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Multiple choice question for engineering

Set 1

1. Find the total voltage applied in a series RLC circuit when i=3mA, VL=30V, VC=18V and R=1000 ohms.
a) 3.95V
b) 51V
c) 32.67V
d) 6.67V

View Answer

Answer: b [Reason:] Total voltage= VR+VL+VC. VR=1000*3*10-3=3V. Therefore, total voltage= 30+18+3=51V.

2. In an RLC circuit, which of the following is always used as a vector reference?
a) Voltage
b) Resistance
c) Impedance
d) Current

View Answer

Answer: a [Reason:] In an RLC circuit, the voltage is always used as a reference and according to the phase of the voltage, the phase of the other parameters is decided.

3. In an RLC circuit, the power factor is always ____________
a) Positive
b) Negative
c) Depends on the circuit
d) Zero

View Answer

Answer: c [Reason:] In an RLC series circuit, the power factor depends on the number of resistors and inductors in the circuit, hence it depends on the circuit.

4. In an RLC series phasor, we start drawing the phasor from which quantity?
a) Voltage
b) Resistance
c) Impedance
d) Current

View Answer

Answer: d [Reason:] 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.

5. 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)

View Answer

Answer: a [Reason:] from the impedance triangle we get tanφ=(XL-XC)/R. Hence φ=tan-1 (XL-XC)/R.

6. When is tanφ positive?
a) When inductive reactance is less than capacitive reactance
b) When inductive reactance is greater than capacitive reactance
c) When inductive reactance is equal to capacitive reactance
d) When inductive reactance is zero

View Answer

Answer: a [Reason:] tanφ is positive when inductive reactance is greater than capacitive reactance because current will lag the voltage.

7. When is tanφ negative?
a) When inductive reactance is less than capacitive reactance
b) When inductive reactance is greater than capacitive reactance
c) When inductive reactance is equal to capacitive reactance
d) When inductive reactance is zero

View Answer

Answer: c [Reason:] tanφ is positive when inductive reactance is less than capacitive reactance because current will lead the voltage.

8. When is current in phase with the voltage?
a) When XL>XC
b) When XL<XC
c) When XL=XC
d) When XC=infinity

View Answer

Answer: c [Reason:] The current is in phase with the voltage when the capacitive reactance is in phase with the inductive reactance.

9. What is resonance condition?
a) When XL>XC
b) When XL<XC
c) When XL=XC
d) When XC=infinity

View Answer

Answer: c [Reason:] The current is in phase with the voltage when the capacitive reactance is in phase with the inductive reactance. This is known as resonance condition.

10. What is the frequency in resonance condition?
a) Minimum
b) Maximum
c) Cannot be determined
d) Zero

View Answer

Answer: b [Reason:] At resonance condition, the frequency is maximum since the inductive reactance is equal to the capacitive reactance and the voltage and current are in phase.

Set 2

1. What is the time constant of an inductive circuit?
a) LR
b) R/L
c) 1/LR
d) L/R

View Answer

Answer: d [Reason:] The time constant in an inductive circuit is the time taken for the voltage across the inductor to become 63 percent of its initial value. It is given by: Time constant= L/R.

2. Among the following, which is the right formula for decay in an inductive circuit?
a) i=I(1-e-t/time constant)
b) i=I(1-et /time constant)
c) i=(1-e-t /time constant)
d) i=I(e-t /time constant)

View Answer

Answer: d [Reason:] The correct formula for decay in an inductive circuit is i=I(e-t /time constant). As the time increases, the current in the inductor decreases, the voltage also increases.

3. The discharging time constant of a circuit consisting of an inductor is the time taken for the voltage in the inductor to become __________% of the initial voltage.
a) 33
b) 63
c) 37
d) 36

View Answer

Answer: d [Reason:] We know that: V=V0(e-t/time constant). When time constant=t, we have: V=V0(e-1)= 0.36*V0. Hence the time constant is the time taken for the charge in an inductive circuit to become 0.36 times its initial charge.

