Deck 21: AC Circuits

Question 1

What is the average power if the current is I_o sin( $\omega$ t) and the voltage is V_o sin( $\omega$ t) ?

Answer 1

(I_o V_o) /2

Question 2

The average power dissipated in an RLC circuit is I² rms R where R is the Reactance.

Answer 2

False

Question 3

Impedance is the total, or effective, opposition to current that takes into account both resistances and reactances.

Answer 3

True

Question 4

The resonance frequency of an RLC circuit is the frequency at which the circuit dissipates minimum power.

Question 5

The polarity of the voltage in a circuit operating at 60 Hz reverses

A) 90 times per second.
B) 10 $\pi$ times per second.
C) 30 times per second.
D) 60 times per second.
E) 120 times per second.

Question 6

The rms value of a "square" wave is ________ the rms value of a "sine" wave of similar amplitude and frequency. (see Figure 21-1)

A) equal to
B) less than
C) more than

Question 7

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

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

Question 8

If the frequency of the AC voltage across a capacitor is doubled, the capacitive reactance of that capacitor

A) becomes zero.
B) increases to twice its original value.
C) increases to 4 times its original value.
D) decreases to one-fourth its original value.
E) decreases to one-half its original value.

Question 9

As the frequency of the AC voltage across a capacitor approaches zero, the capacitive reactance of that capacitor

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

Question 10

Inductive Reactance is actually a consequence of

A) Coulomb's Law.
B) Newton's Law.
C) Faraday's Law.
D) Gauss's Law.

Question 11

If I have a circuit which has unwanted current variations (with time) (want a more steady current), which one of these should be added in series?

A) inductor
B) resistance
C) battery
D) capacitor

Question 12

If the frequency of the AC voltage across an inductor is doubled, the inductive reactance of that inductor

A) increases to 4 times its original value.
B) decreases to one-half its original value.
C) decreases to one-fourth its original value.
D) increases to twice its original value.

Question 13

The average power delivered by an AC generator in an RLC circuit is ________ the product IV where I & V are respectively the current and voltage amplitudes.

A) less than
B) equal to
C) more than

Question 14

Which of the following doesn't have ohm units?

A) L
B) 11eea40d_cc1e_6c53_97f4_f597a21d7c25_TB9720_11 C
C) (X_cX_c)/R
D) (X_L - X_C)
E) R

Question 15

In a series RLC circuit the total impedance

A) never exceeds R.
B) equals R + X_L + X_C.
C) always exceeds R.
D) never is less than R.

Question 16

Consider an RLC circuit. The impedance of the circuit increases if R increases.

A) always true
B) true only if X_L is less than or equal to X_C
C) never true
D) true only if X_L is greater than or equal to X_C

Question 17

Consider an RLC circuit. The impedance of the circuit increases if X_L increases.

A) true only if X_L is less than or equal to X_C
B) always true
C) true only if X_L is greater than or equal to X_C
D) never true

Question 18

The power factor of an RLC circuit is defined as cos = R/Z, but the phase angle can also be calculated from

A) sin 11eea40e_0adb_84f4_97f4_991b903cac4d_TB9720_11= Z/( X_L - X_C).
B) sin 11eea40e_0adb_84f4_97f4_991b903cac4d_TB9720_11 = (X_L - X_C)/Z.
C) sin 11eea40e_0adb_84f4_97f4_991b903cac4d_TB9720_11 = RZ/( X_L - X_C).
D) sin 11eea40e_0adb_84f4_97f4_991b903cac4d_TB9720_11 = (X_L - X_C)/RZ.

Question 19

Compare the voltage waves from two different AC circuits, as shown in Figure 21-2.

The absolute value of the phase difference is

A) 45°.
B) -90°.
C) 90°.
D) zero.
E) 180°.

Question 20

Resonance in a series RLC circuit occurs when

A) X_L is greater than X_C.
B) X_L equals X_C.
C) (X_L - X_C )² is equal to R².
D) X_C is greater than X_L.

Question 21

If the inductance and the capacitance both double in an LRC series circuit, the resonant frequency of that circuit will

A) decrease to one-half its original value.
B) increase to four times its original value.
C) increase to twice its original value.
D) decrease to one-fourth its original value.
E) decrease to one-eighth its original value.

Question 22

American power plants usually supply 120. volts AC:
(a) at what frequency?
(b) at what maximum voltage?

Question 23

For an rms current of 5.0 amperes determine the maximum and peak-to-peak values.

Question 24

The output voltage of a power supply is V = Vo sin (120. t) volts. At t = 0.010 s the output voltage is 4.2 V. What is the rms voltage of the generator?

Question 25

A 100. Watt light bulb is powered by 120. v AC, 60. Hz household connection. Determine the rms current and the peak current.

Question 26

The peak output of a generator is 20. A and 240. V. What average power is provided by the generator?

Question 27

In an AC circuit the voltage varies as V = 96. sin (277 t) volts.
(a) What is the rms voltage?
(b) What is the frequency?

Question 28

The potential applied to a 20. $\Omega$ resistor is 60. cos (33. t) volts. What is the:
(a) instantaneous current?
(b) rms current?

