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Physics: Principle and Applications, 7e (Giancoli)

Chapter 18 Electric Currents

18.1 Conceptual Questions

1) If a quantity you calculated has units of A ∙ s, what is that quantity?

A) potential

B) resistivity

C) resistance

D) capacitance

E) charge

Answer: E

Var: 1

2) When current is flowing in an ordinary metal wire, the magnitude of the average velocity of the electrons is closest to

A) 10 m/s.

B) the speed of light.

C) 1 mm/s.

D) 1 km/s.

E) 1 m/s.

Answer: C

Var: 1

3) When a current flows through a metal wire, the moving charges are

A) only protons.

B) only electrons.

C) both protons and electrons.

D) positive metal ions.

E) negative metal ions.

Answer: B

Var: 1

4) When a current flows through an ionic liquid such as salty water, the moving charges are

A) only protons.

B) only electrons.

C) only negative ions.

D) only positive ions.

E) both positive and negative ions.

Answer: E

Var: 1

5) If a quantity you calculated has units of Ω ∙ m what is that quantity?

A) potential

B) resistivity

C) resistance

D) capacitance

E) charge

Answer: A

Var: 1

6) If a quantity you calculated has units of [pic], what is that quantity?

A) potential

B) resistivity

C) resistance

D) capacitance

E) charge

Answer: A

Var: 1

7) The figure shows electrons passing through a resistor. The arrow shows the direction in which the electrons are moving. Which of the following statements are correct? (There could be more than one correct choice.)

[pic]

A) The electrons are moving slower at point b than at point a.

B) The electric potential is higher at point b than at point a.

C) The electric potential is lower at point b than at point a.

D) The electrons are losing electric potential energy as they move through the resistor from a to b.

E) The speed of the electrons at point b is the same as it is at point a.

Answer: B, D, E

Var: 1

8) The figure shows conventional current passing through a resistor. The arrow shows the direction in which this conventional current is flowing. Which of the following statements are correct? (There could be more than one correct choice.)

[pic]

A) The charges are moving slower at point b than at point a.

B) The electric potential is lower at point b than at point a.

C) The electric potential is higher at point b than at point a.

D) The current at point b is the same as the current at point a.

E) The electric potential at point b is the same as it is at point a.

Answer: B, D

Var: 1

9) For the graph shown in the figure, what physical quantity does the slope of the graph represent for ohmic material?

[pic]

A) power

B) resistivity

C) 1/(resistivity)

D) resistance

E) 1/(resistance)

Answer: D

Var: 1

10) For the graph shown in the figure, what physical quantity does the slope of the graph represent for ohmic material?

[pic]

A) power

B) resistivity

C) 1/(resistivity)

D) resistance

E) 1/(resistance)

Answer: E

Var: 1

11) For the graph shown in the figure, what physical quantity does the slope of the graph represent for ohmic material?

[pic]

A) current

B) resistivity

C) 1/(current)

D) power

E) 1/(resistivity)

Answer: A

Var: 1

12) For the graph shown in the figure, what physical quantity does the slope of the graph represent for ohmic material?

[pic]

A) current

B) resistivity

C) 1/(current)

D) power

E) 1/(resistivity)

Answer: C

Var: 1

13) 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: C

Var: 1

14) 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: A

Var: 1

15) Copper wire #1 has a length L and a radius b. Copper wire #2 has a length 2L and a radius 2b. Which statement about the resistance across the ends of the wires is true?

A) The resistance of wire #1 is twice as high as that of wire #2.

B) The resistance of wire #1 is equal to that of wire #2.

C) The resistance of wire #1 is half that of wire #2.

D) The resistance of wire #1 is four times higher than that of wire #2.

Answer: A

Var: 1

16) The figure shows a graph of the resistance of a wire as a function of its length. What physical quantities does the slope of this graph represent?

[pic]

A) the cross-sectional area of the wire

B) the resistivity of the material of which the wire is made

C) the product of the resistivity and the cross-sectional area of the wire

D) the resistivity of the material divided by the cross-sectional area of the wire

E) the reciprocal of the resistivity of the material

Answer: D

Var: 1

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

A) be the same as the original diameter.

B) be one-half the original diameter.

C) be one-fourth the original diameter.

D) be two times the original diameter.

E) be four times the original diameter.

Answer: D

Var: 1

18) The length of a certain wire is kept same while its radius is doubled. What is the new resistance of this wire?

