1
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Define: Capacitance
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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
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2
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Explain the term : Relative Permeability
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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.
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3
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Which parameters affect the value of capacitance?
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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 )
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4
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How dielectric constant affects the value of
capacitance?
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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.
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5
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Which type of capacitor have positive, negative and
zero value of temperature co – efficient?
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Ceramic capacitor
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6
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Which type of capacitor has smallest size ( of the
same rating )? Why?
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The ceramic capacitor has smallest size to that of
other types of capacitor due to high permittivity of the ceramic dielectric
material.
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7
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Which type of capacitor works only on the DC supply?
What is happened when it is connected with incorrect polarity?
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- 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.
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8
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Why energy is stored in the capacitor during the
charging?
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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.
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9
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Define: Time constant in the RC series circuit
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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.
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10
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Explain : The unit of RC time constant is second.
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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
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11
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Give reason : The angle between voltage vector and
current vector for the capacitor circuit is not 900.
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Due to dielectric loss
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12
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Describe the effect of frequency on the dielectric
loss.
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Effect of frequency on dielectric loss
- The dielectric loss directly proportional to supply
frequency.
- As the frequency increases the dielectric loss also increases.
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13
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Is there any capacitance possible for an isolated
sphere?
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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 )
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14
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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?
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Increases
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19
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The voltage across capacitor is directly
proportional to current. True or false
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The voltage across capacitor is directly
proportional to charge not current.
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20
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Explain : The capacitor behaves as open circuit on
DC supply.
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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.
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21
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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?
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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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