St. John's University
ELECTRICAL 123
1. A 50 Hz alternating voltage has amplitude of 100 V. Find the instantaneous value of
the wave 1 ms after the wave passes through zero in a positive direction.
a. 30.9 V
b. 50.5 V
c. 40.2 V
d. 70.7 V
ANS. A
2. An alternating current is given by i = 10 sin314t. with reference to time t = 0, how
long will it take to
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1. A 50 Hz alternating voltage has amplitude of 100 V. Find the instantaneous value of
the wave 1 ms after the wave passes through zero in a positive direction.
a. 30.9 V
b. 50.5 V
c. 40.2 V
d. 70.7 V
ANS. A
2. An alternating current is given by i = 10 sin314t. with reference to time t = 0, how
long will it take to reach + 10A for the second time?
a. 0.01 s
b. 0.014 s
c. 0.025 s
d. 0.035 s
3. The voltage v = 90 cos ( T – 161.5°) maybe expresses as sine function by ______.
a. 90 sin ( t + 18.5°)
b. 90 sin ( t + 71.5°)
c. 90 sin ( t – 71.5°)
d. 90 sin ( t – 18.5°)
ANS. C
4. A sinusoidal current has a maximum value of 20 amperes. Find its instantaneous
value at 135°
a. 12.12 A
b. 14.14 A
c. 10.65 A
d. 15.73 A
ANS. B
5. A 25 – cps sinusoidal emf wave has a maximum value of 340 V. Determine the
instantaneous value of the emf, 0.01 sec after the wave passes through zero in a
negative direction.
a. – 320 V
b. – 340 V
c. – 330 V
d. – 325 V
ANS. B
6. A sinusoidal voltage of 50 Hz has a maximum value of 282.8 V. At what time
measured from a positive maximum value will the instantaneous voltage be V?
a. 1.67 ms
b. 2.45 ms
c. 3.33 ms
d. 2.08 ms
ANS. A
7. A 100V peak ac is as effective as ____ dc.
a. 50 V
b. 70.7 V
c. 41.2 V
d. 63.6 V
ANS. C
8. Three loads, units A, B, and C are connected in parallel and take currents that are
12, 10 and 15A respectively. Assuming Ia by 65°, calculate average value of the
resulting current.
a. 25.5
b. 40.1
c. 30.3
d. 45.2
ANS. B
9. Determine the average value of a current if the instantaneous value at 30° is 35 A.
a. 44.52 A
b. 40.41 A
c. 70 A
d. 49.6 A
ANS. C
10. What is the average voltage output of a half wave rectifier with an output of 100
volts peak?
a. 56.7 V
b. 75 V
c. 63.7 V
d. 50 V
ANS. C
11. The average value of a sinusoidal current is 100 A. Its rms value is ____.
a. 63.7 A
b. 1.1 A
c. 70.7 A
d. 111 A
ANS. A
12. Find the form factor for a half wave rectified alternating current.
a. 1.38
b. 1.414
c. 1.57
d. 1.11
ANS. C
13. A 10 A current is lagging the voltage by 47° and a 12 A current is lagging the voltage
by 58°. What is the total current in this situation?
a. 20.28 A
b. 21.89 A
c. 22.65 A
d. 23.72 A
ANS. B
14. The equations of the current in two parallel connected devices are i1 = 11.3 sin (377t
+ π/4) and i2 = 8.5 sin (377t – π/4). What is the rms value of the resultant current?
a. 10 A
b. 14 A
c. 19.8 A
d. 14.14 A
ANS. A
15. A non-inductive resistance is in series with a condenser takes 7 A and absorbs 300
W when connected across a 110 – V, 25 Hz mains. What non-conductive resistance
must be connected in series with the circuit in order that it may take the same
current from a 110 – V, 60 Hz supply?
a. 8.38
b. 6.12
c. 6.87
d. 7.72
ANS. B
16. A circuit has a resistance of R ohms in series with an inductance of L henries. With a
supply of 240-V, 50 Hz, the power of the circuit is 300 W and the voltage across R is
100 V. Find the value of L.
a. 0.302 H
b. 0.231 H
c. 0.262 H
d. 0.142 H
ANS. B
17. An impedance coil is connected in series with a 6 – ohm resistance. The combination
is connected across a 120V, 60 Hz source. If the voltage across the coil is 100 V while
that across the resistance is 30V, determine the inductance of the coil. AC/DC EE
REVIEW CENTER
a. 0.043 H
b. 0.022 H
c. 0.075 H
d. 0.068 H
ANS. D
18. A series RC circuit is supplied with 150V, 50 Hz supply. The current and power
drawn are 2.5 A and 100 W respectively. Determine the capacitance of the capacitor.
a. 58
b. 67
c. 55
d. 62
ANS. C
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