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# Basic Electrical Engineering MCQ’s – Growth in an Inductive Circuit

In a pure inductive circuit, the power factor is?

a) Maximum

b) Minimum

c) 0

d) Infinity

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Answer: c

Explanation: In a pure inductive circuit, the current is lagging by 90 degrees from the voltage. The power factor is the cosine of the angle between the voltage and the current. If the angle between the voltage and the current is 90, then cos90=0. Hence, the power factor is zero.

Among the following, which is the right formula for growth in an inductive circuit?

a) VL=V(1-e^{-tR/L})

b) VL=(e^{-tR/L})

c) VL=(1-e^{-tR/L})

d) VL=V(e^{-tR/L})

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Answer: d

Explanation: The correct formula for growth in an inductive circuit is VL=V(e^{-tR/L}). As the time increases, voltage decreases.

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

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Answer: c

Explanation: We know that: V=V_{0}(1-e^{-tR/L}).

When time constant=t, we have: V=V_{0}(1-e^{-1}) = 0.37*V_{0}.

Hence the time constant is the time taken for the charge in an inductive circuit to become 0.37 times its initial charge.

What is the time constant of an inductive circuit?

a) LR

b) R/L

c) 1/LR

d) L/R

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Answer: d

Explanation: 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.

Calculate the time constant of an inductive circuit having resistance 5 ohm and inductance 10H.

a) 2s

b) 4s

c) 5s

d)10s

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Answer: a

Explanation: We know that: Time constant = L/R

Substituting the values from the given question, we get a time constant = 2s.

Calculate the resistance in an inductive circuit whose time constant is 2s and the inductance is 10H.

a) 7ohm

b) 10ohm

c) 2ohm

d) 5ohm

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Answer: d

Explanation: We know that: Time constant = L/R

Substituting the values from the given question, we get R=5ohm.

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

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Answer: a

Explanation: We know that: Time constant = L/R

Substituting the values from the given question, we get L=10H.

The charging time constant of a circuit consisting of an inductor is the time taken for the current in the inductor to become __________% of the initial current.

a) 33

b) 63

c) 37

d) 36

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Answer: b

Explanation: We know that: i=i_{0}(1-e^{-tR/L}).

When t=L/R, we have: i=i_{0}(1-e^{-1}) = 0.63*i_{0}.

Hence the time constant is the time taken for the current in an inductive circuit to become 0.63 times its initial current.