Question 29

The current through a 50. $\Omega$ resistor is I = 0.80 sin (240. t) amperes.
(a) What is the peak current?
(b) What is the rms current?

Question 30

The current through a 50. $\Omega$ resistor is I = 0.80 sin (240. t) amperes.
(a) At what frequency does the current vary?
(b) How much power is dissipated in the resistor?

Question 31

A 150. W lamp is placed into a 120. V AC outlet.
(a) What are the peak and rms currents?
(b) What is the resistance of the lamp?

Question 32

The potential applied to a 20. $\Omega$ resistor is 60. cos (33. t) volts. What is the:
(a) frequency?
(b) average power?

Question 33

An AC voltage source has an amplitude of 96. V and frequency 400. Hz. The voltage was zero at t = 0 and was increasing.
(a) What is the effective rms voltage?
(b) What is the instantaneous voltage at 1.00 ms?
(c) When will the voltage be 40. V?

Question 34

A series RC circuit has resistance of 150 $\Omega$ and capacitance of 40. $\mu$ F. It is driven by a 120. V, 60. Hz source.
(a) How much power is dissipated in the resistor?
(b) How much power is dissipated in the capacitor?

Question 35

What is the reactance of a 1.0 mH inductor at 60 Hz?

Question 36

The current in a coil increases from 2.0 A to 12. A in 50. ms. Calculate the voltage induced in the coil if its self-inductance is 0.120 H.

Question 37

At what frequency will the Inductive Reactance of 44. mH be equal to the Capacitive Reactance of a 27. pF capacitor?

Question 38

For what frequency range is the reactance of a 3.3 $\mu$ F larger than a 3.3 $\Omega$ resistance where the capacitor, resistance, and a 3.3 mH inductor are in series?

Question 39

A 50. $\Omega$ resistor is placed in series with a 40. mH inductor. At what frequency will the current in such a circuit lag the applied voltage by exactly 45°?

Question 40

What current flows in a 60. mH inductor when 12. V AC at a frequency of 20. kHz is applied to it?

Question 41

A series RL circuit is driven by a 120. V, 60. Hz source. The values of the resistance and inductance are R = 20.0 $\Omega$ and L = 160. mH. Determine:
(a) the current.
(b) the phase angle between the current and the applied voltage.

Question 42

Given 30. $\mu$ H and a 60. $\Omega$ resistor, what range of capacity allows tuning this series circuit to the FM band (88-108 MHz)?

Question 43

What capacitance will have the same reactance as a 100. mH inductance in a 120. V, 60. Hz circuit?

Question 44

A 120. mH inductor is in series with a 20. $\Omega$ resistor and a variable capacitor ( 0.110 $\mu$ F to 0.400 $\mu$ F)
(a) What is the range of resonant frequencies possible?
(b) Connected to a 24. volt rms source, what current flows at resonance?

Question 45

A resistor is connected to normal 60. Hz, 120. volt AC. How many times a second does the instantaneous dissipated POWER reach a maximum?

A) 30
B) 0
C) 240
D) 120
E) 60

Question 46

In an RLC circuit, the resistance is 10. ohms, the capacitive reactance is 30. ohms, and the inductive reactance is 50. ohms. What is the circuit impedance?

A) 63.
B) 22. 11eea40e_be8e_7ac5_97f4_516006ddc11a_TB9720_11
C) 10. 11eea40e_be8e_7ac5_97f4_516006ddc11a_TB9720_11
D) 81. 11eea40e_be8e_7ac5_97f4_516006ddc11a_TB9720_11
E) 90. 11eea40e_be8e_7ac5_97f4_516006ddc11a_TB9720_11

Question 47

Consider a 4500. ohm resistor in series with a 133. mH inductor. What capacitance should be added in series so that the impedance is a minimum at 500. KHz?

A) 0.627 pF
B) 7.26 $\mu$ F
C) 0.762 pF
D) 2.67 $\mu$ F
E) 0.267 pF

Question 48

A series RLC circuit has R = 20. $\mu$ , L = 200. mH, C = 10. $\mu$ F. At what frequency should the circuit be driven in order to have maximum power transferred from the driving source?

A) 0.28 kHz
B) 0.96 kHz
C) 0.17 kHz
D) 0.11 kHz
E) 0.45 kHz

Question 49

A resistance of 55. , a capacitor of capacitive reactance 30. 11eea40e_f9c3_5706_97f4_d114cf755d2c_TB9720_11, and an inductor of inductive reactance 30.11eea40e_f9c3_5706_97f4_d114cf755d2c_TB9720_11 are connected in series to a 110. V, 60. Hz power source. What current flows in this circuit?

A) more than 4. A
B) 2. A
C) more than 2. A but less than 4. A
D) 4. A
E) less than 2. A

Question 50

What size capacitor must be placed in series with a 30.resistor and a 40. mH coil if the resonant frequency of the circuit is to be 1000. Hz?

A) 6.0 $\mu$ F
B) 3.3 $\mu$ F
C) 0.63 $\mu$ F
D) 2.0 $\mu$ F
E) 4.5 $\mu$ F