Capacitors MCQs with Explanatory Answers

Answer:     ( 1 )

Capacitive Reactance is given as:

X_c = 1/2 πfC

Where, f is the frequency of applied voltage to the capacitor plates and C is the capacitance.

For DC, the value of f is zero and hence the capacitive reactance is infinite. Since capacitive reactance denotes the opposition to flow of current just like resistance, this implies the capacitor blocks passage of DC current.

Since frequency of AC signals is anything but zero or less than that, capacitive reactance has a finite value for AC supply and allows to pass AC.

Answer:      ( 2 )

Capacitors are of two types based on their polarity – Electrolyte and Non-Polarized Capacitors. While Electrolyte capacitors have polarized leads, the non-polarized capacitors like ceramic, mica etc. have equivalent leads.

Answer:      ( 2 )

As per the colour code for disk ceramic capacitor, the first two colours represent the value of capacitance, the third colour represents the multiplier and the last colour represents the tolerance.

Thus, referring the colour chart, we come to the following conclusion:

1. First colour: Red. Hence first digit is 2
2. Second colour: Green. Hence second digit is 5
3. Thus, value of capacitance is 25
4. Third colour: Brown. Hence multiplier is 10pF
5. Fourth colour: Red. Hence tolerance is +/-2%

Therefore, value of the capacitor is 25 x 10pF, +/-2%,   i.e.  250pF, +/-2%  tolerance.

Answer:       (  1  )

As per the colour code for tubular capacitor, the first two colours represent the value of capacitance, the third colour represents the multiplier, fourth colour represents the tolerance and last colour represents the maximum permissible voltage across the capacitor.

Thus, referring the colour chart, we come to the following conclusion:

1.   First colour:       Yellow. Hence first digit is 4
2.   Second colour:   Blue. Hence second digit is 6
3.   Thus, value of capacitance is 46.
4.   Third colour:      Brown. Hence multiplier is 10pF
5.   Fourth colour:   Green. Hence tolerance is +/-5%
6.   Fifth colour:       Red. Hence voltage is 250 Volts

Thus, value of the capacitor is: 46 x 10pF, +/-5% tolerance, 250 Volts.

Answer:       (  2  )

Consider the below parallel connection of capacitors

Applying Kirchhoff’s current law, we get 

Now we know current through capacitor N is,

Also, voltage across each capacitor is same.

Substituting in equation 1, we get:

Thus, for parallel connection of capacitors, the total capacitance is the summation of individual capacitances.

Answer:       (  1  )

Consider the circuit given below

Applying Kirchhoff’s voltage law, we get:

Now, voltage across capacitor N, 

Since current through the loop is same. Hence equation 1 becomes:

Thus, total capacitance for capacitors in series is same as total resistances in parallel.

Answer:      ( 2 )

When voltage, v(t) is applied to a capacitor, current I flows through it. The energy stored in the capacitor is the integral of the instantaneous power.

Thus, energy,

Thus, energy stored in capacitor, E = 1/2CV²

Answer:       ( 1 )

Even though theoretically the dielectric material between plates of a capacitor has infinite resistivity, it has some finite resistivity which leads to flow of current known as leakage current. The Equivalent Series Resistance (ESR) is the total resistance offered by the dielectric material and other metal elements in the capacitor and denotes the total losses incurred. It typically ranges from mili-ohms to ohms.

Answer:        ( 2 )

Given: Voltage across capacitor, C1 = 0

Hence energy stored in capacitor, C1 = 0 Joules

Voltage across capacitor, C2 = 12V

Hence, energy stored in capacitor, C2 = 1/ 2CV² = 144 x 10^-5 Joules

Voltage across capacitor, C3 = 12V

Hence, energy stored in capacitor, C3 = 1/ 2CV² = 72 x 10^-5 Joules

Given:

Voltage, v(t) = 30sin ( 100 πt )

Capacitance, C1 = 50uF

Thus current through capacitor, i(t) = Cdv (t)/ dt = 50 x 10^-6 d( 30sin 100πt ) /dt = 50 x 10^-6 x 30 x 100 x πcos 100 πt = 15 x 10^-2cos 100πt

Answer:        ( 1 )

A capacitor consists of two electric conductors separated by a distance d with a dielectric material. When voltage is applied to the conductors, the potential difference between the conductors charges the conductors, which creates a uniform electric field between the conductors. This electric field is defined as the force per unit charge. Thus, a capacitor works in electric field.

Answer:       ( 2 )

Capacitance is the measure of a capacitor to store charge for a given electric potential. If it charged to ‘V’ volts, it stores charge +Q at one plate and charge -Q at another plate. Capacitance of 1 Farad denotes storage of 1 Coulomb charge for 1 volt potential difference.

Answer:      ( 1 )

Consider the circuit given below:

When the DC voltage is applied to the capacitor, initially high current passes through the capacitor plates as it starts getting charged. After time ‘t’, the value of current drops to zero as the capacitor plates get fully charged. This time ‘t’ is given as the reciprocal of RC, where R is the value of resistor R1.

Answer:     ( 4 )

Most of the electric power supply systems have inductive loads, which causes lagging effect on the supply current. This causes an increase in apparent or reactive power and increases the supply current.