A) It is increased by a factor of 2.

B) It is increased by a factor of 4.

C) It is reduced by a factor of 2.

D) It is reduced by a factor of 4.

E) It is reduced by a factor of 8.

Answer: D

Var: 1

19) The length of a certain wire is kept same while its radius is doubled. What is the new resistivity of this wire?

A) It is increased by a factor of 2.

B) It is increased by a factor of 4.

C) It is reduced by a factor of 2.

D) It is reduced by a factor of 4.

E) It does not change.

Answer: E

Var: 1

20) The length of a certain wire is doubled while its radius is kept constant. What is the new resistance of this wire?

A) It stays the same.

B) It is 2 times as large.

C) It is 3 times as large.

D) It is 4 times as large.

E) It is ½ as large.

Answer: B

Var: 1

21) The length of a certain wire is doubled and at the same time its radius is also doubled. What is the new resistance of this wire?

A) It stays the same.

B) It is 2 times as large.

C) It is 4 times as large.

D) It is ½ as large.

E) It is ¼ as large.

Answer: D

Var: 1

22) The length of a certain wire is doubled and at the same time its radius is reduced by a factor of 2. What is the new resistance of this wire?

A) It is 2 times as large.

B) It is 4 times as large.

C) It is 6 times as large.

D) It it 8 times as large.

E) It is ½ as large.

Answer: D

Var: 1

23) If a quantity you calculated has units of [pic] what is that quantity?

A) potential

B) resistivity

C) resistance

D) capacitance

E) current

Answer: C

Var: 1

24) If a quantity you calculated has units of [pic] what is that quantity?

A) potential

B) resistivity

C) resistance

D) capacitance

E) current

Answer: B

Var: 1

25) Which one of the following quantities is equivalent to 1 Ω?

A) 1 J/s

B) 1 W/A

C) 1 V ∙ A

D) 1 V/A

E) 1 A ∙ s

Answer: D

Var: 1

26) If the length and diameter of a wire of circular cross section are both tripled, the resistance will be

A) unchanged.

B) tripled.

C) increased by a factor of 9.

D) 1/3 of what it originally was.

E) 1/9 of what it originally was.

Answer: D

Var: 1

27) Consider two copper wires of equal cross-sectional area. One wire has 3 times the length of the other. How do the resistivities of these two wires compare?

A) Both wires have the same resistivity.

B) The longer wire has 3 times the resistivity of the shorter wire.

C) The longer wire has 9 times times the resistivity of the shorter wire.

D) The longer wire has 27 times times the resistivity of the shorter wire.

Answer: A

Var: 1

28) Consider two copper wires of equal cross-sectional area. One wire has 3 times 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 1/3 the resistance of the shorter wire.

C) The longer wire has 3 times the resistance of the shorter wire.

D) The longer wire has 9 times times the resistance of the shorter wire.

E) The longer wire has 27 times times the resistance of the shorter wire.

Answer: C

Var: 1

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

A) The thicker wire has twice the resistance of the thinner wire.

B) The thicker wire has eight times the resistance of the thinner wire.

C) The thicker wire has one-half the resistance of the thinner wire.

D) The thicker wire has four times the resistance of the thinner wire.

E) The thicker wire has one-fourth the resistance of the thinner wire.

Answer: C

Var: 1

30) Consider two copper wires with circular cross-sections and equal lengths. One wire has 3 times the diameter of the other. How do the resistances of these two wires compare?

A) The thicker wire has 1/3 the resistance of the thinner wire.

B) The thicker wire has 1/9 the resistance of the thinner wire.

C) The thicker wire has 3 times the resistance of the thinner wire.

D) The thicker wire has 9 times the resistance of the thinner wire.

E) The thicker wire has [pic] times the resistance of the thinner wire.

Answer: B

Var: 1

31) When the current through a resistor is increased by a factor of 4, the power dissipated by the resistor

A) decreases by a factor of 4.

B) decreases by a factor of 16.

C) increases by a factor of 16.

D) increases by a factor of 4.

E) increases by a factor of 2.

Answer: C

Var: 1

32) Which one of the following quantities is equivalent to 1 W?

A) 1 V/A

B) 1 Ω ∙ m

C) 1 V ∙ A

D) 1 V/Ω

E) 1 A ∙ s

Answer: C

Var: 1

33) A kilowatt-hour is equivalent to

A) 1000 W.

