Deck 22: Alternating-Current Circuits and Machines

Question 1

In the typical household AC voltage of 120 V, what is the peak voltage?

A) 170 V
B) 240 V
C) 85 V
D) 120 V

Answer 1

170 V

Question 2

An AC voltage source, with a peak output of 120 V, results in dissipation of energy in a resistor at rate of 50.0 W. What is the value of the resistance?

A) 100 $\Omega$
B) 120 $\Omega$
C) 144 $\Omega$
D) 72.0 $\Omega$

Answer 2

144 $\Omega$

Question 3

The frequency in an AC series circuit is halved. By what factor does this change the capacitive reactance?

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

Answer 3

2

Question 4

A 17.0- $\mu$ F capacitor is connected to an AC source with an rms voltage of 120 V and a frequency of 60.0 Hz. What is the rms current in the capacitor?

A) 1.41 A
B) 0.543 A
C) 0 A
D) 0.769 A

Question 5

In a capacitor in an AC circuit, the voltage:

A) lags the current by 90°.
B) may lead or lag the current depending on the frequency.
C) leads the current by 90°.
D) is in phase with 70.7% of the current.

Question 6

The rms current is equal to the direct current that:

A) produces the same average voltage across a resistor as in an AC circuit.
B) provides the same average current in a resistor as in an AC circuit.
C) results in the same peak power in a resistor as in an AC circuit.
D) dissipates an equal amount of energy in a resistor at the same rate as in an AC circuit.

Question 7

A 400- $\Omega$ resistor and a capacitor are connected in parallel across a 300-Hz source. If the amplitude of the current in the resistor is equal to that in the capacitor, what is the value of the capacitance?

A)
B)
C)
D) 0.212 F

Question 8

The output voltage of an AC generator is of the form A typical frequency in household circuits is 60 Hz. Using radians for angular measure in this equation, what is the factor in radians per second for this frequency?

A) 2 $\pi$
B) 120
C) 377
D) 60

Question 9

In an AC series circuit, the current in a pure resistor differs in phase with the applied voltage by what angle?

A) 180°
B) zero
C) 90°
D) 45°

Question 10

An AC voltage source, with a peak output of 200 V, is connected to a 25- $\Omega$ resistor. What is the effective (or rms) current in the circuit?

A) 2.8 A
B) 5.7 A
C) 4.0 A
D) 2.0 A

Question 11

The peak voltage of an AC source is 400 V. What is the rms voltage?

A) 100 V
B) 200 V
C) 141 V
D) 283 V

Question 12

In the inductor of a 60-Hz AC series circuit, the peak voltage precedes the peak current in each cycle by what time interval?

A) 2.1 *10^-3 s
B) 8.3 *10^-3 s
C) 4.2 *10^-3 s
D) 1.7 *10^-3 s

Question 13

In some countries the AC frequency is 50 Hz. What is the time interval for one cycle at this frequency?

A) 0.0050 s
B) 0.020 s
C) 2 s
D) 50 s

Question 14

The rate of heat dissipation in an AC circuit with resistance, R, and effective current, I_rms, is given by which of the following?

A) (I_rms)²R
B) 4.0 * (I_rms)²R
C) 2.0 * (I_rms)²R
D) 0.5*(I_rms)²R

Question 15

In an AC series circuit the capacitive reactance is 253 $\Omega$ and frequency is 100 Hz. What is the capacitance?

A) 6.29 $\mu$ F
B) 8.0 $\mu$ F
C) 50.0 $\mu$ F
D) 3.2 $\mu$ F

Question 16

What is the effective (rms) current value for an AC current with an amplitude of 20 A?

A) 28 A
B) 7.1 A
C) 3.1 A
D) 14 A

Question 17

When a 50- $\mu$ F capacitor is attached to an AC source, its capacitive reactance is 60 $\Omega$ . If instead a 200- $\mu$ F capacitor is attached to the same source, what will be its capacitive reactance?

