Review Exam: ELEC02 :AC

Question 1

What is the main reason that alternating current replaced direct current for general use?

Answer 1

AC voltage may be conveniently transformed.

Question 2

The rmsvalue of an ac current is:

Answer 2

That steady current that produces the same rate of heating in a resistor as the actual current.

Question 3

An inductor and a resistor are in series with a sinusoidal voltage source. The frequency is set so that the inductive reactance is equal to the resistance. If the frequency is increased, then:

Answer 3

V_L>V_R

Question 4

To decrease the phase angle below 45°, the following condition must exist:

Answer 4

R>Xc

Question 5

In residential house wiring the hot wire is usually color­ coded

Answer 5

black or red

Question 6

Capacitive reactance, Xc:

Answer 6

Applies only to sine waves.

Question 7

In a sinusoidally driven series RLC circuit, the inductive reactance is X_L = 200 ohm, the capacitive reactance is Xc= 100 ohm, and the resistance is R = 50 ohm. The current and applied emf would be in phase if:

Answer 7

The capacitance is halved, with no other changes.

Question 8

In an RLC series circuit, which is connected to a source of emf E_m cos(wt), the current lags the voltage by 45° if:

Answer 8

R= wL-1/wC

w=omega

Question 9

For a sine wave, one-half cycle is often called a/an ______ .

Answer 9

alternation

Question 10

When the frequency is increased, the impedance of a parallel RL circuit ______.

Answer 10

increases

Question 11

Determine the power factor angle in the series circuit which consists of R = 25 ohms, L = 0.2 H across a power supply of 200 volts, 30 Hz?

Answer 11

56.4°

Question 12

A series circuit composed of a 0.2 H inductor and a 74-µF capacitor is connected to a 60 V variable frequency source. At what frequency will the current be 4 amperes with a lagging power factor?

Answer 12

48 Hz

Question 13

A load of 20 + j35 ohms is connected across a 220 volts source. Determine the power factor and the VARS.

Answer 13

49.6%, 1042 VARS

Question 14

A 5O-microfarad is connected in series with a coil having 50 ohms resistance and 150 mH inductance. The source voltage is 100 sin (wt- 30°) V. What is the maximum power?

Answer 14

199W

Question 15

A series RC circuits is connected to an emf source having angular frequency w. The current:

Answer 15

leads the source emf by an tan^-1 (1/wCR)

Question 16

When the frequency of the voltage applied to a series RC is increased, the impedance _______

Answer 16

decreases

Question 17

What is the unit of the power factor?

Answer 17

None of the choices

Question 18

In a purely capacitive circuit, the current:

Answer 18

Leads the voltage by one­ fourth of a cycle.

ELI

Question 19

A coil ol 25.0 µHand capacitor of 100 pF are connected in series. The frequency is 5.00 MHz. What is the impedance vector, R + jX?

Answer 19

0 + j467

Question 20

A series circuit composed of 100-ohm resistor and a 20- microfarad capacitor connected across a 240 V, 60 Hz line. Which of the following answers is WRONG?

Answer 20

The resulting current is 0.732 ampere.

Question 21

When the frequency of the voltage applied to a series RC circuit is decreased, the phase angle

Answer 21

increases

Question 22

A capacitance is connected to a 115 V, 25 Hz mains and takes 5 A. What current will it take when the capacitance

Answer 22

20A

Question 23

In a sinusoidally driven series RLC circuit the current lags the applied emf. The rate at which energy is dissipated in the resistor can be increased by:

Answer 23

Decreasing the capacitance and making no other changes.

Question 24

In a purely inductive circuit, the current lags the voltage by:

Answer 24

One-fourth of a cycle.

Question 25

Determine the phase angle in the series circuit which consistsof R = 25 ohms, L = 0.2 H across a power supply of
200 volts, 30 Hz?

Answer 25

56.4°