4. A coil has a resistance of 4 ohm and an inductance of 2H. Calculate its time constant.
a) 1s
b) 2s
c) 0.5s
d) 5s

View Answer

Answer: c [Reason:] The expression for time constant in an inductive circuit is: Time constant= L/R Substituting the values from the question given, we get time constant= 0.5s.

5. In case of Inductive circuit, Frequency is ______________ to the current.
a) Directly proportional
b) Inversely proportional
c) Unrelated
d) Much greater than

View Answer

Answer: b [Reason:] Inductance is inversely proportional to current since, as the inductance increases, current decreases.

6. Calculate the time constant of an inductive circuit having resistance 5 ohm and inductance 10H.
a) 2s
b) 4s
c) 5s
d)10s

View Answer

Answer: a [Reason:] We know that: Time constant= L/R Substituting the values from the given question, we get time constant= 2s.

7. Calculate the inductance in an inductive circuit whose time constant is 2 and the resistance is 5 ohm.
a) 10H
b) 20H
c) 5H
d) 15H

View Answer

Answer: a [Reason:] We know that: Time constant= L/R Substituting the values from the given question, we get L=10H.

8. A coil has a resistance of 4 ohm and an inductance of 2H. It is connected to a 20V dc supply. Calculate the final value of current in the circuit.
a) 5A
b) 10A
c) 15A
d) 20A

View Answer

Answer: a [Reason:] The final value of current in the circuit is: I=V/R= 5A.

9. What happens to the inductance when the current in the coil becomes double its original value?
a) Becomes half
b) Becomes four times
c) Becomes infinity
d) Becomes double

View Answer

Answer: d [Reason:] The formula for magnetic field strength in a coil is: H=iN/l The inductance is: directly proportional to magnetic field strength, hence as the current value doubles, the inductance also doubles.

10. What is the total applied voltage in an inductive circuit?
a) V=Ri+Ldi/dt
b) V=Ri+di/dt
c) V=i+Ldi/dt
d) V=R+Ldi/dt

View Answer

Answer: a [Reason:] The total voltage in an inductive circuit is the sum of the voltage due to the resistor which is Ri and the voltage due to the inductor which is Ldi/dt. Hence V=Ri+Ldi/dt.

Set 3

1. What is the total applied voltage in an inductive circuit?
a) V=Ri+Ldi/dt
b) V=Ri+di/dt
c) V=i+Ldi/dt
d) V=R+Ldi/dt

View Answer

Answer: a [Reason:] The total voltage in an inductive circuit is the sum of the voltage due to the resistor which is Ri and the voltage due to the inductor which is Ldi/dt. Hence V=Ri+Ldi/dt.

2. What is Helmholtz equation?
a) i=I(1-eRt/L)
b) i=I(1-e-Rt/L)
c) i=I(1+e-Rt/L)
d) i=I(e-Rt/L)

View Answer

Answer: b [Reason:] Helmholtz equation is an equation which gives the formula for the growth in an inductive circuit. Hence the Helmholtz formula is: i=I(1-e-Rt/L).

3. Among the following, which is the right formula for growth in an inductive circuit?
a) VL=V(1-e-t/time constant)
b) VL=V(1-et /time constant)
c) VL=(1-e-t /time constant)
d) VL=V(e-t /time constant)

View Answer

Answer: a [Reason:] The correct formula for growth in an inductive circuit is VL=V(1-e-t /time constant). As the time increases, the current in the inductor increases hence the voltage also increases.

4. The charging time constant of a circuit consisting of an inductor is the time taken for the voltage in the inductor to become __________% of the initial voltage.
a) 33
b) 63
c) 37
d) 36

View Answer

Answer: b [Reason:] We know that: V=V0(1-e-t /time constant). When time constant=t, we have: V=V0(1-e-1)= 0.63*V0. Hence the time constant is the time taken for the charge in an inductive circuit to become 0.63 times its initial charge.

