**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 90**^{0}.

^{0}.

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 =
4πε_{0}r

r = Radius of sphere ( Meter )

ε_{0}
= Absolute permittivity ( Farad / meter )

**The
capacitance of two parallel plates with uniform air as dielectric material is
C. What is effect of capacitance is dielectric material is inserted between two
plates? **

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.

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