We know that, E = V/d
So, as the
voltage on the capacitor is increased with a given thickness (d) or the
thickness (d) is reduced with a given voltage (V), the electric intensity E
increases.
This intensity
represents the force exerted on the charge on the molecules or the dielectric
material.
As E is
increased, the centre of the positive charges is pushed in the direction of E
and centre of the negative charges in the opposite direction.
Now, every
dielectric medium has its capacity to withstand the increasing E. If the
applied voltage and hence E is increased beyond a certain limit, then forces on
the molecules become sufficiently large. The electrons break away from the
molecules causing ionization and free charges.
The material
then conducts due to ionization and the charge recombine, thereby vanish from
the capacitor plates. The capacitor can no more hold the charge and is said to
be breakdown. The dielectric medium is said to be punctured and becomes useless
from using it as a dielectric.
The ability of
an insulating medium to resist its breakdown when a voltage is increased across
it, is called its dielectric strength.
This depends
upon the temperature of the material and presence of air pockets and
imperfections in the molecules arrangement of that material. It is generally
expressed in kV/cm of kV/mm.
Note: The
voltage at which the dielectric medium of the capacitor breakdown is known as
breakdown voltage of the capacitor.
The factors
affecting the dielectric strength are,
1. Temperature
2. Type of
material
3. Size,
thickness and shape of the plates.
4. Presence of
air pockets in the material.
5. Moisture
content of the material.
6. Molecules
arrangement of the material.
Dielectric
strength and dielectric constants of some materials are quoted below from
published literature.
The dielectric
strength varies as thickness of dielectric material hence the range of values
are given in the table . the value indicates that if material is subjected to
electric field more than specified dielectric strength then it will breakdown.
1.1 Dielectric
Leakage and Losses
If there is no
leakage of current in the dielectric and the insulation is perfect, then the
charge on the capacitor plates can be held on for hours.
The fact
however remains that the insulation resistance of most of the dielectric
materials is only of the order of megaohms and hence charge on the capacitor
leaks leaks away through the insulating material in a few minutes.
Note: In any
case, it is dangerous to touch a charge capacitor even after it is disconnect
from the supply.
In case of
d.c. a practical capacitor is considered to be a charge strong device in
parallel with a leakage resistance(R) as shown in the Fig 2.
Further, when
the voltage applied to the capacitor is alternating, due to molecular friction
of dipoles created in the material, the value of R becomes frequency dependent.
The loss due to molecular friction is called dielectric loss.
Read also
Capacitance of Three Core Cables
Capacitance Grading
Read also
Capacitance of Three Core Cables
Capacitance Grading