This set of Basic Electrical Engineering Interview Questions and Answers for freshers focuses on “Discharge of a Capacitor Through a Resistor”.

1. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the time constant.

a) 1s

b) 2s

c) 3s

d) 4s

View Answer

Explanation: The time constant is the product of the resistance and capacitance in a series RC circuit.

Therefore, time constant= 8*10

^{-6}*4*10

^{6}=4s.

2. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the initial charging current.

a) 100 microA

b) 500 microA

c) 400 microA

d) 1000microA

View Answer

Explanation: In a series RC circuit, the initial charging current is:

I=V/R= 200/(0.5*10

^{6s})= 400*10

^{-6}A= 400 microA.

3. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the time taken for the potential difference across the capacitor to gow to 160V.

a) 6.44s

b) 7.77s

c) 2.33s

d) 3.22s

View Answer

Explanation: From the previous explanations, we know that the initial current is 400mA and the time constant is 4s. Substituting the values of capacitor voltage, initial voltage, initial current and time constant in the equation: Vc=V(1-e

^{-t}/RC), we get t=6.44s.

4. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the voltage in the capacitor 4s after the power is supplied.

a) 123.4V

b) 126.4V

c) 124.5V

d) 132.5V

View Answer

Explanation: We can get the value of the potential difference across the capacitor in 4s, from the following equation:

Vc=V(1-e

^{-t}/RC). Substituting the values in the given equation, we get Vc= 126.4V.

5. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the current in the capacitor 4s after the power is supplied.

a) 150 microA

b) 149 microA

c) 148 microA

d) 147 microA

View Answer

Explanation: In the given question, the time constant is equal to the time taken= 4s. Hence the value of current will be 63% of its initial value= I=0.63*400= 147 microA.

6. The discharging time constant of a circuit consisting of a capacitor is the time taken for the charge in the capacitor to become __________% of the initial charge.

a) 33

b) 63

c) 37

d) 36

View Answer

Explanation: We know that: Q=Q0(e

^{-t}/RC).

When RC=t, we have: Q=Q0(e

^{-1})= 0.37*Q0.

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

7. The discharging time constant of a circuit consisting of a capacitor is the time taken for the charge in the capacitor to become __________% of the initial charge.

a) 33

b) 63

c) 37

d) 36

View Answer

Explanation: We know that: Q=Q0(e

^{-t}/RC).

When RC=t, we have: Q=Q0(e

^{-1})= 0.37*Q0.

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

8. A circuit has a resistance of 2 ohm connected in series with a capacitance of 6F. Calculate the discharging time constant.

a) 3

b) 1

c) 12

d) 8

View Answer

Explanation: The discharging time constant in a circuit consisting of a capacitor and resistor in series is the product of the resistance and capacitance= 2*6=12.

9. What is the value of current in a discharging capacitive circuit if the initial current is 2A at time t=RC.

a) 0.74A

b) 1.26A

c) 3.67A

d) 2.89A

View Answer

Explanation: At time t=RC, that is the time constant, we know that the value of current at that time interval is equal to 37% of the initial charge in the discharging circuit. Hence, I=2*0.37= 0.74A.

10. While discharging, what happens to the current in the capacitive circuit?

a) Decreases linearly

b) Increases linearly

c) Decreases exponentially

d) Increases exponentially

View Answer

Explanation: The equation for the value of current in a discharging capacitive circuit is:

I=I0*e-t /RC. From this equation, we can see that the current is exponentially decreasing since e is raised to a negative power.

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

To practice all areas of Basic Electrical Engineering for Interviews, __here is complete set of 1000+ Multiple Choice Questions and Answers__.