Explore topic-wise InterviewSolutions in .

Right choice is (b) Conduction current

For explanation I WOULD say: When a large amount of voltage is applied between the PLATES of a capacitor, the DIELECTRIC between the plates does not BEHAVE as an insulator anymore and starts conducting and conduction currents flow through it.

Correct CHOICE is (d) Electrolyte capacitor

To EXPLAIN: Electrolyte capacitors use chemical processes like pyrolysis to STORE CHARGE between its plates.

Right CHOICE is (a) INCREASES

The explanation is: The FORCE of attraction between the two plates of the capacitor is directly proportional to the SQUARE of potential gradient, HENCE as a potential gradient increases, the force of attraction also increases.

Right OPTION is (c) 220V

For explanation: As soon as the switch is CLOSED at t=0, there is no charge in the capacitor, HENCE the voltage across the capacitor is zero and all the 220V voltage is the voltage across the RESISTOR.

The CORRECT option is (c) 50MV

For EXPLANATION: Dielectric strength is the potential gradient required to cause a breakdown in the MATERIAL. Potential gradient is the ratio of VOLTAGE and thickness.

V=t*dielectric strength= 5*10=50MV.

The correct choice is (b) 1.26A

For EXPLANATION: At TIME t=RC, that is the time CONSTANT, we know that the value of current at that time interval is equal to 63% of the initial charge in the discharging CIRCUIT. Hence, I = 2*0.63 = 1.26A.

The correct choice is (b) MICA capacitor

Explanation: Mica CAPACITORS are preferred for high frequency circuits because they have low OHMIC LOSSES and less reactance.

The correct choice is (a) Fixed capacitor

The BEST I can EXPLAIN: PAPER capacitors are fixed capacitors because, like fixed capacitors, its capacitance value remains CONSTANT. In paper capacitors, paper is used as the DIELECTRIC.

The correct option is (c) IMPURITIES

Easy explanation: Ideally, dielectrics are insulators and do not CONTAIN any free electrons. But no dielectric is a perfect dielectric, hence the free electrons are due to impurities PRESENT in each dielectric.

The correct answer is (b) 10^-3A/s^2

The BEST 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^2i(0)/dt^2+10di(0)/dt=0.

Substituting the values of di/dt from the previous explanation, we get d^2i(0)/dt^2=10^-3A/s^2.

Right answer is (b) 10A

The 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.

The correct answer is (d) Increases exponentially

Best explanation: The equation for the VALUE of current in a DISCHARGING CAPACITIVE circuit is:

I=I0(1-e^-t/RC). From this equation, we can see that the current is exponentially INCREASING.

The correct choice is (d) Polyester capacitor

For EXPLANATION I would SAY: Polyester capacitors can operate at HIGH voltages, that is, a few thousand volts and the leakage resistance is high, that is, USUALLY 100 M.

Correct OPTION is (a) Turning the rotatable plates in or out

Best explanation: As the plates are rotated, the AREA of the plates between which the field exists, will vary. CAPACITANCE DEPENDS on area, hence as area varies, capacitance also varies.

Correct ANSWER is (b) 50

Best explanation: The discharging TIME constant in a circuit consisting of a CAPACITOR and resistor in series is the product of the resistance and capacitance = 5*10 = 50.

The CORRECT ANSWER is (b) 0A

To explain: When the capacitor is charging, the final voltage of the capacitor becomes EQUAL to the voltage of source. HENCE, the current becomes equal to zero.

The correct option is (B) 7m

Explanation: Dielectric strength is the POTENTIAL gradient REQUIRED to CAUSE a breakdown in the material. Potential gradient is the ratio of voltage and thickness.

V/dielectric strength= t= 28/4=7m.

Right ANSWER is (b) 100mm

To EXPLAIN: From the given data:

A=pi*d^2/4=pi*a^2/4

Potential GRADIENT = V/x = 10^6V/m

F=epsilon*A*(V/x)^2/2

Substituting the given values, we GET d=100mm.

