Deck 28: Direct Current Circuits

Question 1

By convention, the direction of current is defined as

A) the direction a negative charge would flow in.
B) the direction a positive charge would flow in.
C) either of the previous two answers is correct, depending on the sign of the charge carrier in a particular problem.
D) none of the above.

Answer 1

the direction a positive charge would flow in.

Question 2

The current density j has the dimensions of

A) ampere.
B) ampere/second.
C) ampere/meter².
D) ampere/meter³.
E) ampere/second·meter³.

Answer 2

ampere/meter².

Question 3

When current "flows" in an ordinary wire, all of the following are true except

A) there is an electric field in the wire and hence an electrostatic force on the electrons.
B) the electrons move faster and faster with time.
C) electrostatic equilibrium does not occur.
D) all of the above are true; there are no exceptions.

Answer 3

electrostatic equilibrium does not occur.

Question 4

The typical random speed of electrons in a metal at room temperature is

A) much smaller than the speed of sound in air.
B) approximately the same as the speed of sound in air.
C) much larger than the speed of sound in air, but much smaller than the speed of light in air.
D) approximately the same as the speed of light in air.

Question 5

The typical drift speed of electrons in a metal at room temperature is

A) much smaller than the speed of sound in air.
B) approximately the same as the speed of sound in air.
C) much larger than the speed of sound in air, but much smaller than the speed of light in air.
D) approximately the same as the speed of light in air.

Question 6

When you turn on a light switch, the light comes on instantly. This happens because

A) the electric field in the wire is propagated at a speed close to the speed of light.
B) the electric field in the wire is propagated at a speed close to the speed of sound in air.
C) a very small number of charges are moving very fast through the wire.
D) in the absence of an electric field, the average velocity of the charges in the wire is very large.
E) none of the above is true.

Question 7

The resistivity of a material depends on all of the following quantities associated with the wire except

A) the free electron density.
B) the free electron mass.
C) the electron mean free path.
D) All of the above are true; there are no exceptions.
E) None of the above is true; there are no exceptions.

Question 8

As its temperature increases, the resistivity of a conducting material generally

A) increases.
B) remains unchanged.
C) decreases.
D) all of the above may be true; the answer depends on the specific geometry of the material.

Question 9

Two wires, A and B, are made of the same material and have the same length. Wire A has half the diameter of wire B. If the resistance of wire A is R, the resistance of wire B is

A) 4R.
B) 2R.
C) R.
D) R/2.
E) R/4.

Question 10

A long cylindrical copper wire has a resistance of 3.0 $\Omega$ . If the same mass of copper is fashioned into a wire of twice the length of the 3.0- $\Omega$ wire, the new wire's resistance is

A) 1.5 $\Omega$
B) 3.0 $\Omega$
C) 6.0 $\Omega$
D) 9.0 $\Omega$
E) 12 $\Omega$

Question 11

A copper bar has dimensions of 2.0 cm $\times$ 3.0 cm $\times$ 5.0 cm. To get the largest resistance for the bar when it is used as a resistor, we should attach the leads to the two opposite sides that have the dimensions

A) 2.0 cm $\times$ 5.0 cm.
B) 3.0 cm $\times$ 5.0 cm.
C) 2.0 cm $\times$ 3.0 cm.
D) any of the above; the resistance of the bar would be the same.

Question 12

A copper wire is stretched to become 10% longer. Its resistance

A) stays the same.
B) becomes 1.2 times larger.
C) becomes 1.1 times larger.
D) becomes 1.1 times smaller.
E) becomes 1.2 times smaller.

Question 13

The total resistance of two resistors depends on how they are connected. The smaller value of the total resistance occurs when the two are connected

A) in parallel.
B) in series.
C) in certain cases the total resistance is the same whether the two resistors are connected in series or in parallel.
D) none of the above is true; it depends on the specific values of the two resistors.

Question 14

In the circuits shown here all the batteries have the same emf, and all the bulbs are identical. The circuit with the greatest current is

A) A
B) B
C) C
D) all have the same current because the batteries have the same emf.

Question 15

In the circuits shown here all the batteries have the same emf, and all the bulbs are identical. The circuit that provides the greatest voltage across the equivalent resistor for the circuit is

A) A
B) B
C) C
D) all provide the same voltage across the equivalent resistor for the circuit because the batteries have the same emf.

Question 16

To measure the current through a resistor in an electric circuit, an ammeter should be connected

A) in parallel with the resistor.
B) in series with the resistor.
C) either in parallel or in series with the resistor, depending on the geometry of the circuit.
D) neither in parallel nor in series with the resistor because the current cannot be measured with an ammeter.

Question 17

An ideal ammeter has

A) a very small resistance.
B) a resistance comparable to the resistances of the other resistors in the circuit.
C) a very large resistance.

Question 18

To measure the potential difference across a resistor in an electric circuit, a voltmeter should be connected

A) in parallel with the resistor.
B) in series with the resistor.
C) either in parallel or in series with the resistor, depending on the geometry of the circuit.
D) neither in parallel nor in series with the resistor because the potential difference cannot be measured with a voltmeter.

