## Important Questions of Capacitance

In this post, some of the important questions of capacitance is given. It includes definition of capacitance, definition of relative permeability, factor affecting capacitance, ceramic capacitor, capacitor of smallest size, capacitor on DC supply, energy stored in capacitor, time constant of RC circuit, unit of time constant, dielectric loss, capacitance of isolated sphere.

### Define:  Capacitance

Capacitance

It is defined as the two conducting plates are separated by the dielectric material. The charge across the dielectric material is directly proportional to the potential difference across them.

q α v

q = Cv

Where

q  =  Charge stored in the dielectric material

v  =  Potential difference across the dielectric material

C =  Constant, a capacitance

### Explain the term : Relative Permeability

Relative permittivity

The ability of the dielectric material to concentrate electric lines of force between conducting plates of the capacitor is called as relative permittivity or dielectric constant.

OR

It is defined as the ratio of the capacitance of the given dielectric material to the capacitance of the air.

The relative permittivity is unit less.

### Which parameters affect the value of capacitance?

Parameters affecting the capacitance

The area of conducting plates, distance between two conducting plates ( thickness of the dielectric material ) and relative permittivity of the dielectric affecting the value of the capacitance

( C α A /d )

### How dielectric constant affects the value of capacitance?

Dielectric constant

The value of the capacitance is directly proportional to the dielectric constant of the dielectric material placed between two conducting plates.

Therefore higher the value of the dielectric constant means large value of capacitance and vice versa.

### Which type of capacitor have positive, negative and zero value of temperature co – efficient?

Ceramic capacitor

### Which type of capacitor has smallest size ( of the same rating )? Why?

The ceramic capacitor has smallest size to that of other types of capacitors due to high permittivity of the ceramic dielectric material.

Which type of capacitor works only on the DC supply? What is happened when it relates to incorrect polarity?

The electrolyte capacitor works only on the DC supply. If it is connected with incorrect polarity, gas forms within the electrolyte and it may burn out.

### Why energy is stored in the capacitor during the charging?

Energy stored in the capacitor

When the potential difference is applied to the two parallel plates, charges moves from positive plate to the negative plate.

However these charges have to work done against repulsive force due to some charge already collected on the capacitor plates.

Therefore some energy is stored in the capacitor during charging of capacitor.

### Define: Time constant in the RC series circuit

Time constant

It is defined as the time during which the voltage across the capacitor rises to 63.2% of the final steady value.

OR

It is defined as the time during which the current falls to 36.8% of its initial maximum value.

The time constant affect the charging time of the RC series circuit. Higher the value of the time constant, longer time to charge the capacitor and vice versa.

### Explain : The unit of RC time constant is second.

The time constant of the capacitor  λ = RC

= ( V / I )( Q / V )

= Q / I

=  I × t / I

= t

Therefore the unit of time constant is second

### Give reason : The angle between voltage vector and current vector for the capacitor circuit is not 900.

Due to dielectric loss

### Describe the effect of frequency on the dielectric loss.

Effect of frequency on dielectric loss

The dielectric loss directly proportional to supply frequency. As the frequency increases the dielectric loss also increases.

### Is there any capacitance possible for an isolated sphere?

Isolated sphere

The isolated sphere acts as one plate only whereas the earth acts as second plate or reference plate. Therefore there is always capacitance of an isolated sphere.

C = 4πε0r

r = Radius of sphere ( Meter )

ε0 = Absolute permittivity ( Farad / meter )

Increases

### The voltage across capacitor is directly proportional to current. True or false

The voltage across capacitor is directly proportional to charge not current.

### Explain : The capacitor behaves as open circuit on DC supply.

Capacitor as open circuit

When the DC supply is given to the capacitor, the current through it becomes zero.

Therefore it acts as open circuit.

i = C ( dv/dt )

The current in the capacitor flows only when the voltage is change with time.

### The charging of capacitor C is done through 24 V battery through series resistance R. Find out time for the capacitor to charge 24 V?

The capacitor charged up to 100% supply voltage in 5 times time constant. Therefore the charging time is 5λ = 5RC.