6.0 kJ
D)1.9 MJ
E)6.9 MJ

Question 57

A carbon resistor has a resistance of 18 Ω at a temperature of 20°C. What is its resistance at a temperature of 120°C? (The temperature coefficient of resistivity for carbon is -5.0 × 10^-4 /C°.)

A)18 Ω
B)17 Ω
C)16 Ω
D)15 Ω
E)14 Ω

Question 58

The temperature coefficient of resistivity for copper is 0.0068°C^-1. If a copper wire has a resistance of 104 Ω at 20°C, what is its resistance 80°C?

A)192 Ω
B)42 Ω
C)71Ω
D)161 Ω
E)146 Ω

Question 59

A 120-m long copper wire (resistivity 1.68 × 10^-8 Ω∙m) has resistance 6.0 Ω. What is the diameter of the wire?

A)0.065 mm
B)0.65 mm
C)0.65 cm
D)0.65 m
E)6.5 m

Question 60

The resistance of a 100-cm wire of cross sectional area 2 × 10^-6 m² is 400 Ω. What is the resistivity of the material of this wire?

A)0.8 × 10^-4 Ωm
B)8 × 10^-4 Ωm
C)0.8 × 10^-5 Ωm
D)0.8 × 10^-6 Ωm
E)8 × 10^-6 Ωm

Question 61

When a current I₁ = 1.3 A passes through a wire of cross-sectional area A₁ = 5 mm² the drift velocity of the charge carriers is v₁ = 2.7 × 10^-5 m/s. What is the drift velocity of the charges when a current I₂ = 0.5 A passes through a wire, made of the same material of cross-sectional area A₂ = 3 mm²?

A)8.4 × 10^-5 m/s
B)6.4 × 10^-5 m/s
C)1.7 × 10^-5 m/s
D)3.2 × 10^-5 m/s
E)2.1 × 10^-5 m/s

Question 62

A 500-W device is connected to a 120-V ac power source. What rms current flows through this device?

A)4.2 A
B)5.9 A
C)120 A
D)150 A
E)170 A

Question 63

A 400-W computer (computer plus monitor) is turned on 8.0 hours per day. If electricity costs 10 cents per kWh, how much does it cost to run the computer annually?

A)$116.80
B)$1168.00
C)$14.60
D)$16.80
E)$146.00

Question 64

FIGURE 25-3
Electric charges flow through a wire shaped as shown in Fig. 25-3. The cross-sectional areas are A₁= 4 mm² and A₂ = 1 mm² respectively. What is the drift velocity of the electrons in the narrow section of the wire if their velocity is 0.08 m/s in the wider region?

A)0.16 m/s
B)0.02 m/s
C)0.04 m/s
D)0.32 m/s
E)0.08 m/s

Question 65

A 500-W device is connected to a 120-V ac power source. What peak current flows through this device?

A)4.2 A
B)5.9 A
C)120 A
D)150 A
E)170 A

Question 66

A silver wire has a cross sectional area A = 2mm². A total of of 9.4 × 10¹⁸ electrons pass through the wire in a time t = 3s. Silver has n_q = 5.8 × 10²⁸ electrons/m³. What is the drift velocity of these electrons?

A)2.7 × 10^-5 m/s
B)9.1 × 10^-5 m/s
C)5.2 × 10^-5 m/s
D)1.1 × 10^-5 m/s
E)7.4 × 10^-5 m/s

Question 67

If a current of 2.4 A is flowing in a wire of diameter 2.0 mm, what is the average current density?

A)7.6 × 10⁵ A/m²
B)5.2 × 10^-6 A/m²
C)1.9 × 10⁵ A/m²
D)3.6 × 10⁵ A/m²
E)21 × 10^-6 A/m²

Question 68

The monthly (30 days) electric bill included the cost of running a central air-conditioning unit for 2.0 hr/day at 5000 W, and a series connection of ten 60 W light bulbs for 5.0 hr/day. How much did these items contribute to the cost of the monthly electric bill if electricity costs 8.0¢ per kWh?

A)$21.30
B)$39.50
C)$31.20
D)$13.20
E)$12.30

Question 69

The diameter of no. 12 copper wire is 0.081 in. The maximum safe current it can carry (in order to prevent fire danger in building construction) is 20 A. At this current, what is the drift velocity of the electrons? (The number of electron carriers in one cubic centimeter of copper is 8.5 × 10²².)

A)0.044 mm/s
B)0.44 mm/s
C)0.44 cm/s
D)0.44 m/s
E)4.4 m/s

Question 70

If the current density in a wire is given by j = kr, 0 < r < R, where R is the radius of the wire, the current in the wire is

A)2πkR³/3.
B)3πkR³/2.
C)kR³/3.
D)kR².
E)kR²/2.

Question 71

A 500-W device is connected to a 120-V ac power source. What is the peak voltage across this device?

A)4.2 V
B)5.9 V
C)120 V
D)150 A
E)170 V