This set of Basic Electrical Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Capacitance”.
1. A power factor of a circuit can be improved by placing which, among the following, in a circuit?
Explanation: Power factor = Real power/Apparent power = kW/kVA
By adding a capacitor in a circuit, an additional kW load can be added to the system without altering the kVA. Hence, the power factor is improved.
2. When the supply frequency increases, what happens to the capacitive reactance in the circuit?
c) Remains the same
d) Becomes zero
Explanation: The expression for capacitive reactance is: Xc=1/(2*pi*f*C). This relation shows that frequency is inversely related to capacitive reactance. Hence, as supply frequency increases, the capacitive reactance decreases.
3. Calculate the time constant of a series RC circuit consisting of a 100microF capacitor in series with a 100ohm resistor.
a) 0.1 sec
b) 0.1 msec
c) 0.01 sec
d) 0.01 msec
Explanation: The time constant of a RC circuit= R*C= 100*10-6*100=0.01 sec.
4. Capacitors charge and discharge in __________ manner.
Explanation: 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 which can be written in the form of exponent through euler formula.
5. Air has a dielectric constant of ___________
Explanation: Dielectric constant of air is the same as that of a vacuum which is equal to unity. Dielctric constant of air is taken as the reference to measure the dielectric constant of all other materials.
6. What is the value of capacitance of a capacitor which has a voltage of 4V and ha 8C of charge?
Explanation: 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.
7. Unit of capacitance is___________
Explanation: Volts is the unit of voltage, Henry for inductance and Newton for a force. Hence the unit for capacitance is Farad.
8. 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
Explanation: As soon as the source is removed, the capacitor does not start discharging it remains in the same charged state.
9. Which among the following equations is incorrect?
Explanation: Q is directly proportional to V. The constant of proportionality in this case is C, that is, the capacitance. Hence Q=CV. From the given relation we can derive all the equations except for Q=C/V.
10. Capacitance is directly proportional to__________
a) Area of cross section between the plates
b) Distance of separation between the plates
c) Both area and distance
d) Neither area nor distance
Explanation: The relation between capacitance, area and distance between the plates is: C=epsilon*A/D. According to this relation, the capacitance is directly proportional to the area.
Sanfoundry Global Education & Learning Series – Basic Electrical Engineering.
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