Deck 26: Ac Electricity

Question 1

In a RLC circuit, the values of the inductance and capacitance are both halved. As a result of this change, the resonance frequency of the circuit is

A)the same as before.
B)reduced to one-half the original value.
C)reduced to one-fourth the original value.
D)twice the original value.
E)four times the original value.

Answer 1

D

Question 2

A resistor and an inductor are connected in series to a battery. The time constant for the circuit represents the time required for the current to reach

A)25% of the maximum current.
B)37% of the maximum current.
C)63% of the maximum current.
D)75% of the maximum current.
E)100% of the maximum current.

Answer 2

C

Question 3

A resistor and an inductor are connected in series to an ideal battery of constant terminal voltage. At the moment contact is made with the battery, the voltage across the resistor is

A)greater than the battery's terminal voltage.
B)equal to the battery's terminal voltage.
C)less than the battery's terminal voltage, but not zero.
D)zero.
E)equal to the voltage across the inductor.

Answer 3

D

Question 4

In an RLC ac circuit, the values of the inductance and capacitance are both doubled. As a result of this change, the resonance frequency of the circuit is

A)the same as before.
B)reduced to one-half the original value.
C)reduced to one-fourth the original value.
D)twice the original value.
E)four times the original value.

Question 5

If the primary coil of a transformer contains more loops than the secondary coil then it is a

A)step-up transformer.
B)step-down transformer.

Question 6

If the secondary coil of a transformer contains more loops than the primary coil, then it is a

A)step-up transformer.
B)step-down transformer.

Question 7

Consider a capacitor connected across the ac source. As the capacitance is increased, the current through the capacitor will

A)increase.
B)remain constant.
C)decrease.

Question 8

As the frequency of the ac voltage across an inductor approaches zero, the inductive reactance of that coil

A)approaches zero.
B)approaches one.
C)approaches infinity.
D)becomes negative.

Question 9

A pure inductor is connected to an ac power supply. In this circuit, the current

A)leads the voltage by 90°.
B)is in phase with the voltage.
C)lags the voltage by 45°.
D)lags the voltage by 90°.
E)leads the voltage by 45°.

Question 10

Which of the following statements is true for a series RLC ac circuit in resonance? (There could be more than one correct choice).

A)The inductive reactance and the capacitive reactance are both equal to zero.
B)The inductive reactance is equal to the capacitive reactance.
C)The current in the circuit is a minimum.
D)The current in the circuit is a maximum.
E)The rms current is equal to the peak current.

Question 11

What is the phase angle between the voltages of the inductor and capacitor in a RLC series circuit?

A)0°
B)45°
C)90°
D)180°
E)270°

Question 12

A transformer is a device used to

A)transform an alternating current into a direct current.
B)transform a direct current into an alternating current.
C)increase or decrease an ac voltage.
D)increase or decrease a dc voltage.

Question 13

An ideal solenoid has a self-inductance L. If you now double its radius and its length, but do not change the number of coils, what will its self-inductance be?

A)4L
B)2L
C)L
D)L/2
E)L/4

Question 14

A resistor and an inductor are connected in series to an ideal battery of constant terminal voltage. At the moment contact is made with the battery, the voltage across the inductor is

A)greater than the battery's terminal voltage.
B)equal to the battery's terminal voltage.
C)less than the battery's terminal voltage, but not zero.
D)zero.
E)equal to the voltage across the resistor.

Question 15

In the figure, which of the phasor diagrams represents a series RLC circuit driven at resonance?

A)1
B)2
C)3
D)4
E)5

Question 16

A simple ac circuit is composed of a capacitor connected across the terminals of an ac power source. If the frequency of the source is doubled, what happens to the reactance of the capacitor?

A)It increases by a factor of 4.
B)It increases by a factor of 2.
C)It increases by a factor of $\sqrt { 2 }$
D)It decreases by a factor of 2.
E)It decreases by a factor of 4.