5. What is the time constant of an inductive circuit?
a) LR
b) R/L
c) 1/LR
d) L/R

View Answer

Answer: d [Reason:] The time constant in an inductive circuit is the time taken for the voltage across the inductor to become 63 percent of its initial value. It is given by: Time constant= L/R.

6. A coil has a resistance of 4 ohm and an inductance of 2H. Calculate its time constant.
a) 1s
b) 2s
c) 0.5s
d) 5s

View Answer

Answer: c [Reason:] The expression for time constant in an inductive circuit is: Time constant= L/R Substituting the values from the question given, we get time constant= 0.5s.

7. A coil has a resistance of 4 ohm and an inductance of 2H. It is connected to a 20V dc supply. Calculate the final value of current in the circuit.
a) 5A
b) 10A
c) 15A
d) 20A

View Answer

Answer: a [Reason:] The final value of current in the circuit is: I=V/R= 5A.

8. A coil has a resistance of 4 ohm and an inductance of 2H. It is connected to a 20V dc supply. Calculate the value of current 1s after the switch is closed.
a) 5.44A
b) 4.32A
c) 6.56A
d) 2.34A

View Answer

Answer: b [Reason:] We know that: i=I(1-eRt/L) I=V/R=5A Substituting the remaining values from the given question, we get i=4.32A.

9. What happens to the inductance when the current in the coil becomes double its original value?
a) Becomes half
b) Becomes four times
c) Becomes infinity
d) Becomes double

View Answer

Answer: d [Reason:] The formula for magnetic field strength in a coil is: H=iN/l The inductance is: directly proportional to magnetic field strength, hence as the current value doubles, the inductance also doubles.

10. Calculate the inductance in an inductive circuit whose time constant is 2s and the resistance is 5 ohm.
a) 10H
b) 20H
c) 5H
d) 15H

View Answer

Answer: a [Reason:] We know that: Time constant= L/R Substituting the values from the given question, we get L=10H.

Set 4

1. Ammeters and voltmeters are calibrated to read?
a) RMS value
b) Peak value
c) Average value
d) Instantaneous value

View Answer

Answer: a [Reason:] Ammeters and voltmeters are calibrated to read the rms value because the rms value is the most accurate average value.

2. The rms value is _________ times he maximum value
a) 1.414
b) 0.5
c) 2
d) 0.707

View Answer

Answer: d [Reason:] We know that the rms value is i/root(2) time the maximum value, hence the rms vale is 0.707 times the maximum value.

3. The rms value is 0.707 times the _________ value.
a) Peak
b) Instantaneous
c) Average
d) DC

View Answer

Answer: a [Reason:] We know that the rms value is i/root(2) time the peak value, hence the rms vale is 0.707 times the peak value.

4. If the phasors are drawn to represent the rms values, instead of the maximum values, what would happen to the phase angle between quantities?
a) Increases
b) Decreases
c) Remains constant
d) Becomes zero

View Answer

Answer: c [Reason:] When phasors are drawn representing the rms values instead of the maximum value, the shape of the diagram remains unaltered and hence the phase angle remains the same.

5. Usually phasor diagrams are drawn representing?
a) RMS value
b) Peak value
c) Average value
d) Instantaneous value

View Answer

Answer: a [Reason:] Ammeters and voltmeters are calibrated to read the rms value, hence the phasors are drawn representing the rms values.

6. A phasor has frozen at 30 degrees, find the value of the phase angle?
a) 30 degrees
b) 60 degrees
c) 120 degrees
d) 180 degrees

View Answer

Answer: a [Reason:] The value of the phase angle is the value at which the phasor stops or freezes. Here, it freezes at 30 degree, hence the phase angle is 30 degrees.

7. The time axis of an AC phasor represents?
a) Time
b) Phase angle
c) Voltage
d) Current

View Answer

Answer: b [Reason:] The time axis while measuring an AC sinusoidal voltage or current represents the phase angle when converting it to a phasor.

