This set of Basic Electrical Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Transients in LR Networks”.

1. An RL network is one which consists of ____________

a) Resistor and capacitor in parallel

b) Resistor and capacitor in series

c) Resistor and inductor in parallel

d) Resistor and inductor in series

View Answer

Explanation: An R-L network is a network which consists of a resistor which is connected in series to an inductor.

2. If the switch is opened at t=0, what is the current in the circuit?

a) 0A

b) 1A

c) 2A

d) 3A

View Answer

Explanation: Initially when switch was closed,current in the inductor was 60/30=2A.

Current in inductor doesn’t change suddenly so when switch is opened, current in inductor remains same i.e. 2A.

3. In an RL series circuit, when the switch is closed and the circuit is complete, what is the response?

a) Response does not vary with time

b) Decays with time

c) Increases with time

d) First increases, then decrease

View Answer

Explanation: In an RL series circuit, the response decays with time because according to the equation, there is an exponential decrease in the response.

4. If the switch is closed at t=0, what is the current in the circuit?

a) 0A

b) 10A

c) 20A

d) 30A

View Answer

Explanation: Initially, when the switch is open, the current in the circuit is 0. As soon as the switch is closed at t=0

^{+}, the inductor acts as an open circuit, hence the current in the circuit is zero. Since the current in the circuit is zero, there is no voltage drop across the resistor and hence voltage across the inductor is equal to the supply voltage, i.e. 60V.

5. What is the voltage across the inductor at t=0?

a) 0V

b) 20V

c) 60V

d) 58V

View Answer

Explanation: Initially, when the switch is open, the current in the circuit is 0. As soon as the switch is closes at t=0+, the inductor acts as an open circuit, hence the current in the circuit is zero. Since the current in the circuit is zero, there is no voltage drop across the resistor and the voltage across the inductor is equal to the supply voltage, which is equal to 60V.

6. What is the expression for current in the given circuit?

a) i=2(e^{-2t})A

b) i=2(1-e^{-2t})A

c) i=2(e^{2t})A

d) i=2(1+e^{-2t})A

View Answer

Explanation: Applying KVL in above circuit, we get

60-30i-15di/dt =0

i=2(1-e

^{-2t})A

7. What is the expression for voltage in the given circuit?

a) V=60e^{-0.5t}

b) V=30e^{-0.5t}

c) V=60e^{-2t}

d) V=30e^{-2t}

View Answer

Explanation: Applying KVL in above circuit, we get

60-30i-15di/dt = 0

i=2(1-e

^{-2t})A

di/dt = 4e

^{-2t}

V=Ldi/dt=15*4e-

^{-2t}=60e

^{-2t}.

8. At steady state, the current in the inductor is?

a) Maximum

b) Minimum

c) Zero

d) Infinity

View Answer

Explanation: At steady state maximum current flows in the inductor because it acts as an open circuit.

9. Initially, when the switch in a series RL circuit is closed, the inductor acts as?

a) Open circuit

b) Short circuit

c) Resistor

d) Capacitor

View Answer

Explanation: Before switch is closed, current in inductor is zero. When the switch in a series RL circuit is closed, current in the inductor remains zero since current in inductor doesn’t change suddenly. So, the inductor acts as an open circuit.

10. Initially, when the switch in a series RL circuit is closed, the current in the inductor is?

a) Maximum

b) Minimum

c) Zero

d) Infinity

View Answer

Explanation: Initially, when the switch in a series RL circuit is closed, the inductor acts as an open circuit. Current in an open circuit is zero, hence the inductor current is zero.

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