B) 3600 J.

C) 3,600,000 J/s.

D) 3,600,000 J.

E) 3600 J/s

Answer: D

Var: 1

34) If the resistance in a constant voltage circuit is doubled, the power dissipated by that circuit will

A) increase by a factor of two.

B) increase by a factor of four.

C) decrease to one-half its original value.

D) decrease to one-fourth its original value.

Answer: C

Var: 1

35) If the voltage across a circuit of constant resistance is doubled, the power dissipated by that circuit will

A) be four times as large.

B) be two times as large.

C) decrease to one-half the original power.

D) decrease to one-fourth the original power.

Answer: A

Var: 1

36) If we double the resistance in a circuit but keep the current in it constant, the power dissipated by that circuit will

A) be four times as great.

B) be two times as great.

C) be one-half as great.

D) be one-fourth as great.

Answer: B

Var: 1

37) If the current flowing through a circuit of constant resistance is doubled, the power dissipated by that circuit will

A) quadruple in magnitude.

B) double in magnitude.

C) decrease to one-half of what it was.

D) decrease to one-fourth of what it was.

Answer: A

Var: 1

38) During a period of high power demand, the voltage output of the power company is reduced by 5.0%. By what percentage is the power in a resistor decreased?

A) 2.5%

B) 5.0%

C) 10%

D) 15%

E) 90%

Answer: C

Var: 1

39) The figure shows a graph of the power dissipated in a resistor as a function of the resistance. What quantity does the slope of this graph represent?

[pic]

A) the current in the resistor

B) the potential difference across the resistor

C) the reciprocal of the current in the resistor

D) the square of the current in the resistor

E) the resistivity of the resistor

Answer: D

Var: 1

40) For the graph shown in the figure, what physical quantity does the slope of the graph represent for a dc circuit?

[pic]

A) resistivity

B) resistance

C) energy

D) current

E) capacitance

Answer: D

Var: 1

41) For the graph shown in the figure, what physical quantity does the slope of the graph represent for a dc circuit?

[pic]

A) 1/(current)

B) resistance

C) 1/(resistance)

D) current

E) resistivity

Answer: A

Var: 1

42) For the graph shown in the figure, what physical quantity does the slope of the graph represent for a dc circuit?

[pic]

A) 1/(resistance)

B) resistance

C) potential

D) (potential)2

E) 1/(potential)

Answer: C

Var: 1

43) For the graph shown in the figure, what physical quantity does the slope of the graph represent for a dc circuit?

[pic]

A) 1/(resistance)

B) resistance

C) potential

D) (potential)2

E) 1/(potential)

Answer: E

Var: 1

44) Over ordinary temperature ranges, the resistance of most metals such as copper

A) is independent of the temperature of the metal.

B) decreases as the metal gets hotter.

C) increases as the metal gets hotter.

D) is a maximum at 20°C.

Answer: C

Var: 1

18.2 Problems

1) A 10-A current flows through a wire for 2.0 min. (e = 1.60 × 10-19 C)

(a) How much charge has passed through this wire?

(b) How many electrons have passed any point in the wire?

Answer: (a) 1200 C (b) 7.5 × 1021

Var: 1

2) If a charge of 11.4 C passes through a computer in 1.75 min, what is the average current through the computer?

Answer: 0.109 A

Var: 1

3) A current of 5.0 A flows through an electrical device for 10 seconds. How many electrons flow through this device during this time? (e = 1.60 × 10-19 C)

A) 0.20

B) 20

C) 2.0

D) 3.1 x [pic]

E) 31 x [pic]

Answer: D

Var: 5

4) What current is flowing in a wire if 0.67 C of charge pass a point in the wire in 0.30 s?

A) 2.2 A

B) 0.67 A

C) 0.30 A

D) 0.20 A

Answer: A

Var: 3

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

A) 0.10 A

B) 0.60 A

C) 1.0 A

D) 6.0 A

Answer: A

Var: 1

6) How much charge must pass by a point in a wire in 10 s for the current in the wire to be 0.50 A?