Question 17

A pure capacitor is connected to an ac power supply. In this circuit, the current

A)leads the voltage by 90°.
B)lags the voltage by 90°.
C)lags the voltage by 180°.
D)is in phase with the voltage.
E)None of the given answers are correct.

Question 18

A transformer is a device that normally

A)operates on either dc or ac.
B)operates only on ac.
C)operates only on dc.

Question 19

A resistor and an inductor are connected in series to a battery. The battery is suddenly removed from the circuit and replaced by a wire to complete the circuit. The time constant for of the new circuit represents the time required for the current to decrease to

A)25% of the original value.
B)37% of the original value.
C)63% of the original value.
D)75% of the original value.
E)zero.

Question 20

A simple ac circuit consists of an inductor connected across the terminals of an ac power source. If the frequency of the ac source is decreased by a factor of four, what happens to the reactance of the inductor?

A)It decreases by a factor of four.
B)It decreases by a factor of eight.
C)It increases by a factor of two.
D)It increases by a factor of four.
E)It increases by a factor of eight.

Question 21

When 5.0 A at 110 V flows in the primary of an ideal transformer, how many amps at 24 V can flow in the secondary?

A)1.1 A
B)4.6 A
C)5.0 A
D)23 A

Question 22

The primary of an ideal transformer has 100 turns and its secondary has 200 turns. Neglecting frictional losses, if the power input to the primary is 100 W, we can expect the power output of the secondary to be

A)50 W.
B)100 W.
C)200 W.
D)none of the given answers.

Question 23

The primary coil of an ideal transformer has 100 turns and its secondary coil has 400 turns. If the ac voltage applied to the primary coil is 120 V, what voltage is present in its secondary coil?

A)100 V
B)30 V
C)70 V
D)480 V
E)400 V

Question 24

An ideal transformer has 60 turns on its primary coil and 300 turns on its secondary coil. If 120 V at 2.0 A is applied to the primary,
(a)what voltage is present in the secondary?
(b)what current is present in the secondary?

Question 25

A generator produces 60 A of current at 120 V. The voltage is usually stepped up to 4500 V by an ideal transformer and transmitted through a power line of total resistance 1.0 Ω. Find the number of turns in the secondary if the primary has 200 turns.

A)5
B)200
C)4500
D)7500

Question 26

In an ideal transformer, how many turns are necessary in a 110-V primary if the 24-V secondary has 100 turns?

A)458
B)240
C)110
D)22

Question 27

You need an ideal transformer to reduce a voltage of 150 V in the primary circuit to 25 V in the secondary circuit. The primary circuit has 130 windings and the secondary circuit is completed through a 55-? resistor. How many windings should the secondary circuit contain?

Question 28

The primary of an ideal transformer has 100 turns and its secondary has 200 turns. If the input voltage to the primary is 100 V, we can expect the output voltage of the secondary to be

A)50 V.
B)100 V.
C)200 V.
D)none of the given answers.

Question 29

The primary coil of an ideal transformer has 600 turns and its secondary coil has 150 turns. If the current in the primary coil is 2 A, what is the current in its secondary coil?

A)8 A
B)1/2 A
C)1/4 A
D)2 A
E)4 A

Question 30

What is the self-inductance of an ideal solenoid that is 300 cm long with a cross-sectional area of 1.00 × 10^-4 m² and has 1000 turns of wire? (μ₀ = 4π × 10^-7 T ∙ m/A)

A)4.19 nH
B)4.19 pH
C)4.19 μH
D)41.9 nH
E)41.9 µH

Question 31

An ideal transformer has 60 turns on its primary coil and 300 turns on its secondary coil. If 120 V at 2.0 A is applied to the primary, what voltage and current are present in the secondary?

Question 32

A current of 2.0 A in the 100-turn primary of an ideal transformer causes 14 A to flow in the secondary. How many turns are in the secondary?