A) 80 $\Omega$
B) 15 $\Omega$
C) 30 $\Omega$
D) 120 $\Omega$

Question 18

An AC voltage source, with a peak output of 200 V, is connected to a 25- $\Omega$ resistor. What is the rate of energy dissipated due to heat in the resistor?

A) 566 W
B) 400 W
C) 800 W
D) 200 W

Question 19

If the rotating coil used in a generator has its radius doubled, what change results in the amplitude of the voltage produced?

A) It doubles.
B) It quadruples.
C) It is increased by a factor 2^3/2.
D) It stays the same.

Question 20

In an AC series circuit, the voltage in the inductor differs in phase with the voltage in the capacitor by what angle?

A) 180°
B) zero
C) 90°
D) 45°

Question 21

What is the impedance of an AC series circuit that is constructed of a 10.0- $\Omega$ resistor along with 24.0- $\Omega$ inductive reactance and 7.0- $\Omega$ capacitive reactance?

A) 19.7 $\Omega$
B) 11.2 $\Omega$
C) 41.0 $\Omega$
D) 27.7 $\Omega$

Question 22

An AC series circuit has 8.00- $\Omega$ resistance, 15.00- $\Omega$ inductive reactance, and 10.00- $\Omega$ capacitive reactance. If an effective (rms) emf of 120 V is applied, what is the effective (rms) current value?

A) 26.0 A
B) 5.31 A
C) 9.23 A
D) 12.7 A

Question 23

An LCR circuit has resistance R₀, inductance L₀, and capacitance C₀. If the resistance value is now tripled, which values of inductance and capacitance will result in the same resonant frequency as before.

A) 3L₀, C₀
B) L₀/3, C₀/3
C) 3L₀, 3C₀
D) 3L₀, C₀/3

Question 24

Consider an AC series circuit containing a coil, capacitor, and resistor. Doubling the frequency will change the circuit's impedance by what factor?

A) 2.0
B) 1.0
C) 1/2
D) Information is insufficient to determine answer.

Question 25

Consider an AC series circuit containing a coil, capacitor, and resistor. Doubling the frequency will change the capacitive reactance by what factor?

A) 1.41
B) 1.0
C) 2.0
D) 1/2

Question 26

A 400- $\Omega$ resistor and an inductor are connected in parallel across a 300-Hz source. If the amplitude of the current in the resistor is equal to that in the inductor, what is the value of the inductance?

A) 0.212 H
B) 0.666 H
C)
D) 0.333 H

Question 27

At what frequency does a 0.32-H inductor have the same reactance as a 0.22- $\mu$ F capacitor?

A)
B)
C) 24 Hz
D)

Question 28

A resistor and capacitor are connected in series with an applied AC voltage source. Separate voltmeter readings across the resistor and capacitor give values of 50 V and 60 V (rms), respectively. What is the effective (rms) voltage of the source?

A) 78 V
B) 90 V
C) 110 V
D) 25 V

Question 29

An FM radio tuning circuit has a coil with an inductance of 0.0015 mH. What is the value of the capacitance if the set is tuned to 98 MHz?

A) 1.8 *10^-6 $\mu$ F
B) 0.88 *10^-6 $\mu$ F
C) 0.98 *10^-6 $\mu$ F
D) 12 *10^-6 $\mu$ F

Question 30

In an LCR circuit, the maximum current is 1 amp. What is the average current?

A) (1/2) A
B) A
C) A
D) None of these choices.

Question 31

An AM radio tuning circuit has a coil with an inductance of 4.00 mH and a capacitor set at 7.50 *10^-6 $\mu$ F. What frequency will it detect?

A) 1520 kHz
B) 919 kHz
C) 550 kHz
D) 750 kHz

Question 32

Consider an AC series circuit containing a coil, capacitor, and resistor. Doubling the frequency will change the inductive reactance by what factor?

A) 1.0
B) 1.41
C) 1/2
D) 2.0

Question 33

In an RL circuit, which of the following is a possible magnitude of a phase angle?

A) 45^°
B) 315^°
C) 225^°
D) 135^°

Question 34

A resistor, inductor, and capacitor are connected in series, each with effective (rms) voltage of 65 V, 150 V, and 80 V, respectively. What is the value of the effective (rms) voltage of the applied source in the circuit?

