1

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

2

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.

3

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 )

4

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.

5

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


Ceramic capacitor

6

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


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

7

Which type of capacitor works only on the DC supply?
What is happened when it is connected with 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.

8

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.

9

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.

10

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

11

Give reason : The angle between voltage vector and
current vector for the capacitor circuit is not 90^{0}.


Due to dielectric loss

12

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.

13

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 )

14

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

19

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


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

20

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.

21

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 = 5 RC.

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