A) 20 C

B) 2.0 C

C) 5.0 C

D) 0.050 C

Answer: C

Var: 3

7) A total of 2.0 × 1013 electrons pass a given point in a wire in 15 s. What is the current in the wire? (e = 1.60 × 10-19 C)

A) 1.3 mA

B) 1.3 A

C) 0.21 μA

D) 3.2 μA

Answer: C

Var: 1

8) What current is flowing in a resistor if 4.0 × 1016 electrons pass a point in the resistor in 0.50 s? (e = 1.60 × 10-19 C)

A) 0.013 A

B) 0.31 A

C) 6.3 A

D) 78 A

Answer: A

Var: 1

9) If 3.0 × 1015 electrons flow through a section of a wire of diameter 2.0 mm in 4.0 s, what is the current in the wire? (e = 1.60 × 10-19 C)

A) 0.12 mA

B) 0.24 mA

C) 7.5 × 107 A

D) 7.5 × 1014 A

Answer: A

Var: 1

10) A electric heater that draws 13.5 A of dc current has been left on for 10 min. How many electrons that have passed through the heater during that time? (e = 1.60 × 10-19 C)

A) 1.5 × 1022

B) 5.1 × 1022

C) 1.8 × 103

D) 8.1 × 103

E) 1.0 × 1023

Answer: B

Var: 1

11) In an electroplating process, it is desired to deposit 40 mg of silver on a metal part by using a current of 2.0 A. How long must the current be allowed to run to deposit this much silver? The silver ions are singly charged, and the atomic mass of silver is 108 g/mol. (e = 1.60 × 10-19 C, NA = 6.02 × 1023 atoms/mol)

A) 16 s

B) 18 s

C) 20 s

D) 22 s

Answer: B

Var: 1

12) A jeweler needs to electroplate gold, having an atomic mass of 196.97 g/mol, onto a bracelet. He knows that the charge carriers in the ionic solution are singly-ionized gold ions, Au+, and has calculated that he must deposit [pic] of gold to reach the necessary thickness. How much current does he need to plate the bracelet in 3.0 hours? (e = 1.60 × 10-19 C, NA = 6.02 × 1023 atoms/mol)

A) 9.1 mA

B) 540 mA

C) 33 A

D) 1800 mA

Answer: A

Var: 50+

13) What potential difference is required across an 8.0-Ω resistor to cause 2.0 A to flow through it?

Answer: 16 V

Var: 1

14) The current through a piece of lab equipment must be limited to 2.75 A when it is run by a 120-V dc power supply. What must be the resistance of this equipment?

Answer: 43.6 Ω

Var: 1

15) What potential difference is required to cause 4.00 A to flow through a resistance of 330 Ω?

A) 12.1 V

B) 82.5 V

C) 334 V

D) 1320 V

Answer: D

Var: 2

16) What is the voltage drop across a 5.0-Ω resistor if the current through it is 5.0 A?

A) 100 V

B) 25 V

C) 4.0 V

D) 1.0 V

Answer: B

Var: 2

17) A 4000-Ω resistor is connected across a 220-V power source. What current will flow through the resistor?

A) 0.055 A

B) 1.8 A

C) 5.5 A

D) 18 A

Answer: A

Var: 1

18) 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) 154 Ω

Answer: B

Var: 1

19) A 12-V battery is connected across a 100-Ω resistor. How many electrons flow through the wire in 1.0 min? (e = 1.60 × 10-19 C)

A) 1.5 × 1019

B) 2.5 × 1019

C) 3.5 × 1019

D) 4.5 × 1019

Answer: D

Var: 1

20) The graph shown in the figure shows the results of measurements of the dc current through a circuit device for various potential differences across it. Assume that all the numbers shown are accurate to two significant figures. What is the resistance of this device?

[pic]

Answer: 25 Ω

Var: 1

21) The graph shown in the figure shows the results of measurements of the dc current through a circuit device for various potential differences across it. Assume that all the numbers shown are accurate to two significant figures. What is the resistance of this device?

[pic]

Answer: 2.0 Ω

Var: 1

22) When a thin copper wire that is 178 m long is connected between a 1.2-V potential difference, a current of 2.0 amps flows through the wire. What is the diameter of this wire? The resistivity of copper is 1.72 × 10-8 Ω ∙ m.

Answer: 2.5 mm

Var: 1

23) A 25-m wire of diameter 0.30 mm draws 0.499 A when connected across a 3.0-V potential difference.

(a) What is the resistance of the wire?

(b) What is the resistivity of the material from which the wire is made?