8. The length of the phasor represents?
a) Magnitude of the quantity
b) Direction of the quantity
c) Neither magnitude nor direction
d) Either magnitude or direction

View Answer

Answer: a [Reason:] The length of the phasor arrow represents the magnitude of the quantity, whereas the angle between the phasor and the reference represents the phase angle.

9. What is the type of current obtained by finding the square of the currents and then finding their average and then fining the square root?
a) RMS current
b) Average current
c) Instantaneous current
d) Total current

View Answer

Answer: a [Reason:] RMS stands for Root Mean Square. This value of current is obtained by squaring all the current values, finding the average and then finding the square root.

10. For a direct current, the rms current is ________ the mean current.
a) Greater than
b) Less than
c) Equal to
d) Not related to

View Answer

Answer: c [Reason:] For a direct current, that is, a non-sinusoidal current, the mean current value is same as that of the rms current.

Set 5

1. Find the average value of current when the current that are equidistant are 4A, 5A and 6A.
a) 5A
b) 6A
c) 15A
d) 10A

View Answer

Answer: a [Reason:] The average value of current is the sum of all the currents divided by the number of currents. Therefore average current= (5+4+6)/3=5A.

2. What is the current found by finding the current in n equidistant regions and dividing by n?
a) RMS current
b) Average current
c) Instantaneous current
d) Total current

View Answer

Answer: b [Reason:] The average value of current is the sum of all the currents divided by the number of currents.

3. RMS stands for ________
a) Root Mean Square
b) Root Mean Sum
c) Root Maximum sum
d) Root Minimum Sum

View Answer

Answer: a [Reason:] RMS stands for Root Mean Square. This value of current is obtained by squaring all the current values, finding the average and then finding the square root.

4. What is the type of current obtained by finding the square of the currents and then finding their average and then fining the square root?
a) RMS current
b) Average current
c) Instantaneous current
d) Total current

View Answer

Answer: a [Reason:] RMS stands for Root Mean Square. This value of current is obtained by squaring all the current values, finding the average and then finding the square root.

5. __________ current is found by dividing the area enclosed by the half cycle by the length of the base of the half cycle.
a) RMS current
b) Average current
c) Instantaneous current
d) Total current

View Answer

Answer: b [Reason:] The average value of current is the sum of all the currents divided by the number of currents. Hence it can also be found by dividing the area enclosed by the half cycle by the length of the base of the half cycle.

6. What is the effective value of current?
a) RMS current
b) Average current
c) Instantaneous current
d) Total current

View Answer

Answer: a [Reason:] Effective current is also known as the effective current. RMS stands for Root Mean Square. This value of current is obtained by squaring all the current values, finding the average and then finding the square root.

7. In a sinusoidal wave, average current is always _______ rms current.
a) Greater than
b) Less than
c) Equal to
d) Not related

View Answer

Answer: b [Reason:] The average value of current is the sum of all the currents divided by the number of currents whereas RMS current is obtained by squaring all the current values, finding the average and then finding the square root. Hence RMS current is greater than average current.

8. For a rectangular wave, average current is ______ rms current.
a) Greater than
b) Less than
c) Equal to
d) Not related

View Answer

Answer: c [Reason:] The rms value is always greater than the average except for a rectangular wave, in which the heating effect remains constant so that the average and the rms values are the same.

9. Peak value divided by the rms value gives us?
a) Peak factor
b) Crest factor
c) Both peak and crest factor
d) Neither peak nor crest factor

View Answer

Answer: c [Reason:] Peak and crest factor both mean the same thing. Hence the peak value divided by the rms value gives us the peak or crest factor.

10. Calculate the crest factor if the peak value of current is 10A and the rms value is 2A.
a) 5
b) 10
c) 5A
d) 10A

View Answer

Answer: a [Reason:] We know that: Crest factor= Peak value/RMS value. Substituting the values from the given question, we get crest factor=5.