Materials which do not allow current to flow through them are called insulators or dielectrics. While the
electrons in such materials remain bound within their individual molecules, thus preserving the overall neutrality of each molecule, they are affected by external electric fields because the positive and negative charges tend to shift in opposite directions.
Dielectric substances are of two types
1. Non-polar dielectrics : In such dielectrics the centre of mass of all the positive charges (protons) in a
molecule coincides with the centre of mass of all the negative charges (electrons). Therefore, they are
not only electrically neutral but also possess zero dipole moments.
In the presence of an external field, the two centres of charge get slightly separated and each molecule
becomes a dipole, having a small dipole moment. This is because the protons experience a force in the
direction of the applied field while the electrons experience a force in the opposite direction. Thus in the
presence of a field the dielectric gets polarized.
2. Polar Dielectrics : In polar dielectrics the centre of mass of the protons in a molecule do not coincide
with the centre of mass of the electrons. This is due to the asymmetric shape of the molecule. Thus
each molecule behaves as a dipole having a permanent dipole moment.
In the presence of an external applied field, these dipoles tend to align themselves along the field and
their dipole moments may also increase.
(a) Dielectric Constant : Suppose a dielectric slab is placed in a uniform electric field E0 The electric field will polarize the slab, i.e., the positive charges of the molecules will be shifted slightly towards the right and the negative charges towards the left. The right surface of the slab gets positivelycharged and the left surface negatively charged. This sets up a field Ep in the opposite direction which is less than E0. The net field inside the slab becomes E = E0 - Ep The ratio
K = (E0 /E)
is called the dielectric constant of the material.
(b) Dielectric Strength : The dielectric strength of a dielectric is the maximum value of the electric field that can be applied to the dielectric without its electric breakdown, i.e., without liberating electrons from its atoms (or molecules).
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