Answer: (a) 6.0 Ω (b) 1.7 × 10-8 Ω ∙ m

Var: 1

24) A certain metal wire has a cross-sectional area of 1.0 cm2 and a resistivity of 1.7 × 10-8 Ω ∙ m. How long would it have to be to have a resistance of 1.0 Ω?

A) 5.9 × 106 m

B) 5.9 m

C) 5.9 × 104 m

D) 590 m

E) 5.9 km

Answer: E

Var: 1

25) What is the resistance of 1.0 m of a solid cylindrical metal cable having a diameter of 0.40 inches and a resistivity of 1.68 × 10-8 Ω ∙ m?

A) 0.00012 Ω

B) 0.00021 Ω

C) 0.0012 Ω

D) 0.0021 Ω

Answer: B

Var: 1

26) What is the resistance of a cylindrical metal rod 1.0 cm in diameter and 45 m long, if the resistivity of the metal is 1.4 × 10-8 Ω ∙ m?

A) 0.0063 Ω

B) 0.0080 Ω

C) 0.80 Ω

D) 6.3 Ω

Answer: B

Var: 1

27) A certain metal has a resistivity of 1.68 × 10-8 Ω ∙ m. You have a long spool of wire made from this metal. If this wire has a diameter of 0.15 mm, how long should you cut a segment so its resistance will be 15 Ω?

A) 16 mm

B) 16 cm

C) 1.6 m

D) 16 m

Answer: D

Var: 1

28) A 120-m long metal wire having a resistivity of 1.68 × 10-8 Ω ∙ m has a resistance of 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

Answer: B

Var: 1

29) A rod is 4.0 m long and has a square cross-section that is 1.5 cm on each side. An ohmmeter measures 0.040 Ω across its ends. What is the resistivity of the material from which this rod is made?

A) 0.023 Ω ∙ m

B) 0.015 Ω ∙ m

C) 1.5 × 10-4 Ω ∙ m

D) 2.3 × 10-6 Ω ∙ m

Answer: D

Var: 1

30) Calculate the current through a 10.0-m long 22-gauge nichrome wire with a radius of 0.321 mm if it is connected across a 12.0-V battery. The resistivity of the nichrome is 1.00 × 10-6 Ω ∙ m.

A) 30.9 A

B) 61.8 A

C) 0.388 A

D) 0.776 A

Answer: C

Var: 1

31) A metal bar is 20 cm long and has a rectangular cross-section measuring 1.0 cm × 2.0 cm. What is the voltage drop along its length when it carries a 4000-A current? The resistivity of the metal is 1.68 × 10-8 Ω ∙ m.

A) 0.67 V

B) 0.34 V

C) 0.067 V

D) 0.034 V

Answer: C

Var: 1

32) A 1.0-m length of nichrome wire has a radius of 0.50 mm and a resistivity of 1.0 × 10-6 Ω ∙ m. When this wire carries a current of 0.50 A, what is the voltage across its ends?

A) 0.0030 V

B) 0.32 V

C) 0.64 V

D) 1.6 V

Answer: C

Var: 1

33) A 1.0-mm diameter extension cord is made of metal having a resistivity of 1.68 × 10-8 Ω ∙ m. When it carries a current of 15 A, what is the potential difference between two points in the cord that are 100 m apart?

A) 12 V

B) 23 V

C) 32 V

D) 41 V

Answer: C

Var: 1

34) The resistivity of gold is [pic] at a temperature of 20°C. A gold wire that is 0.50 mm in diameter and 44 cm long carries a current of 380 mA. How many electrons each second pass a given cross section of the wire at 20°C? (e = 1.60 × 10-19 C)

A) 2.4 × 1018

B) 2.4 × 1017

C) 1.2 × 1022

D) 2.8 × 1014

E) 6.3 × 1015

Answer: A

Var: 50+

35) How much current will be flowing through a [pic] length of copper wire with radius [pic] if it is connected to a source supplying [pic] The resistivity of the metal is 1.68 × 10-8 Ω ∙ m.

A) 0 A

B) 0 × 108 A

C) 0 nA

D) 0 A

Answer: A

Var: 50+

36) When a 1.0-m length of metal wire is connected to a [pic] battery, a current of [pic] flows through it. What is the diameter of the wire? The resistivity of the metal is 2.24 × 10-8 Ω ∙ m.