A) 295 V
B) 48 V
C) 96 V
D) 88 V

Question 35

What is the magnitude of the phase angle of an AC series circuit that is constructed of a 10.0- $\Omega$ resistor along with 20.00- $\Omega$ inductive reactance and 7.00- $\Omega$ capacitive reactance?

A) 18.4°
B) 26.6°
C) 52.4°
D) 59.5°

Question 36

In an AC series circuit the inductive reactance is 150 $\Omega$ and the frequency is 100 Hz. What is the inductance in the circuit?

A) 740 mH
B) 80 mH
C) 500 mH
D) 240 mH

Question 37

Resonance occurs in an AC series circuit when which of the following conditions is met?

A) capacitive reactance equals zero
B) capacitive reactance equals inductive reactance
C) resistance equals capacitive reactance
D) resistance equals inductive reactance

Question 38

Which of the following combinations of circuit components can be used to make a tuner for a radio, to select the desired frequency?

A) fixed capacitor, variable resistor
B) fixed resistor, variable inductor
C) fixed inductor, variable capacitor
D) fixed inductor, variable resistor

Question 39

An AC circuit consists of a 60-Hz source, a 200- $\Omega$ resistor, and a 1.06-H inductor, all in series. What is the impedance of this circuit?

A) 400 $\Omega$
B) 280 $\Omega$
C) 600 $\Omega$
D) 450 $\Omega$

Question 40

A series LC circuit in a radio is in resonance with AM 1200 kHz. If the radio station is changed to AM 600 kHz, by what factor must the capacitance be multiplied to again achieve resonance?

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

Question 41

The primary winding of an electric train transformer has 200 turns, and the secondary has 50. If the input voltage is 120 V(rms), what is the output voltage?

A) 30 V
B) 15 V
C) 10 V
D) 480 V

Question 42

A resistor, inductor, and capacitor are connected in series, each with an effective (rms) voltage of 65 V, 140 V, and 90 V, respectively. What is the magnitude of the phase angle between the current and applied voltage phasors in this circuit?

A) 38°
B) 28°
C) 22°
D) 43°

Question 43

An ideal transformer is one that:

A) has an output frequency of 60 Hz.
B) has a turn ratio, N₂/N₁, equal to 1.
C) experiences no power loss.
D) works with direct current.

Question 44

A series LCR circuit has a inductive reactance of 2 $\Omega$ and a capacitive reactance of 1/2 $\Omega$ . By what factor should the AC frequency be changed to put this circuit into resonance?

A)
B) 4
C)
D) This circuit cannot be made to achieve resonance.

Question 45

In an LCR series circuit, if the AC frequency is increased to a very large value, what value does the magnitude of the angle between the current and voltage phasors approach?

A) 0°
B) 45°
C) 180°
D) 90°

Question 46

An AC series circuit contains a resistor of 20 $\Omega$ , a capacitor of 0.75 $\mu$ F, and an inductor of 80 mH. What frequency should be used to create a resonance condition in the circuit?

A) 650 Hz
B) 530 Hz
C) 160 Hz
D) 320 Hz

Question 47

An AC power generator produces 100 A (rms) at 3600 V. The voltage is stepped up to 100,000 V by an ideal transformer, and the energy transmitted through a long-distance power line with a total resistance of 100 $\Omega$ . What is the rms current in the long-distance line?

A) 3.6 A
B) 1.0 A
C) 1000 A
D) 1.8 A

Question 48

In an LCR series circuit, if the AC frequency is decreased to a very small value, what value does the magnitude of the angle between the current and voltage phasors approach?

A) 90°
B) 180°
C) 0°
D) 45°

Question 49

A series circuit has equal values for resistance, capacitive reactance, and inductive reactance. The phase angle for this circuit is:

A) 180°.
B) -60°.
C) 60°.
D) not given.

Question 50

In an LCR series circuit with X_L > X_C, the phase angle between the current and voltage phasors is 30°. If the AC frequency doubles, what is the magnitude of the resulting phase angle?

