21. Which of the following depends on the charging and discharging rate of a capacitor?

A. Time constant
B. Current
C. Power
D. Voltage

Answer: A

The time constant in a circuit consisting of a capacitor is the product of the resistance and the capacitance. The smaller the time constant, the faster the charging and discharging rate and vice versa.

22. What is the initial current while charging a capacitor?

A. High
B. Low
C. 0
D. Cannot be determined

Answer: A

The initial current of a capacitor is very high because the voltage source will transport charges from one plate of the capacitor to the other plate.

23. What is the final current while charging a capacitor?

A. High
B. Zero
C. Infinity
D. Low

Answer: B

The final current is almost equal to zero while charging a capacitor because the capacitor is charged up to the source voltage.

24. A capacitor is charged to a voltage of 400V and has a resistance of 20ohm. Calculate the initial value of the charging current.

A. 10A
B. 0A
C. Infinity
D. 20A

Answer: D

When the capacitor is charging the initial value of the current is V/R = 400/20 = 20A.

25. A capacitor is charged to a voltage of 400V and has a resistance of 20ohm. Calculate the initial value of the discharge current.

A. 10A
B. 0A
C. Infinity
D. 20A

Answer: B

When the capacitor is discharging the value of the initial current is zero.

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26. A capacitor is charged to a voltage of 400V and has a resistance of 20ohm. Calculate the final value of the discharge current.

A. 10A
B. 0A
C. Infinity
D. 20A

Answer: D

In a discharging circuit, the final voltage is equal to zero for the capacitor. For a resistor, the final voltage is 400V.

So,final current = V/R = 400/20 = 20A.

27. When will be capacitors fully charged?

A. When voltage is zero
B. When the supply voltage is equal to the capacitor voltage
C. When voltage is infinity
D. When capacitor voltage is equal to half the supply voltage

Answer: B

When the capacitor voltage is equal to the source voltage, it means that all the charges have moved from one plate of the capacitor to the other.

28. What happens to the capacitor when the capacitor voltage is equal to the source voltage?

A. The charging phase of the capacitor is over
B. The discharging phase of the capacitor is over
C. The capacitor is switched off
D. The capacitor is switched on

Answer: C

When the capacitor voltage is equal to the source voltage, it means that all the charges have moved from one plate of the capacitor to the other. Hence the capacitor is fully charged and we say it gets switched off.

29. A capacitor is charged to a voltage of 400V and has a resistance of 20ohm. Calculate the final value of the charging current.

A. 10A
B. 0A
C. Infinity
D. 20A

Answer: B

When the capacitor is charging, the final voltage of the capacitor becomes equal to the voltage of the source. Hence, the current becomes equal to zero.

30. The charging 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

Answer: B

We know that

Q = Q0(1-e-t/RC).

When RC = t, we have

Q = Q_{0}(1-e^{-1}) = 0.63 × Q0.

Hence the time constant is the time taken for the charge in a capacitive circuit to become 0.63 times its initial charge.