A) 12 μm

B) 6.0 μm

C) 24 μm

D) 2.2 μm

Answer: A

Var: 38

37) The resistance of a 100-cm wire of cross sectional area 2 × [pic] [pic] is 400 Ω. What is the resistivity of the material from which this wire is made?

A) 0.80 × [pic] Ω ∙ m

B) 8.0 × [pic] Ω ∙ m

C) 0.80 × [pic] Ω ∙ m

D) 0.80 × [pic] Ω ∙ m

E) 8.0 × [pic] Ωv

Answer: B

Var: 5

38) The resistivity of the material of a wire is 1.76 × 10-8 Ω ∙ m. If the diameter of the wire is 2.00 mm and its length is 2.00 m, what is its resistance?

A) 112 Ω

B) 11.2 Ω

C) 1.12 Ω

D) 0.112 Ω

E) 0.0112 Ω

Answer: E

Var: 1

39) The resistivity of a 1.0 m long copper wire is 1.72 × 10-8 Ω ∙ m 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

Answer: C

Var: 1

40) How much current will flow through a [pic] length of metal wire with a radius of [pic] if it is connected to a power source supplying [pic] The resistivity of the metal is 1.68 × 10-8 Ω ∙ m.

A) 0 A

B) 0 x 108 A

C) 0 nA

D) 0 A

Answer: A

Var: 50+

41) When a potential difference is applied across a piece of wire made of metal A, a [pic] current flows. If the metal-A wire is replaced with a wire made of metal B having twice the diameter of the metal-A wire, how much current will flow through the metal-B wire? The lengths of both wires are the same, and the voltage difference remains unchanged. The resistivity of metal A is 1.68 × 10-8 Ω ∙ m, and the resistivity of metal B is 1.59 × 10-8 Ω ∙ m.

A) 21 mA

B) 19 mA

C) 11 mA

D) 5.3 mA

Answer: A

Var: 9

42) A tube of mercury with resistivity 9.84 × 10-7 Ω ∙ m has a uniform electric field of 23 N/C inside the mercury. How much current is flowing in the tube, if the radius of the tube is 0.495 mm?

A) 18 A

B) 180 A

C) 29 A

D) 280 A

Answer: A

Var: 1

43) The power rating of a 400-Ω resistor is 0.800 W.

(a) What is the maximum safe voltage across this resistor?

(b) What is the maximum current the resistor can safely draw?

Answer: (a) 17.9 V (b) 44.7 mA

Var: 1

44) A resistor operated at 120 V dc is rated at 1.40 kW.

(a) What is the normal operating current through the resistor?

(b) What is the resistance of the resistor?

Answer: (a) 11.7 A (b) 10.3 Ω

Var: 1

45) A flashlight draws 0.133 A from a 3.0-V battery pack. In 2.0 minutes (a) how much charge flows from the battery, (b) how much energy does the battery supply, and (c) how many electrons have passed any point in the circuit every second?

Answer: (a) 16 C (b) 48 J (c) 8.3 × 1017

Var: 1

46) An instrument is rated at 250 W if it is connected across a 120-V dc power supply.

(a) What current does it draw under normal operation?

(b) What is its resistance?

(c) How many kilowatt-hours does it use in a day if it is left on all the time?

Answer: (a) 2.1 A (b) 58 Ω (c) 6.0 kWh

Var: 1

47) A 200-W light bulb is connected across a 110-V dc power supply. 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

Answer: E

Var: 1

48) A 100-W resistance heater is connected to a 110-V dc source. What current flows through the heater?

A) 1.1 A

B) 2.2 A

C) 3.3 A

D) 4.4 A

E) 0.91 A

Answer: E

Var: 1

49) A 100-W light bulb is operated by a 110-V dc source. What is the resistance of this bulb?

A) 100 Ω

B) 8.0 × 10-3 Ω

C) 6.0 × 10-3 Ω

D) 120 Ω

E) 240 Ω

Answer: D

Var: 1

50) If the power rating of a 400-Ω resistor is 0.800 W, what is the maximum voltage that can safely be connected across the resistor?

A) 110 V

B) 17.9 V

C) 170 V

D) 1.80 V

Answer: B

Var: 1

51) If the power rating of a 400-Ω resistor is 0.80 W, what is the maximum current it can safely draw?

A) 45 mA

B) 18 mA

C) 4.4 mA

D) 2.0 mA

E) 320 mA

Answer: A

Var: 1

52) A light bulb operating at a dc voltage of 120 V has a power rating of 60 W. How much current is flowing through this bulb?