A)700
B)114
C)14
D)4

Question 33

An ideal step-down transformer is needed to reduce a primary voltage of 120 V to 6.0 V. What must be the ratio of the number of turns in the secondary to the number of turns in the primary?

Question 34

An ideal step-up transformer doubles a primary voltage of 110 V. What is the ratio of the number of turns in its primary coil to the number of turns in the secondary coil?

A)1:4
B)4:1
C)2:1
D)1:2
E)1:8

Question 35

The primary coil of an ideal transformer has 100 turns and its secondary coil has 400 turns. If the ac current in the secondary coil is 2 A, what is the current in its primary coil?

A)2 A
B)8 A
C)1/2 A
D)1/4 A
E)4 A

Question 36

An ideal transformer with 120 turns in its secondary supplies 12 V at 220 mA to a toy train. The primary is connected across a 120-V wall outlet.
(a)How many turns are in the primary?
(b)What is the primary current?
(c)What power is delivered by the wall outlet?

Question 37

An ideal transformer consists of a 500-turn primary coil and a 2000-turn secondary coil. If the current in the secondary is 3.0 A, what is the current in the primary?

A)0.75 A
B)1.3 A
C)12 A
D)48 A

Question 38

The primary of an ideal transformer has 100 turns and its secondary has 200 turns. If the input current at the primary is 100 A, we can expect the output current at the secondary to be

A)50 A.
B)100 A.
C)200 A.
D)none of the given answers.

Question 39

An ideal transformer steps down 120 V to 12. V and the 2630.-turn secondary supplies 12. A.
(a)Determine the current in the primary.
(b)Determine the turns ratio.
(c)What is the ratio of output power to input power?

Question 40

The secondary coil of an ideal neon sign transformer provides 7500 V at 10.0 mA. The primary coil operates on 120 V. What current does the primary draw?

A)0.625 A
B)0.625 mA
C)0.160 A
D)1.66 A

Question 41

What size capacitor must be placed in series with a 40-mH coil so the circuit will oscillate with a frequency of 1000 Hz?

A)0.63 μF
B)0.50 μF
C)0.22 μF
D)0.17 μF

Question 42

The potential applied to a 20-? resistor is v = (60 V)cos(33t). What is the rms current through this resistor?

Question 43

A 100-W light bulb is powered by 120 V ac 60.0-Hz household connection. Determine the rms current and the current amplitude.

Question 44

A series circuit contains an 80-?F capacitor, a 0.020-H inductor, and a switch. The resistance of the circuit is negligible. Initially, the switch is open, the capacitor voltage is 50 V, and the current in the inductor is zero. At time t = 0 s, the switch is closed.

-How long after closing the switch does the magnetic energy in the inductor first reach its maximum value?

A)2.0 ms
B)4.0 ms
C)6.0 ms
D)8.0 ms
E)10 ms

Question 45

A coil with a self-inductance of 6.0 H has a constant current of 2.0 A flowing through it for 2.0 s. What is the emf induced in this coil?

A)6.0 V
B)12 V
C)0.0 V
D)4.0 V
E)8.0 V

Question 46

A simple series circuit consists of just a 3.4-µF capacitor and an 80-mH inductor, with no appreciable resistance. At t = 0 the capacitor has charge 5.4 µC and the current in the inductor is zero.
(a)How long after t = 0 will the current in the circuit be maximum?
(b)What will be this maximum current?

Question 47

A series circuit contains an 80-?F capacitor, a 0.020-H inductor, and a switch. The resistance of the circuit is negligible. Initially, the switch is open, the capacitor voltage is 50 V, and the current in the inductor is zero. At time t = 0 s, the switch is closed.

-What is the maximum current in the circuit?

A)2.2 A
B)2.4 A
C)2.8 A
D)3.0 A
E)3.2 A

Question 48

American power plants usually supply 120 V ac.
(a)At what frequency is this voltage supplied?
(b)What is the maximum voltage?