The CORRECT answer is (a) 1kV

Explanation: From the given DATA:

A=pi*d^2/4=0.007854m^2

Potential gradient = V/x = 1000*V

F=epsilon*A*(V/x)^2/2

Substituting the given VALUES in the above EXPRESSION, we get V=1kV.

Correct answer is (b) 4J

Easy explanation: The expression for finding the VALUE of ENERGY is:

U=Q^2/2C = 4*4/(2*2) = 4J.

Right choice is (a) 0V

To elaborate: 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.

The correct option is (C) 37

To explain: We know that: Q=Q0(1-e^-t/RC).

When RC=t, we have: Q=Q0(1-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.

Correct option is (d) 20A

The EXPLANATION: In a discharging circuit, the final voltage is equal to ZERO for capacitor. For a RESISTOR, final voltage is 400V.So,final CURRENT = V/R = 400/20 = 20A.

Right ANSWER is (b) 0A

For explanation I WOULD SAY: When the capacitor is discharging the value of the INITIAL CURRENT is zero.

The correct CHOICE is (C) 400 microA

For explanation: In a series RC circuit, the initial charging CURRENT is:

I=V/R = 200/(0.5*10^6s) = 400*10^-6A = 400 microA.

Right choice is (c) Charging CURRENT

For explanation I would say: The current in conductors connecting the voltage source to the plates of a capacitor is the charging current and not the CONDUCTION or LEAKAGE current.

The correct option is (b) CONDUCTION current

Easiest explanation: Conduction current is the current caused by the actual flow of electrons and DISPLACEMENT current is the current where no CHARGE CARRIERS are involved.

Correct option is (a) 2V

The best I can EXPLAIN: The expression for FINDING the VALUE of ENERGY is:

U=Q^2/2C.

Substituting the values of U and Q, we get C=2F.

V=Q/C, hence V=4/2=2V.

Right choice is (b) 126.4V

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.

The correct OPTION is (a) Displacement current

For EXPLANATION I WOULD say: Displacement current is the current where no charge CARRIERS are involved. It is caused due to variations in the electric field.

Right CHOICE is (B) i=(V/R)e^-t/RC

Easy explanation: APPLYING KVL to the GIVEN circuit, we get:

i=i0e^-t/RC = (100/10)e^-t/100

i=10 e^-t/100.

Right option is (B) 63

The best I can EXPLAIN: We know that: Q=Q0(1-e^-t/RC).

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

Right OPTION is (C) Tuning CAPACITOR

Easy explanation: Ceramic capacitor, electrolyte capacitor and paper capacitor are FIXED capacitors WHEREAS tuning capacitors is a variable capacitor, hence it is the odd one out.

The correct choice is (a) Displacement current

The explanation: Under NORMAL conditions capacitors contain an insulating material called DIELECTRIC sandwiched between the plates of the capacitor. SINCE insulators can CARRY only an electric field but not moving carriers, therefore NORMALLY a capacitor has displacement current and not conduction current.

The correct CHOICE is (B) Zero

To elaborate: When a capacitor is fully charged, there is no CONDUCTION of electrons from ONE plate of the capacitor to ANOTHER, hence there is no conduction current and conduction current is equal to zero.

Correct OPTION is (a) TRANSISTORS

The explanation: Leakage is primarily caused DUE to electronic devices, such as transistors, connected to the CAPACITORS. Transistors conduct a small amount of current even when they are turned off, hence they are RESPONSIBLE for leakage current.

Correct OPTION is (b) Decreases

The explanation: The FORCE of attraction between the TWO PLATES of the capacitor is inversely proportional to the square of the distance between the plates, hence as distance increases, the force of attraction decreases.

The correct OPTION is (C) 0.035N

To explain: From the given data:

A=pi*d^2/4=0.007854m^2

Potential GRADIENT = V/x = 10^6V/m

F=epsilon*A*(V/x)^2/2

Therefore, F=0.035N.