A) 1.5 A

B) 2.5 A

C) 0.50 A

D) 2.0 A

E) 1.0 A

Answer: C

Var: 1

53) A light bulb operating at a dc voltage of 120 V has a resistance of 200 Ω. How much power is dissipated in this bulb?

A) 100 W

B) 60 W

C) 72 W

D) 14 mW

E) 7.2 W

Answer: C

Var: 1

54) The power rating of a 400-Ω resistor is 0.25 W. What is the maximum voltage you can safely connect across its ends?

A) 10 V

B) 20 V

C) 30 V

D) 40 V

E) 50 V

Answer: A

Var: 1

55) When 5.00 A is flowing through an 10.0-Ω device, how much power is being dissipated in the device?

A) 50.0 W

B) 250 W

C) 500 W

D) 2.50 kW

Answer: B

Var: 2

56) A resistance heater is rated at 1200 W when operating at 110 V dc. What current will it draw?

A) 0.090 A

B) 1.0 A

C) 11 A

D) 12 A

Answer: C

Var: 1

57) A 150-W light bulb is designed to operate at 110 V dc. How much current does it draw?

A) 0.73 A

B) 1.4 A

C) 2.0 A

D) 15 A

Answer: B

Var: 2

58) What is the resistance of a 0.100-kW light bulb designed to be used in a 120-V circuit dc?

A) 12.0 Ω

B) 144 Ω

C) 1.2 Ω

D) 0.83 Ω

Answer: B

Var: 1

59) A toaster is rated at 800 W when operating at 120 V dc. What is the resistance of its heating element?

A) 16 Ω

B) 18 Ω

C) 6.7 Ω

D) 0.15 Ω

Answer: B

Var: 1

60) A 200-Ω resistor is rated at 1/4 W. What is the maximum current it can safely draw?

A) 0.035 A

B) 0.35 A

C) 50 A

D) 0.25 A

Answer: A

Var: 1

61) A 25-W soldering iron runs on 110 V dc. What is its resistance?

A) 0.0020 Ω

B) 4.4 Ω

C) 0.48 kΩ

D) 2.8 kΩ

Answer: C

Var: 1

62) How much does it cost to operate a 25-W soldering iron for 8.0 hours if energy costs 8.0¢/kWh?

A) $1.50

B) 25¢

C) 16¢

D) 1.6¢

Answer: D

Var: 1

63) 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) 800 kWh

Answer: B

Var: 1

64) 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

Answer: D

Var: 1

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

A) 1.9 kJ

B) 6.0 kJ

C) 1.9 MJ

D) 6.9 MJ

Answer: D

Var: 1

66) An electronic component with a 17-Ω resistor is rated for use at power levels not exceeding [pic] How much current can safely flow through the component?

A) 0.91 A

B) 1.21 A

C) 238 A

D) 0.22 A

Answer: A

Var: 50+

67) A 100-W driveway light bulb is on 10 hours per day. If the power company charges 10¢ for each kilowatt-hour of electricity used, estimate the yearly cost to operate the bulb if it is used every day for a 365-day year.

A) $3.65

B) $7.30

C) $37

D) $73

Answer: C

Var: 1

68) A 400-W computer (including its monitor) is turned on for 8.0 hours per day. If electricity costs 10¢ per kWh, how much does it cost to run the computer annually for a 365-day year?

A) $120

B) $1200

C) $15

D) $150

Answer: A

Var: 1

69) The nichrome heating element in an electric drier operates on 240 V and generates heat at the rate of 2.0 kW. The heating element shorts out and, in repairing it, the repairman shortens the nichrome wire by 10%. (Assume the temperature is unchanged. In reality, the resistivity of the wire will depend on its temperature.) What effect will the repair have on the power dissipated in the heating element?

A) The power is still 2.0 kW.

B) The power increases to 2.2 kW.

C) The power decreases to 1.8 kW.

D) None of the given answers is correct.

Answer: B

Var: 1

70) When a 14.0-A current flows through an 8.00-Ω device for 24.0 hours, how much does this cost if energy costs $0.0900 per kW ∙ h?

A) $0.24

B) $1.04

C) $2.16

D) $3.39

Answer: D

Var: 1

71) A 9.0-V battery costs $1.49 and will run a portable CD player for 6.0 hours before running down and needing to be replaced. If this battery supplies a current of 25 mA to the player, what is the cost of the energy provided by the battery in dollars per kWh?