A) 30°
B) less than 30°
C) more than 30°
D) Additional information is needed to reach a determination.

Question 51

A transformer is to be designed to increase the 30 kV (rms) output of a generator to the transmission line voltage of 345 kV (rms). If the primary winding has 60 turns, how many turns must the secondary have?

A) 57
B) 6900
C) 690
D) 11

Question 52

When an LCR series circuit is in resonance, its impedance is:

A) "a maximum."
B) "equal to its resistance."
C) "zero."
D) " $\pi$ 2 $\Omega$ ."

Question 53

A series LCR AC circuit is at resonance. It contains a resistor of 16 $\Omega$ , a capacitor of 0.35 $\mu$ F, and an inductor of 90 mH. If an effective (rms) voltage of 150 V is applied, what is the effective (rms) current when the circuit is in resonance?

A) 9.4 A
B) 3.3 A
C) 5.0 A
D) 16.1 A

Question 54

In an LCR series circuit the capacitive reactance is 24 $\Omega$ . If the AC frequency halves, what will be the resulting capacitive reactance?

A) 24 $\Omega$
B) 48 $\Omega$
C) 12 $\Omega$
D) Additional information is need to find the value.

Question 55

An AC series circuit contains a resistor of 20 $\Omega$ , an inductor of 50 mH, and a variable capacitor. If the frequency of the applied voltage is 500 Hz, to what setting should the capacitor be set if resonance is achieved?

A) 2.4 µF
B) 3.4 µF
C) 2.0 µF
D) 0.8 µF

Question 56

In an AC circuit operating at a frequency where the phase angle with respect to the current is 30°, what happens to the phase angle if the capacitance of the circuit is increased?

A) It increases.
B) It decreases.
C) nothing
D) It decreases and can become zero.

Question 57

In an AC circuit, the ratio of average current to maximum current is:

A) zero.
B) 0.707.
C) 1.0.
D) 0.5.

Question 58

A transformer consists of a 1000-turn primary coil and a 4000-turn secondary coil. If the current in the secondary is 3.00 A, what is the primary current?

A) 6.00 A
B) 0.750 A
C) 48.0 A
D) 12.0 A

Question 59

Find the resonant frequency for a series LCR circuit where R = 20.0 $\Omega$ , C = 10.0 µF, and L = 8.0 mH.

A) 1.59 kHz
B) 563 Hz
C) 5.00 kHz
D) 796 Hz

Question 60

An AC power generator produces 25 A (rms) at 3600 V. The energy is transmitted through a long-distance power line, which has a total resistance of 50 $\Omega$ . What percentage of the power delivered by the generator is dissipated as heat in the long-distance line if a step-up transformer is not used?

A) 69%
B) 35%
C) 0.09%
D) 1.8%

Question 61

An electric motor produces 2.0 hp in a V_rms = 120 V circuit. Using 1 hp = 746 W, how long would it take this motor to do 1.0 kWh of work?

A) 0.67 min
B) 0.67 h
C) 1.3 min
D) 1.3 h

Question 62

A transformer is used to step up the voltage to a power line. If the stepped-up voltage is 10 times the original value, by how much will the power loss in the line decrease?

A) more than 95%
B) between 10% and 20%
C) between 20% and 95%
D) There will be no change since the power into a transformer equals the power out.

Question 63

Transmitting power at high voltage, e.g., 400,000 V, reduces the power loss for transmission lines. But such a voltage would destroy electrical devices in household use. How then can such 400,000 V electricity be used in a normal house?

A) The ground prongs on household plugs reduce the voltage to 120 V.
B) Before reaching households, transformers are used to reduce the voltage appropriately.
C) No matter the incoming voltage, a 120 V device only lets 120 V in.
D) The 400,000 V is just theoretical; long distance transmission lines actually use 120 V.

Question 64

An electric motor produces 2.0 hp in a V_rms = 120 V circuit. What is I_rms for this motor? (1 hp = 746 W)

A) 12 A
B) 6.2 A
C) 0.62 A
D) 1.6 A