This set of Basic Electrical Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Analysis of Growth & Decay”.

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

Explanation: 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(e^{Rt/L})

b) i=I(1-e^{-Rt/L})

c) i=I(1+e^{-Rt/L})

d) i=I(e^{-Rt/L})

View Answer

Explanation: 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. A coil has a resistance of 4 ohms and an inductance of 2H. It is connected to a 20V dc supply. Calculate the initial value of the current in the circuit.

a) 5A

b) 10A

c) 0 A

d) 20A

View Answer

Explanation: Initially, inductor behave as open circuit for dc current so, i=0.

4. A coil has a resistance of 4 ohms and an inductance of 2H. It is connected to a 20V dc supply. Calculate the final value of the current in the circuit.

a) 5A

b) 10A

c) 15A

d) 20A

View Answer

Explanation: The final value of the current in the circuit is:

I=V/R = 5A.

5. A coil has a resistance of 4 ohms 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

Explanation: We know that:

i=I(1-e

^{Rt/L})

I=V/R=5A

Substituting the remaining values from the given question, we get i=4.32A.

6. A coil has a resistance of 4 ohms and an inductance of 2H. It is connected to a 20V dc supply. Calculate the value of voltage 1s after the switch is closed.

a) 5.4V

b) 10.8V

c) 0 V

d) 2.7V

View Answer

Explanation: V=V

_{0}e

^{-Rt/L}

V=20e

^{-2}=2.7V.

7. 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-e^{t} /time constant)

c) i=(1-e^{-t} /time constant)

d) i=I(e^{-t} /time constant)

View Answer

Explanation: 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.

8. 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

Explanation: We know that: V=V

_{0}(e

^{-tR/L}).

When t=L/R, we have: V=V

_{0}(e

^{-1}) = 0.37*Vsub>0.

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

9. A coil has a resistance of 4 ohms and an inductance of 2H. It is connected to a 20V dc supply. Calculate the initial value of the voltage across the inductor.

a) 5V

b) 10V

c) 0 V

d) 20V

View Answer

Explanation: Initially, inductor behave as open circuit for dc current so, V = V0 = 20V i.e. same as voltage source.

10. A coil has a resistance of 4 ohms and an inductance of 2H. It is connected to a 20V dc supply. Calculate the final value of the voltage across the inductor.

a) 5V

b) 10V

c) 0 V

d) 20V

View Answer

Explanation: At steady state, inductor behaves as a short circuit for dc current so, V=0

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