A) $11/kWh

B) $110/kWh

C) $1100/kWh

D) $11,000/kWh

Answer: C

Var: 1

72) A 200-Ω resistor is rated at 1/4 W. What is the maximum voltage that can safely be connected across it?

A) 0.71 V

B) 7.1 V

C) 50 V

D) 0.25 V

Answer: B

Var: 1

73) A simple circuit has a total resistance of 30 Ω. If a 2.0-A current is maintained in this circuit, how much energy is dissipated in this circuit in 4.0 seconds?

A) 24 J

B) 4.8 J

C) 48 J

D) 480 J

E) 6.0 J

Answer: D

Var: 5

74) The resistivity of gold is [pic] at a temperature of 20°C. A gold wire, 1.8 mm in diameter and 14 cm long, carries a current of 480 mA. What power is dissipated in this wire at 20°C?

A) 0.31 mW

B) 0.077 mW

C) 0.14 mW

D) 0.19 mW

E) 0.25 mW

Answer: A

Var: 50+

75) A 120-Ω laboratory resistor is rated at 0.25 W. How much current can safely flow through the resistor?

A) 30 A

B) 2.1 mA

C) 22 mA

D) 46 mA

Answer: D

Var: 1

76) The voltage drop across a metal bar is [pic] while a current of [pic] flows through it. How much power does this bar dissipate?

A) 50 mW

B) 500 μW

C) 2.5 kW

D) 0.50 kW

Answer: A

Var: 35

77) A battery is rated such that it provides [pic] of power at [pic] when fully charged. How much current can it deliver?

A) 0.11 mA

B) 81 kA

C) 0.33 mA

D) 9.1 kA

Answer: A

Var: 41

78) A device with a resistance of [pic] is connected to a [pic] battery. How much power does the device use?

A) 0.50 mW

B) 20,000 kW

C) 0.050 mW

D) 2000 kW

Answer: A

Var: 10

79) A 6.0-V battery that can store [pic] of energy is connected to a resistor. How much charge must flow between the battery's terminals to completely drain the battery if it is fully charged? Assume that the voltage of the battery remains the same until it is totally drained.

A) 83 C

B) 0.010 C

C) 3000 C

D) 0.07 C

Answer: A

Var: 50

80) The heating element of a toaster is a 5.4-m length of nichrome wire of diameter 0.48 mm. The resistivity of nichrome at the operating temperature of the toaster is 1.3 × 10-6 Ω ∙ m. The toaster is designed to operate at a voltage of 120 V. How much power does it draw in normal operation?

A) 370 W

B) 360 W

C) 380 W

D) 400 W

E) 410 W

Answer: A

Var: 50+

81) A battery supplies 6.0 mA to a 12-Ω resistor for 1.5 h. How much electric energy does this resistor dissipate in this time?

Answer: 2.3 J

Var: 1

82) A 5.0-V battery storing 43.0 kJ of energy supplies [pic] of current to a circuit. How much energy does the battery have left after powering the circuit for [pic]

A) 16 kJ

B) 27 kJ

C) 43 kJ

D) 41 kJ

Answer: A

Var: 50+

83) 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

Answer: B

Var: 1

84) The temperature coefficient of resistivity of platinum is 3.9 × 10-3/C°. If a platinum wire has a resistance of R at a temperature of 23°C, to what temperature must it be heated in order to double its resistance to 2R?

A) 280°C

B) 300°C

C) 730°C

D) 930°C

Answer: A

Var: 1

85) 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 just starts to melt. What is the melting point of indium? The temperature coefficient of resistivity for platinum is 3.927 × 10-3/C°.

A) 116°C

B) 136°C

C) 156°C

D) 351°C

Answer: C

Var: 1

86) A tungsten wire is 1.50 m long and has a diameter of 1.00 mm. At 20°C a current of 50 mA flows through this wire. The temperature coefficient of resistivity for this tungsten is 4.5 × 10-3/C°, and its resistivity at 20°C is 5.6 × 10-8 Ω ∙ m.

(a) What is the potential difference across the ends of this wire at 20°C?

(b) If the wire temperature increases by 100 C°, what potential difference across its ends is now required to produce a current of 50 mA?

Answer: (a) 5.3 mV (b) 7.8 mV

Var: 1

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