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

1. A CR network is one which consists of _________

a) A capacitor and resistor connected in parallel

b) A capacitor and resistor connected in series

c) A network consisting of a capacitor only

d) A network consisting of a resistor only

View Answer

Explanation: A CR network is one which consists of a capacitor connected in series with a resistor. The capacitor discharges or charges through the resistor.

2. At DC, capacitor acts as _________

a) Open circuit

b) Short circuit

c) Resistor

d) Inductor

View Answer

Explanation: At DC, the capacitor acts as open circuit because the capacitive resistance is infinity. The frequency of a DC circuit is 0. The capacitive resistance=1/(2*pi*f*C). Therefore, if the frequency is 0, the capacitive resistance is infinity and it acts as an open circuit.

3. In an RC 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 decreases

View Answer

Explanation: In an RC 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 clocked at t=0, what is the current in the circuit?

a) 0A

b) 10A

c) 20A

d) Infinity

View Answer

Explanation: As soon as the switch is closed at t=0, the capacitor acts as a short circuit. The current in the circuit is:

I=V/R= 100/10= 10A.

5. Calculate the voltage across the capacitor at t=0.

a) 0A

b) 10A

c) 20A

d) Infinity

View Answer

Explanation: When the switch is closed at t=0, the capacitor has no voltage across it since it has not been charged. The capacitor acts as a short circuit and the voltage across it is zero.

6. Calculate di(0)/dt if the switch is closed at t=0.

a) -9.9A/s

b) -10A/s

c) 0A/s

d) -0.1A/s

View Answer

Explanation: Applying KVL to the given circuit, we get:

100+10i(0)+1/10*integral(i(0)dt)=0

Differentiating once, we get:

10di(0)/dt+1/10*i.

From the previous questions, we know that i(0)=10A.

Substituting the values in the equation, we get di(0)/dt=-0.1A/s.

7. Calculate d^{2}i(0)/dt^{2} from the given circuit.

a) 10^{-6}A/s^{2}

b) 10^{-3}A/s^{2}

c) 10^{6}A/s^{2}

d) 10^{3}A/s^{2}

View Answer

Explanation: Applying KVL to the given circuit, we get:

100+10i(0)+1/10*integral(i(0)dt)=0

Differentiating once, we get:

10di(0)/dt+1/10*i.

Differentiating once again, we get:

10d

^{2}i(0)/dt

^{2}+10di(0)/dt=0.

Substituting the values of di/dt from the previous explanation, we get d

^{2}i(0)/dt

^{2}=10

^{-3}A/s

^{2}.

8. The current equation for the given circuit is?

a) i=10e^{(-0.01)}t A

b) i=10e^{(0.01)}t A

c) i=10e^{(-0.001)}t A

d) i=100e^{(-0.01)}t A

View Answer

Explanation: The KVL equation is:

100+10i(0)+1/10*integral(i(0)dt)=0

On applying Laplace transform to this equation, we get:

100/s=I(s)/10s+10I(s)

Solving the equation, we get:

i=10e

^{(-0.01)}t A.

9. The expression for current in an RC circuit is?

a) i=(V/R)e^{(t/RC )}

b) i=(V/R)e^{(-t/RC )}

c) i=(V/R)/e^{(t/RC )}

d) i=(V/R)/e^{(-t/RC )}

View Answer

Explanation: The particular solution of the current equation is zero. Hence, the expression for the current in an RC circuit is:

i=(V/R)e

^{(-t/RC )}.

10. What is the voltage in the resistor as soon as the switch is closed at t=0.

a) 0V

b) Infinity

c) 220V

d) Insufficient information provided

View Answer

Explanation: As soon as the switch is closes at t=0, there is no charge in the capacitor, hence the voltage across the capacitor os zero and all the 220V voltage is the voltage across the resistor.

**Sanfoundry Global Education & Learning Series – Basic Electrical Engineering.**

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