Question

How does polarised dielectric modify the original external field inside it?

Answer 1

Consider a dielectric slab placed inside a uniform electric field E0. This field produces a uniform polarization shown in the figure. Any region inside the dielectric,the net charge is zero. This is due to the cancellation of positive charge of one dipole with negative charge of adjacent dipole. But the positive ends of the dipole do not cancel at right surface and the negative ends at the left surface. This surface charges produce a field opposite to the external field. Hence total electric field is reduced inside the dielectric field.

when a dielectric is placed an electric field is set up between the plates of the capacitor, in a direction opposite to the electric frield which was there before placing the dielectric.

due to this surface charge densities on the either sides of the plate is produced. the dielectric gets polarised due to induction and hence because of polarisation of the dielectric electric field is reduced overall.

Answer 2

Answer:

Explanation:

consider, for simplicity, a rectangular dielectric slab placed in a

uniform external field E0 parallel to two of its faces. The field

causes a uniform polarisation P of the dielectric. Thus every

volume element v of the slab has a dipole moment P v in the

direction of the field. The volume element v is macroscopically

small but contains a very large number of molecular dipoles.

Anywhere inside the dielectric, the volume element v has no net

charge (though it has net dipole moment). This is, because, the

positive charge of one dipole sits close to the negative charge of

the adjacent dipole.

However, at the surfaces of the dielectric normal to the electric

field, there is evidently a net charge density. As seen in Fig 2.23,

the positive ends of the dipoles remain unneutralised at the right

surface and the negative ends at the left surface. The unbalanced

charges are the induced charges due to the external field.

Thus, the polarised dielectric is equivalent to two charged

surfaces with induced surface charge densities, say p and –p.

Clearly, the field produced by these surface charges opposes the

external field. The total field in the dielectric is, thereby, reduced

from the case when no dielectric is present. We should note that

the surface charge density ±p arises from bound (not free charges) in the dielectric.