Question 49

The inductance of a solenoid that is 16.0 cm long and has a cross-sectional area of 1.00 × $10 ^ { - 4 }$ $m ^ { 2 }$ is $1.00 \mathrm { mH } \text {. }$ How many turns of wire does this solenoid have? (?₀ = 4? × 10^-7 T ? m/A)

A)318,000
B)159,000
C)1130
D)282
E)150

Question 50

A simple circuit consists only of of a 1.0-μF capacitor and a 15-mH coil in series. At what frequency will this circuit oscillate?

A)15 kHz
B)1.3 kHz
C)0.77 kHz
D)67 Hz

Question 51

What capacitance is needed in series with a 3.7-mH inductor so the circuit will oscillate at a frequency of 1000 Hz?

A)0.15 mF
B)15 μF
C)6.8 μF
D)0.015 μF

Question 52

The current flowing through a circuit is changing at a rate of 6.0 A/s. If the circuit contains a 190-H inductor, what is the emf across the inductor?

A)1100 V
B)11 mV
C)32 mV
D)32 V

Question 53

What inductance is needed in series with a 4.7-μF capacitor so the circuit will oscillate at a frequency of 10 kHz?

A)21 μH
B)54 μH
C)4.7 mH
D)5.4 mH

Question 54

The current through a 50-? resistor is I = (0.80

A)sin(240t). What are (a)the current amplitude and (b)the rms current?

Question 55

A coil with a self-inductance of 6.0 H is connected to a dc battery through a switch. As soon as the switch is closed, the rate of change of current is 2.0 A/s. What is the emf induced in this coil at this instant?

A)6.0 V
B)3.0 V
C)12 V
D)0.33 V
E)0.0 V

Question 56

The peak current and voltage outputs of a generator are 20 A and 240 V, respectively. What average power is provided by the generator?

Question 57

A series circuit contains an 80-?F capacitor, a 0.020-H inductor, and a switch. The resistance of the circuit is negligible. Initially, the switch is open, the capacitor voltage is 50 V, and the current in the inductor is zero. At time t = 0 s, the switch is closed.

- When the current in the circuit is 1.5 A, what is the charge on the capacitor?

A)2000 µC
B)2500 µC
C)3000 µC
D)3500 µC
E)4000 µC

Question 58

The figure shows a solenoid having no appreciable resistance. When the current in this solenoid is decreasing at a rate of 5.5 A/s, the self-induced emf in the solenoid is measured to be 9.5 V.
((a)What is the self-inductance of this solenoid?
(b)If the current is in the direction from b to a in the figure, which point, a or b, is at higher potential?

Question 59

A series circuit contains an 80-?F capacitor, a 0.020-H inductor, and a switch. The resistance of the circuit is negligible. Initially, the switch is open, the capacitor voltage is 50 V, and the current in the inductor is zero. At time t = 0 s, the switch is closed.

-When the potential across the capacitor is 30 V, what is the rate of change of the current?

A)1000 A/s
B)1500 A/s
C)2000 A/s
D)2500 A/s
E)3000 A/s

Question 60

An ideal solenoid with 3000 turns is 70.0 cm long. If its self-inductance is 25.0 mH, what is its radius? (μ₀ = 4π × 10^-7 T ∙ m/A)

A)0.0222 m
B)0.00199 m
C)327 m
D)52.0 m

Question 61

A 0.150-kW lamp is plugged into a 120-V ac wall outlet. What are (a)the peak current through the lamp, (b)the rms current through the lamp, and (c)the resistance of the lamp?

Question 62

If the maximum voltage of an ac signal is 8.0 V, what is the rms value of this voltage?

A)2.8 V
B)5.7 V
C)6.2 V
D)4.0 V
E)16.0 V

Question 63

A 0.10-?F capacitor is connected to a 120-V rms 60-Hz source.
(a)What is its capacitive reactance?
(b)What is the rms current to the capacitor?
(c)If both the capacitance and the frequency were doubled, what would be the rms current?

