AC circuits

Question 1

In a parallel ac circuit with XL and
XC,

Answer 1

IL and IC are 180 out of phase.

Question 2

What is the term for the out-of-phase
power associated with inductors and
capacitors?

Answer 2

Reactive power.

Question 3

In an AC circuit, if the voltage and
current are in phase, the circuit is

Answer 3

resistive

Question 4

For a capacitor in a sine-wave ac
circuit,

Answer 4

Vc lags ic by 90
ic leads Vc by 90
ic and Vc have the samefrequency.

Question 5

Calculate the r.m.s. value of a
sinusoidal current of maximum value
20A.

Answer 5

14.14 A
Solution:
rms value 20A(0.707)
rms value 14.14 A Ans.

Question 6

An alternating voltage is given by
v=75 sin (200πt −0.25) volts. Find
the phase angle (in degrees and
minutes)

Answer 6

14.32 (lagging)

Question 7

In a parallel RL circuit,

Answer 7

IL lags IR by 90.

Question 7

Calculate the current taken by a 10
F capacitor when connected to a
200 V, 100 Hz supply.

Answer 7

. 1.257 A

Question 8

What is the power factor (PF) of a
purely resistive ac circuit?

Answer 8

1

Question 9

A resistive and a capacitive load of
equal magnitude is connected in
series, determine the phase difference
between the voltage and the current.

Answer 9

current leads the voltage by 45°

Question 9

For an inductor in a sine-wave ac
circuit,

Answer 9

VL leads iL by 90.

Question 10

What will happen when the power
factor of a circuit is increased?

Answer 10

Active power increases

Question 11

Calculate the capacitive reactance of a
capacitor of 20 μF when connected to
an a.c. circuit of frequency 20 Hz

Answer 11

397.9 ohms

Question 11

The impedance in the study of
electronics is represented by resistance and ________.

Answer 11

reactance

Question 11

In a series RLC circuit,

Answer 11

XL and XC are 180 out of phase.

Question 11

A resistive and a capacitive load of
equal magnitude is connected in
series, determine the phase difference
between the voltage and the current.

Answer 11

current leads the voltage by 45°

Question 12

In a series RC circuit,

Answer 12

Vc lags Vr by 90

Question 12

An e.m.f. of 200 V at a frequency of
2 kHz is applied to a coil of pure
inductance 50 mH. Determine the
reactance of the coil and the current
flowing in the coil.

Answer 12

628 Ω; 0.318 A

Question 13

If a circuit has an admittance of Y =
0.2 + j0.6, the circuit is

Answer 13

capacitive

Question 14

What is the significance of
connecting loads in parallel?

Answer 14

It allows independent
operations of loads

Question 14

What is the term used to identify an
AC voltage that would cause the
same heating in a resistor as a
corresponding value of DC voltage?

Answer 14

Root mean square (RMS).

Question 15

When the frequency of the applied
voltage increases in a series RC
circuit,

Answer 15

ZT decreases.

Question 16

In an ac circuit with only series
capacitors,

Answer 16

V 97.39sin 300 t 0.620 V

Question 16

The voltages across three
components in a series circuit when
connected across an a.c. supply are:

Answer 16

Apparent power.

Question 16

The product of the readings of an AC voltmeter and AC ammeter is called:

Answer 16

150 – j100

Question 16

A resistor of 150 Ω, a coil with reactance 100 Ω and a capacitor with reactance - 200 Ω are connected in series. What is the complex impedance R + jX?

Answer 16

VT lags IT by 90.

Question 16

A value of –j500 represents

Answer 16

500  of capacitive reactance

Question 17

.Inductive reactance, XL,

Answer 17

applies only to sine waves.

Question 18

Susceptance, B, is the

Answer 18

reciprocal of reactance.

Question 19

When the frequency of the applied voltage decreases in a parallel RL circuit,

Answer 19

18 Ohms

Question 19

What is the total impedance of a series AC circuit having a resistance of 6 ohms, an inductive reactance of 17 ohms, and zero capacitive reactance?

Answer 19

. ZEQ decreases.

Question 19

An impedance draws a current i = 10 cos (ωt – 30degree) from a voltage, v = 220 sin (ωt + 30degree). What is the impedance?

Answer 19

19.1 – j11.1

Question 19

In a parallel RC circuit

Answer 19

IC leads IR by 90.

Question 20

If a circuit has an admittance of Y = 0.5- j0.9, the circuit is

Answer 20

inductive

Question 20

A coil, resistor and capacitor are in parallel. The resistance is 1 Ω; the capacitive susceptance is 1.0 siemens; the inductive susceptance is - 1.0 siemens. Then the frequency is cut to half its former value. What will be the admittance vector, G + jB, at the new frequency?

Answer 20

1 – j1.5

Question 21

A branch current of +j250 mA represents

Answer 21

250 mA of capacitive current.

Question 21

What happens to reactive power in a circuit that has both inductors and capacitors?

Answer 21

15.92 µF; 0.25 A

Question 22

A capacitor has a capacitive reactance of 400  when connected to a 100 V, 25 Hz supply. Determine its capacitance and the current taken from the supply.

Answer 22

It alternates between magnetic and electric fields and is not dissipated.

Question 23

How do you compute true power (power dissipated in the circuit) in a circuit where AC voltage and current are out of phase?

Answer 23

Multiply apparent power times the power factor.

Question 24

In a parallel RC circuit,

Answer 24

VA and IR are in phase.

Question 25

An alternating voltage given by v = 100 sin 240t volts is applied across a coil of resistance 32  and inductance 100 mH. Determine the circuit impedance.

Answer 25

40 ohms

Question 26

What is a sine wave?

Answer 26

A wave whose amplitude at any given instant can be represented by the projection of a point on a wheel rotating at a uniform speed.

Question 27

A 120 mH inductor has a 50 mA, 1 kHz alternating current flowing through it. Find the p.d. across the inductor

Answer 27

37.7 V

Question 28

What type of wave is characterized by a rise time significantly faster than the fall time (or vice versa)?

Answer 28

Sawtooth wave.

Question 29

What type of wave is made up of sine waves of the fundamental frequency and all the odd harmonics?

Answer 29

Square.

Question 29

When the frequency of the applied voltage increases in a parallel RC circuit,

Answer 29

the phase angel, θ, increases. ZEQ increases. ZEQ decreases.

Question 29

How many degrees are there in one complete sine wave cycle?

Answer 29

360 degrees.

Question 29

In a series RC circuit,

Answer 29

VR and I are in phase.

Question 30

What is the description of a square wave?

Answer 30

A wave that abruptly changes back and forth that abruptly changes back and forth between two voltage levels and stays at these levels for equal amounts of time.

Question 31

What type of wave is made up of sine waves at the fundamental frequency and all the harmonics?

Answer 31

Sawtooth wave.