Alternating Voltage and Current MCQs ( Electrical Engineering ) MCQs – Electrical Engineering MCQs
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Alternating Voltage and Current MCQs ( Electrical Engineering ) MCQs – Electrical Engineering MCQs
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Generation of an Alternating EMF
1. Which, among the following, is the correct expression for alternating emf generated?
a) e=2Blvsin(θ)
b) e=2B2lvsin(θ)
c) e=Blvsin(θ)
d) e=4Blvsin(θ)
Answer: c
Explanation: The correct expression for alternating emf generated is e=Blvsin(θ). Where B stands for magnetic field density, l is the length of each of the parallel sides v is the velocity with which the conductor is moved and θ is the angle between the velocity and the length.
2. What should theta be in order to get maximum emf?
a) 00
b) 900
c) 1800
d) 450
Answer: b
Explanation: The value of θ should be 900 in order to get maximum emf because e = Blvsin(θ) and sin is maximum when θ is 900.
3. Calculate the maximum emf when the velocity is 10m/s, the length is 3m and the magnetic field density is 5T.
a) 150V
b) 100V
c) 300V
d) 0V
Answer: a
Explanation: We know that: emax=Bvl
Substituting the values from the given question, we get e=150V.
4. When a coil is rotated in a magnetic field, the emf induced in it?
a) Is maximum
b) Is minimum
c) Continuously varies
d) Remains constant
Answer: c
Explanation: When a coil is rotated in a magnetic field, cross sectional area varies due to which the number of flux lines crossing it varies, which causes the emf to vary continuously.
5. emf is zero if the angle between velocity and length is _____
a) 00
b) 900
c) 2700
d) 450
Answer: a
Explanation: If the angle between velocity and length is zero, sinθ=0
So, e=Bvlsinθ = 0.
6. In an A.C. generator, increase in number of turns in the coil _________
a) Increases emf
b) Decreases emf
c) Makes the emf zero
d) Maintains the emf at a constant value
Answer: a
Explanation: In an A.C. generator, the emf increases as the number of turns in the coil increases because the emf is directly proportional to the number of turns.
7. The number of cycles that occur in one second is termed as ___________
a) Waveform
b) Frequency
c) Amplitude
d) Period
Answer: b
Explanation: The number of cycles that occur in one second is known as the frequency. It is the reciprocal of the time period.
Waveform Terms and Definitions
1. The variation of a quantity such as voltage or current shown on a graph is known as ___________
a) Waveform
b) Peak value
c) Instantaneous value
d) Period
Answer: a
Explanation: The variation of a quantity, which is voltage or current in this case, shown on a graph with the x-axis as time is known as a waveform.
2. What is the duration of one cycle known as _________
a) Waveform
b) Peak value
c) Instantaneous value
d) Period
Answer: d
Explanation: The duration of one cycle is known as a period. A function which repeats the same waveform at equal intervals of time is known as a periodic function.
3. The repetition of a variable quantity, recurring at equal intervals, is known as ___________
a) Waveform
b) Instantaneous value
c) Cycle
d) Period
Answer: c
Explanation: Each repetition of a variable quantity, recurring at equal intervals, is termed as a cycle.
4. The value of a given waveform at any instant time is termed as ___________
a) Waveform
b) Instantaneous value
c) Cycle
d) Period
Answer: b
Explanation: Instantaneous value is the value of the waveform at that instant. Hence the value of a given waveform at any instant time is termed as instantaneous value.
5. The maximum instantaneous value measured from zero value is known as?
a) Peak value
b) Peak to peak value
c) Cycle
d) Period
Answer: a
Explanation: The maximum instantaneous value measured from the zero value is termed as the peak value.
Inductance In A DC Circuit MCQs
6. The maximum variation between the maximum positive and the maximum negative value is known as?
a) Peak value
b) Peak to peak value
c) Cycle
d) Period
Answer: b
Explanation: The maximum variation between the maximum positive instantaneous value and the maximum negative instantaneous value is the peak-to-peak value.
