21. A power factor of a circuit can be improved by placing Which among the following in a circuit?
A. Inductor
B. Capacitor
C. Resistor
D. Switch
Answer: B
Power factor = Real power/Apparent power = kW/kVA
By adding a capacitor to a circuit, an additional kW load can be added to the system without altering the kVA. Hence, the power factor is improved.
22. When the supply frequency increases, what happens to the capacitive reactance in the circuit?
A. Increases
B. Decreases
C. Remains the same
D. Becomes zero
Answer: B
The expression for capacitive reactance is:
Xc = 1/(2 × π × f × C).
This relation shows that frequency is inversely related to capacitive reactance.
Hence, as supply frequency increases, the capacitive reactance decreases.
23. Calculate the time constant of a series RC circuit consisting of a 100 µF capacitor in series with a 100ohm resistor.
A. 0.1 sec
B. 0.1 msec
C. 0.01 sec
D. 0.01 msec
Answer: C
The time constant of a RC circuit is
= R × C = 100 × 10-6 × 100 = 0.01 sec.
24. Capacitors charge and discharge in __________ manner.
A. Linear
B. Constant
C. Square
D. Exponential
Answer: D
Capacitors charge and discharge in an exponential manner because of the relation:
XC = 1/(2πfC) and Q = CV
∴ Q = V/(2πf XC)
XC is complex and can be written in the form of exponent through the Euler formula.
25. Air has a dielectric constant of ___________
A. Unity
B. Zero
C. Infinity
D. Hundred
Answer: A
The dielectric constant of air is the same as that of a vacuum which is equal to unity.
The dielectric constant of air is taken as the reference to measure the dielectric constant of all other materials.
26. What is the value of capacitance of a capacitor that has a voltage of 4V and ha 8C of charge?
A. 2F
B. 4F
C. 6F
D. 8F
Answer: A
Q is directly proportional to V.
The constant of proportionality, in this case, is C, that is, the capacitance.
Hence Q = CV.
From the relation, C = Q/V = 8/4 = 2F.
27. Unit of capacitance is ___________
A. Volts
B. Farad
C. Henry
D. Newton
Answer: B
Volts are the unit of voltage, Henry for inductance and Newton for force. Hence the unit for capacitance is Farad.
28. What will happen to the capacitor just after the source is removed?
A. It will not remain in its charged state
B. It will remain in its charged state
C. It will start discharging
D. It will become zero
Answer: B
As soon as the source is removed, the capacitor does not start discharging it remains in the same charged state.
29. Which among the following equations is incorrect about correct?
A. Q = CV
B. Q = C/V
C. V = Q/C
D. C = Q/V
Answer: B
Q is directly proportional to V.
The constant of proportionality, in this case, is C, that is, the capacitance.
Hence Q = CV.
We can derive all the equations from the given relation except for Q = C/V.
30. Capacitance is directly proportional to __________
A. Area of a cross-section between the plates
B. Distance of separation between the plates
C. Both area and distance
D. Neither area nor distance
Answer: A
The relation between capacitance, area, and distance between the plates is C = ε × A/D. According to this relation, the capacitance is directly proportional to the area.
31. What is the total capacitance when three capacitors, C1, C2, and C3 are connected in parallel?
A. C1/(C2+C3)
B. C1+C2+C3
C. C2/(C1+C3)
D. 1/C1+1/C2+1/C3
Answer: B
When capacitors are connected in parallel, the total capacitance is equal to the sum of the capacitance of each of the capacitors. Hence Ctotal = C1+C2+C3.
32. Calculate the total capacitance in the given circuit
A. 10F
B. 15F
C. 13F
D. 20F
Answer: C
The equivalent capacitance when capacitors are connected in parallel is the sum of all the capacitors = 1+2+10 = 13F.
33. Calculate the voltage across AB if the total charge stored in the combination is 13C.
A. 1V
B. 2V
C. 3V
D. 4V
Answer: A
The equivalent capacitance when capacitors are connected in parallel is the sum of all the capacitors = 1+2+10 = 13F.
V = Q/C = 13/13 = 1V.
34. Calculate the charge in the 2F capacitor.
A. 200C
B. 100C
C. 300C
D. 400C
Answer: A
Since the capacitors are connected in parallel, the voltage across each is the same, it does not get divided.
Q = CV = 2 × 100 = 200C.
35. Calculate the charge in the 1F capacitor.
A. 200 C
B. 100 C
C. 300 C
D. 400 C
Answer: B
Since the capacitors are connected in parallel, the voltage across each is the same, it does not get divided.
Q = CV = 1 × 100 = 100C.
36. Calculate the total charge of the system in the given circuit.
A. 200 C
B. 100 C
C. 300 C
D. 400 C
Answer: C
The equivalent capacitance when capacitors are connected in parallel is the sum of all the capacitors = 1+2 = 3F.
Q = CV = 3 × 100 = 300V.
37. When capacitors are connected in parallel, the total capacitance is always __________ the individual capacitance values.
A. Greater than
B. Less than
C. Equal to
D. Cannot be determined
Answer: A
When capacitors are connected in parallel, the total capacitance is equal to the sum of the capacitance of each of the capacitors.
Hence Ctotal = C1+C2+C3.
Since it is the sum of all the capacitance values, the total capacitance is greater than the individual capacitance values.
38. When capacitors are connected in parallel, what happens to the effective plate area?
A. Increases
B. Decreases
C. Remains the same
D. Becomes zero
Answer: A
When capacitors are connected in parallel, the top plates of each of the capacitors are connected together while the bottom plates are connected to each other. This effectively increases the top plate area and the bottom plate area.
39. Three capacitors having a capacitance equal to 2F, 4F, and 6F are connected in parallel. Calculate the effective parallel.
A. 10F
B. 11F
C. 12F
D. 13F
Answer: C
When capacitors are connected in parallel, the total capacitance is equal to the sum of the capacitance of each of the capacitors.
Hence Ctotal = C1+C2+C3 = 2+4+6 = 12F.
40. Two capacitors having a capacitance value of 4F, three capacitors having a capacitance value of 2F, and 5 capacitors having a capacitance value of 1F are connected in parallel, calculate the equivalent capacitance.
A. 20F
B. 19F
C. 18F
D. 17F
Answer: B
When capacitors are connected in parallel, the total capacitance is equal to the sum of the capacitance of each of the capacitors.