Question 19

An ideal voltmeter has

A) a very small resistance.
B) a resistance comparable to the resistances of the other resistors in the circuit.
C) a very large resistance.

Question 20

A 10-A current flows through the 3.0- $\Omega$ resistor that is part of a parallel combination of 2.0- $\Omega$ , 3.0- $\Omega$ , and 4.0- $\Omega$ resistors. The current flowing through the 2.0- $\Omega$ resistor is

A) 5.0 A.
B) 10 A.
C) 15 A.
D) 30 A
E) 60 A.

Question 21

A current flows through the lightbulbs in the figure here. When the switch K is closed

A) the same current flows through bulb B as through bulb A.
B) the current through bulb A gets split in two equal parts: half flows through bulb B and half through the wire with the switch.
C) the current through bulb A gets split in two parts: the larger part flows through bulb B and the smaller through the wire with the switch.
D) all the current through bulb A flows through the wire with the switch.
E) none of the above is true.

Question 22

Two identical lightbulbs of resistance 10.0 $\Omega$ are placed in the circuit shown here. Both batteries are 6.0-V batteries. The current in the circuit is

A) 0 A.
B) 0.30 A.
C) 0.60 A.
D) 1.2 A.
E) 2.4 A.

Question 23

A current flows through the two identical lightbulbs shown here. When the switch K is closed, the brightness of bulb A

A) decreases.
B) stays the same.
C) increases.

Question 24

Two identical storage batteries are connected in parallel; then they are placed in a circuit. The combination will provide

A) half the difference in potential and half the total charge provided by one battery.
B) twice the difference in potential and twice the total charge provided by one battery.
C) twice the difference in potential and half the total charge provided by one battery.
D) the same difference in potential and half the total charge provided by one battery.
E) the same difference in potential and twice the total charge provided by one battery.

Question 25

Two identical storage batteries are connected in series, the positive terminal of the first battery to the negative terminal of the second; then they are placed in a circuit. The combination will provide

A) half the difference in potential and half the total charge provided by one battery.
B) twice the difference in potential and twice the total charge provided by one battery.
C) twice the difference in potential and the same total charge as provided by one battery.
D) the same difference in potential and half the total charge provided by one battery.
E) the same difference in potential and twice the total charge provided by one battery.

Question 26

Two identical storage batteries are connected in series, the positive terminal of the first battery to the positive terminal of the second; then they are placed in a circuit. The combination will provide

A) no difference in potential.
B) twice the difference in potential and half the total charge provided by one battery.
C) twice the difference in potential and the same total charge as provided by one battery.
D) the same difference in potential and half the total charge provided by one battery.
E) the same difference in potential and twice the total charge provided by one battery.

Question 27

The current through the 10.0- $\Omega$ resistor in the figure here is

A) 0 A.
B) 0.25 A.
C) 0.33 A.
D) 0.50 A.
E) 0.66 A.

Question 28

The current through the 15.0- $\Omega$ resistor in the figure here is

A) 0 A.
B) 0.25 A.
C) 0.33 A.
D) 0.50 A.
E) 0.66 A.

Question 29

The resistors in the figure here are R_A = 4.0 $\Omega$ , R_B = 4.0 $\Omega$ , and R_C = 2.0 $\Omega$ ; the emf is 8.0 V. The currents through the resistors are

A) 0.80 A, 0.80 A, and 0.80 A respectively.
B) 2.0 A, 2.0 A, and 4.0 A respectively.
C) 3.0 A, 3.0 A, and 2.0 A respectively.
D) 8.0 A, 8.0 A, and 8.0 A respectively.
E) 2.0 A, 4.0 A, and 2.0 A respectively.

Question 30

The resistors in the figure here are R_A = 2.0 $\Omega$ , R_B = 12.0 $\Omega$ , R_C = 12.0 F $\Omega$ , R_D = 6.0 $\Omega$ , and R_E = 5.0 $\Omega$ ; the emf is 20.0 V. The difference in potential across R_B is

A) 20 V.
B) 13 V.
C) 10 V.
D) 6.0 V.
E) 0.54 V.

Question 31

The resistors in the figure here are R_A = 2.0 $\Omega$ , R_B = 12.0 $\Omega$ , R_C = 12.0 $\Omega$ , R_D = 6.0 $\Omega$ and R_E = 5.0 $\Omega$ ; the emf is 20.0 V. The current through R_D is

A) 3.3 A.
B) 2.0 A.
C) 1.5 A.
D) 1.0 A.
E) 0.12 A.

Question 32

In the figure here, the difference in potential V_A - V_B is

A) 4.0 V.
B) 2.0 V.
C) 1.0 V.
D) 0.50 V.
E) 0 V.

Question 33

In the figure here, the difference in potential V_C - V_D is

A) 4.0 V.
B) 2.0 V.
C) 1.0 V.
D) 0.50 V.
E) 0 V.

Question 34

In the figure here, the difference in potential V_C - V_D, when a wire is connected between points C and D, is

A) 4.0 V.
B) 2.0 V.
C) 1.0 V.
D) 0.50 V.
E) 0 V.

