11. In an RLC series phasor, we start drawing the phasor from which quantity?
A. Voltage
B. Resistance
C. Impedance
D. Current
Answer: D
In an RLC series phasor diagram, we start drawing the phasor from the quantity which is common to all three components, that is the current.
12. What is the correct expression for the phase angle in an RLC series circuit?
A. φ = tan-1(XL-XC)/R
B. φ = tan-1 (XL+XC)/R
C. φ = tan(XL-XC)/R
D. φ = tan-1 (XL-XC)
Answer: A
From the impedance triangle, we get
tanφ = (XL-XC)/R.
Hence φ = tan-1 (XL-XC)/R.
13. When is tanφ positive?
A. When inductive reactance is less than the capacitive reactance
B. When inductive reactance is greater than the capacitive reactance
C. When inductive reactance is equal to capacitive reactance
D. When inductive reactance is zero
Answer: B
tanφ is positive when inductive reactance is greater than capacitive reactance because current will lag the voltage.
14. When is tanφ negative?
A. When inductive reactance is less than the capacitive reactance
B. When inductive reactance is greater than the capacitive reactance
C. When inductive reactance is equal to capacitive reactance
D. When inductive reactance is zero
Answer: A
tanφ is negative when inductive reactance is less than capacitive reactance because current will lead to the voltage.
15. Which of the following is not an ac waveform?
A. sinusoidal
B. square
C. constant
D. triangular
Answer: C
AC waveforms are those which periodically change polarity with time. Sinusoidal waves, square waves, and triangular wave change their polarity at regular intervals so they are ac waveforms. A constant wave doesn’t change its polarity so it is not an ac waveform.
16. What is not a frequency for ac current?
A. 50 Hz
B. 55 Hz
C. 0Hz
D. 60 Hz
Answer: C
DC current is a type of constant current so it has a frequency of zero hertz. So, AC current can have rest other frequencies other than zero.
17. Which type of ac waveform is shown in the given figure?
A. sinusoidal
B. triangular
C. square
D. complex waveform
Answer: A
The figure depicts ac waveform of sinusoidal nature changing its polarity after regular intervals sinusoidally.
18. Instantaneous voltage is the product of resistance and _____________ current in a resistive circuit.
A. Instantaneous
B. Average
C. RMS
D. Peak
Answer: A
According to Ohm’s Law
V = IR
So, V(t) = i(t)R
Instantaneous voltage is the product of resistance and instantaneous current in a resistive circuit.
19. Find the value of the instantaneous voltage if the resistance is 2 ohm and the instantaneous current in the circuit is 5A.
A. 5V
B. 2V
C. 10V
D. 2.5V
Answer: C
We know that,
v = iR
substituting the given values from the question,
V = 2 × 5
V = 10V.
20. The power for a purely resistive circuit is zero when?
A. Current is zero
B. Voltage is zero
C. Both current and voltage are zero
D. Either current or voltage is zero
Answer: D
Power in AC Circuit is given by
P = VIcosϕ
Power in a circuit is the product of voltage, current, and the cosine of the phase angle.
The phase angle is 00 for a purely resistive circuit so, P = VI.
Hence if either voltage or current is zero, the power is zero.
