AC CIRCUITS EXCEL SOURCE Flashcards
PROBLEMS AND OBJECTIVE TYPE QUESTIONS
- An incandescent lamp load generally
considered to be made up of resistors
take 4.8 kW from a 120 V AC
source. What is the instantaneous
maximum value of power?
A. 4800 W C. 2400 W
B. 480 W D. 9600W
- D. 9600 W
The p.f. of an RC circuit is ___.
A. often zero
B. between zero and 1
C. always unity
D. between zero and -1
- B. between zero and 1
- A 25-ohm resistor connected in series
with a coil of 50 ohms resistance and
150 mH inductance. What is the
power factor of the circuit?
A. 85% C. 80%
B. 90% D. 75%
- C. 80%
- The length of time between a point in
one cycle and the same point in the
next cycle of an ac wave is the:
A. Frequency C. Magnitude
B. Period D. Polarity
- B. Period
- Which of the following can vary with
ac, but not with dc?
A. Power C. Voltage
B. Frequency D. Magnitude
- B. Frequency
- Three loads, units A, B, and C are
connected in parallel and take
currents that are respectively 12, 10,
and 15 A respectively. Assuming Ia
to be the reference phasor. Ib leads by
30o
and Ic lags behind Ia by 65 degrees
,
calculate the total (resultant) current.
A. 28.33 A C. 30.21 A
B. 26.46 A D. 32.10 A
- A. 28.33 A
- If R increases in an RC circuit, but
the capacitance and the frequency are
nonzero and constant, then the vector
in the RC plane will:
A. Get longer and rotate clockwise
B. Get longer and rotate
counterclockwise
C. Get shorter and rotate clockwise
D. Get shorter and rotate
counterclockwise
- B. Get longer and rotate
counterclockwise
- A 50 and 100-microfarad capacitors
are connected in series and across a
100 sin (ωt + 30o
) voltage. Write the
equation of the current.
A. 1.26 sin (ωt + 120o)
B. 1.26 sin (ωt + 90o)
C. 5.65 sin (ωt + 120o)
D. 5.56 sin (ωt + 90o)
- A. 1.26 sin (ωt + 120o
)
- On a spectrum analyzer, a pure ac
signal, having just one frequency
component, would look like:
A. A single pip
B. A perfect sine wave
C. A square wave
D. A sawtooth wave
- A. A single pip
10.If admittance Y = 0.06 – j0.08 mho,
then conductance G equals,
A. – 0.06 C. 0.06
B. 0.08 D. – 0.08
- C. 0.06
- Three-phase ac:
A. Has waves that add up to three
times the originals
B. Has three waves, all of the same
magnitude
C. Is what you get at a common wall
outlet
D. Is of interest only to physicists
- B. Has three waves, all of the same
magnitude
- A parallel circuit consists of a
resistor having a conductance of 4
mhos, an inductive reactor having a
susceptance of 8 mhos and a
capacitive reactor having a
susceptance of 5 mhos. What is the
impedance of the circuit?
A. 0.11 + j0.13
B. 0.13 + j0.11
C. 0.12 + j0.16
D. 0.16 + j0.12
- D. 0.16 + j0.12
- The period of an ac wave is:
A. The same as the frequency
B. Not related to the frequency
C. Equal to 1 divided by the
frequency
D. Equal to the amplitude divided by
the frequency
- C. Equal to 1 divided by the
frequency
- An industrial coil has a resistance of
32 ohms and a reactance of 24 ohms
and rated 440 volts at 60 Hz. A
factory will connect the coil to a 440
V, 50 Hz supply. Solve for the value
of a series resistor needed to avoid
overcurrent condition.
A. 2.07 Ω C. 2.64 Ω
B. 2.44 Ω D. 2.25 Ω
- C. 2.64 Ω
- A triangular wave:
A. Has a fast rise time and a slow
decay time
B. Has a slow rise time and a fast
decay time
C. Has equal rise and decay rates
D. Rises and falls abruptly
- C. Has equal rise and decay rates
- A non-inductive resistor R is
connected in parallel with an
inductive reactance of 10 ohms. The
combination is then connected in
series with a capacitive reactance of
5 ohms. The whole combination is
connected across a 100-V, 60 Hz AC
source. If R is equal to 5 ohms, solve
for the voltage across the parallel
combination.
A. 87.53 V C. 88.34 V
B. 89.44 V D. 91.87 V
- B. 89.44 V
17.If two waves have the same
frequency and the same amplitude,
but opposite phase, the composite
wave is:
A. Twice the amplitude of either
wave alone
B. Half the amplitude of either wave
alone
C. A complex waveform, but with
the same frequency as the
originals
D. Zero
- D. Zero
18.If R increases in an RC circuit, but
the capacitance and the frequency are nonzero and constant, then the vector
in the RC plane will:
A. Get longer and rotate clockwise
B. Get longer and rotate
counterclockwise
C. Get shorter and rotate clockwise
D. Get shorter and rotate
counterclockwise
- B. Get longer and rotate
counterclockwise
- An advantage of ac over dc in utility
applications is:
A. Ac is easier to transform from one
voltage to another
B. Ac is transmitted with lower loss
in wires
C. Ac can be easily gotten from dc
generators
D. Ac can be generated with less
dangerous by-products
- A. Ac is easier to transform from
one voltage to another
- A 240 V, 25 Hz sinusoidal generator
is connected to a 20 ohms resistor.
Determine the instantaneous current
when elapsed time is 0.01 second.
A. 15.43 A C. 16.30 A
B. 16.97 A D. 12.00 A
- B. 16.97 A
- As the frequency of a wave gets
lower, all other things being equal,
the value of XC for a capacitor:
A. Increases negatively
B. Decreases negatively
C. Does not change
D. Depends on the current
- A. Increases negatively
- Each impedance R + jXL:
A. Corresponds to a unique point in
the RL plane
B. Corresponds to a unique
inductance reactance
C. Corresponds to a unique
resistance
D. All of the above
- D. All of the above
- An impedance draws a current i = 10
cos (ωt – 30o) from a voltage v = sin
ωt. What is the maximum power?
A. 2200 W C. 1100 W
B. 190.5 W D. 1320 W
- C. 1100 W
- A resistor of 6 ohms and an unknown
impedance coil in series draws 12 A
from a 120-V, 60-Hz line. If the real
power taken from the line is 1152
watts, what is the coil inductance?
A. 15.9 mH C. 10.0 mH
B. 20.0 mH D. 1.59 mH
- A. 15.9 mH