Question 35

In the figure here, the equivalent resistance between points A and B_, when a wire is connected between points C and D, is

A) 4.0 $\Omega$
B) 2.4 $\Omega$
C) 2.0 $\Omega$
D) 1.9 $\Omega$
E) 0 $\Omega$

Question 36

The maximum number of 40-W lightbulbs we can connect in parallel in a 120-V circuit without tripping a 20-A circuit breaker is

A) 20
B) 30
C) 40
D) 60
E) 80

Question 37

One hundred identical lightbulbs are connected in parallel. When one bulb burns out,

A) all the other lightbulbs will get much brighter.
B) all the other lightbulbs will get brighter, some more than others.
C) all the other lightbulbs will get much dimmer, some more than others.
D) all the other lightbulbs will go out.
E) the brightness will not change appreciably.

Question 38

A difference in potential is applied across the combination of a 40-W lightbulb and a 60-W lightbulb connected in series. The 40-W bulb

A) glows more brightly than the 60-W bulb.
B) glows less brightly than the 60-W bulb.
C) glows as brightly as the 60-W bulb.
D) and the 60-W bulb do not glow at all.

Question 39

In the circuits shown here all the batteries have the same emf, and all the bulbs are identical. The circuit that provides the largest amount of light is

A) A
B) B
C) C
D) All provide the same amount of light because the batteries have the same emf.

Question 40

A 600-W coffee pot and a 600-W microwave oven are operated at the same time in the household wired to a 120-V circuit. The total current drawn is

A) 1.0 A.
B) 2.0 A.
C) 6.0 A.
D) 8.0 A.
E) 10 A.

Question 41

If electric energy costs 15 cents per kilowatt-hour, the cost of a 100-W lightbulb glowing continuously for 30 days is

A) $ 1.5.
B) $ 11.
C) $ 50.
D) $ 72
E) $ 480.

Question 42

Two identical heating coils are connected to a constant-voltage line. To get the largest amount of heat per minute, the coils should be connected

A) in series because four times as much heat per minute will be dissipated as in the parallel combination.
B) in series because twice as much heat per minute will be dissipated as in the parallel combination.
C) either in series or in parallel; they will dissipate the same amount of heat per minute.
D) in parallel because twice as much heat per minute will be dissipated as in the series combination.
E) in parallel because four times as much heat per minute will be dissipated as in the series combination.

Question 43

In the figure here, the battery has 12.0 V, and the lightbulbs are all identical. The circuit puts the most of light out when

A) switches A, B, and C are closed, and switches D and E are open.
B) switches C, D, and E are closed, and switches A and B are open.
C) switches B, C, and D are closed, and switches A and E are open.
D) switches B, D, and E are closed, and switches A and C are open.
E) all the switches are closed.

Question 44

A 40.0- $\mu$ F capacitor is charged to 120 V and is then connected across a 500- $\Omega$ resistor. The time constant for the circuit is

A) 4.0 ms.
B) 3.0 ms.
C) 2.0 ms.
D) 1.5 ms.
E) 1.0 ms.

Question 45

A 40.0- $\mu$ F capacitor is charged to 120 V and is then connected across a 500- $\Omega$ resistor. Just after the capacitor is connected to the resistor, the current through the resistor is

A) 4.8 A.
B) 4.2 A
C) 300 mA.
D) 240 mA.
E) 120 mA.

Question 46

A series combination of an uncharged capacitor and a resistor is connected to a battery. Just after the combination is connected to the battery,

A) the charge on the capacitor increases and the difference in potential across the resistor decreases.
B) the charge on the capacitor increases and the difference in potential across the resistor stays constant.
C) the charge on the capacitor increases and the difference in potential across the resistor increases.

Question 47

When the resistance is doubled and the capacitance is halved in an RC circuit, the time constant of the circuit

A) becomes four times larger.
B) becomes two times larger.
C) stays the same.
D) becomes two times smaller.
E) becomes four times smaller.

Question 48

A resistor and a capacitor are connected in series to an ideal battery of constant terminal voltage 20.0 V. When the series combination reaches its steady state, the difference in potential across the resistor is

A) larger than 20.0 V.
B) 20.0 V.
C) smaller than 20.0 V.
D) zero.

Question 49

A 40- $\mu$ F capacitor is charged through a 30-k $\Omega$ resistor. The time needed to charge the capacitor to 70% of its full charge is

A) 8.4 s.
B) 0.84 s.
C) 2.1 min.
D) 0.14 s.
E) 1.4 s.

Question 50

A 3.0-k $\Omega$ resistor is connected in series with a 2.0- $\mu$ F initially uncharged capacitor. The RC combination is charged by a 6.0-V battery. The change in voltage between t = RC and t = 3RC is

A) 4.9 V.
B) 3.9 V.
C) 2.9 V.
D) 1.9 V.
E) 0.90 V.

Question 51

The curve that best represents the current in a discharging RC circuit is

A) A
B) B
C) C
D) D
E) E

Question 52

The curve that best represents the charge in a discharging RC circuit is

A) A
B) B
C) C
D) D
E) E