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Dielectric in an Electric Field

Suppose a slab of dielectric material placed in the uniform electric field E0 set up between the plates of a charged parallel plate capacitor. The slab becomes electrically plate capacitor. The slab becomes electrically polarized. The polarization charges inside the body of dielectric cancel other because the negative side of one polarized molecule is adjacent to the positive side of its neighbour. Thus the net charge in the interior of the slab remains zero. The charges at the ends of dielectric do not cancel. These polarization charges are called bound charges as these can. Not be conducted away by a conductor. The charges on the capacitor plates are called free charges. The polarization charges induced on the two faces of the slab produce their own electric field E’, which opposes the external field, E0. The induced electric field E’ is smaller than the applied electric field E0. Hence the resultant field E within the dielectric is smaller than E0, but points in the same direction as E0. The field in the rest of the space is still E0. Hence we conclude that when a dielectric is placed in an electric field, the field ‘within’ the dielectric is weakened.

The reduction in the magnitude of the electric field from E0 to E caused a reduction in the potential difference between the plates of the capacitor. Let V0 and V be the potential difference without dielectric and with dielectric completely filling space between the plates. Then we have
            E0/E = V0/V                     . : E = V/d

But V0/V = C/C0 = K, where K is the dielectric constant of the slab.
            E0/E = K

Thus the electric field within the dielectric is reduced by a factor K.

Dielectric slab

Dielectric in an Electric Field

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