Question 64

What is the peak voltage in an ac circuit where the rms voltage is 120 V?

A)84.8 V
B)120 V
C)170 V
D)240 V

Question 65

The current through a 50-Ω resistor is I = (0.80
A)16 W

A)sin(240t), where t is measured in seconds. How much power on average is dissipated in the resistor?
B)32 W
C)45 W
D)64 W

Question 66

At what frequency does a 10-?F capacitor have a reactance of 0.12 k??

Question 67

A 0.200-H inductor is connected to a 60.0-Hz 120-V rms source.
(a)What is the inductive reactance?
(b)What is the rms current to the inductor?
(c)If both the inductance and the frequency were doubled, what would be the rms current?

Question 68

A 120-V rms voltage is applied across a 6.0-μF capacitor. If the frequency of the generator is 60 Hz, what is the rms value of the current in the circuit?

A)0.47 A
B)0.37 A
C)0.071 A
D)0.17 A
E)0.27 A

Question 69

A 6.0-?F capacitor is connected to an ac signal with a frequency of 60 Hz. If the maximum voltage applied to the capacitor is 8.0 V, what is its capacitive reactance?

A)6.0 ?
B)440 ?
C)7.5 × $10 ^ { - 6 }$ ?
D)740 ?
E)160 ?

Question 70

The reactance of a capacitor is $4.0 \mathrm { k } \Omega$ at a frequency of $0.60 \mathrm { kHz }$ What is the capacitance?

A)0.066 ?F
B)0.42 ?F
C)2.6 ?F
D)0.093 ?F

Question 71

A 120-V rms voltage at 60.0 Hz is applied across an inductor, capacitor and a 100-Ω resistor in series. If the maximum value of the current in this circuit is 1.60 A, what is the rms value of the current in this circuit?

A)1.13 A
B)2.26 A
C)1.82 A
D)1.60 A
E)2.66 A

Question 72

An alternating current is supplied to an electronic component with a rating that the voltage across it can never, even for an instant, exceed $16 \mathrm {~V}$ What is the highest rms voltage that can be supplied to this component while staying below the voltage limit?

A)8 $\sqrt { 2 }$ V
B)16 $\sqrt { 2 }$ V
C)256 V
D)8 V

Question 73

The current through a 50-Ω resistor is I = (0.80
A)0.57 A

A)sin(240t), where t is measured in seconds. What is the rms current?
B)0.80 A
C)1.1 A
D)1.6 A

Question 74

A 150-W lamp is placed into a 120-V ac outlet. What is the peak current?

A)0.80 A
B)0.88 A
C)1.2 A
D)1.8 A

Question 75

At what frequency will the inductive reactance of a 44-mH inductor be equal to the capacitive reactance of a 27-pF capacitor?

Question 76

What capacitance will have the same reactance as a 100-mH inductance of both of them are in a 120-V 60-Hz circuit?

Question 77

In the circuit shown in the figure, the 60-Hz ac source has a voltage amplitude of 120 V, the capacitive reactance is $820 \Omega ,$ the inductive reactance is $330 \Omega$ and the resistance is $550 \Omega$ What is the capacitance C of the capacitor?

A)3.2 µF
B)6.3 µF
C)9.4 µF
D)13 µF
E)20 µF

Question 78

The potential applied to a 20-? resistor is (60 V)cos(33t). What is the average power consumed in the resistor?

Question 79

A 10-Ω resistor is connected to a 120-V ac power supply. What is the peak current through the resistor?

A)12 A
B)17 A
C)0.083 A
D)0.12 A

Question 80

The capacitive reactance of a 44-?F capacitor in an ac circuit is 6.0 × $10 ^ { 2 }$ ?. What is the frequency of the applied signal?

A)80 Hz
B)17 Hz
C)800 Hz
D)6.0 Hz
E)2.2 Hz