7. What is the correct relation between the peak value and peak to peak value for a sinusoidal waveform?
a) Vp=4Vp-p
b) Vp=Vp-p
c) Vp-p=2Vp
d) Vp=2Vp-p
Answer: c
Explanation: The maximum variation between the maximum positive instantaneous value and the maximum negative instantaneous value is the peak-to-peak value. For a sinusoidal waveform, it is twice the peak value. Hence Vp-p=2Vp.
8. If the peak to peak voltage is 10V, calculate the peak voltage.
a) 10V
b) 2V
c) 4V
d) 5V
Answer: d
Explanation: Vp-p=2Vp
Substituting the values from the question, we get Vp=5V.
9. If the peak voltage is 9V, calculate the peak to peak voltage.
a) 9V
b) 20V
c) 18V
d) 12V
Answer: c
Explanation: Vp-p=2Vp
Substituting the values from the question, we get Vp-p= 18V.
Relationship between Frequency, Speed and Number of Pole Pairs
1. The waveform of the emf generated undergoes one complete cycle when?
a) Conductors move past north pole
b) Conductors move past south pole
c) Conductors move past north and south poles
d) Conductors are stationary
Answer: c
Explanation: The waveform of the e.m.f. generated in an a.c. generator undergoes one complete cycle of variation when the conductors move past an N and an S pole.
2. When is the shape of the negative half of the emf waveform equal to the positive half?
a) When the conductors move past north pole
b) When conductors move past south pole
c) When conductors move past both north and south pole
d) When conductors are stationary
Answer: c
Explanation: The waveform of the e.m.f. generated in an a.c. generator undergoes one complete cycle of variation when the conductors move past an N and an S pole and the shape of the wave over the negative half is exactly the same as that over the positive half.
3. Which is the correct formula for frequency in an ac generator?
a) f=p*n
b) f=p/n
c) f=n/p
d) f=n2p
Answer: a
Explanation: The frequency in an ac generator is p*n, where p is pairs of poles and speed is n revolutions per second.
4. What will happen to the frequency if the number of revolutions increases?
a) Increases
b) Decreases
c) Remains the same
d) Becomes zero
Answer: a
Explanation: We know that:
f=p*n, therefore, as n increases, f also increases.
Hence frequency increases if number of revolutions increases.
5. What happens to the frequency if the number of pairs of poles increases?
a) Increases
b) Decreases
c) Remains the same
d) Becomes zero
Answer: a
Explanation: We know that:
f=p*n, therefore, as p increases, f also increases.
Hence frequency increases if number of pair of poles increases.
6. Calculate the frequency if the number of revolutions is 300 and the paired poles are 50.
a) 15kHz
b) 150kHz
c) 1500kHz
d) 150Hz
Answer: a
Explanation: We know that f=p*n
f=50*300=15000 Hz = 15kHz.
7. Calculate the number of revolutions if the frequency is 15kHz and the paired poles are 50.
a) 100
b) 200
c) 300
d) 400
Answer: c
Explanation: We know that f=p*n
f=15kHz=15000Hz, p=50
15000=50*n => n=15000/50=300.
8. Calculate the number of paired poles if the frequency id 15kHz and the number of revolutions is 300.
a) 10
b) 30
c) 50
d) 70
Answer: c
Explanation: We know that f=p*n
f=15kHz=15000Hz, n=300
15000=p*300 => p=15000/300=50.
9. What is the frequency of a two pole machine having n=50?
a) 100Hz
b) 200Hz
c) 50Hz
d) 25Hz
Answer: c
Explanation: For a two pole machine, p=1.
f=pn = 1*50 = 50Hz.
10. What is the minimum number of poles that a machine must have __________
a) 1
b) 2
c) 4
d) 10
Answer: b
Explanation: The minimum number of poles that a machine must have is 2 because a machine must have at least one pair of poles = 2 poles.