Polarisation of Dielectric Material

Consider a parallel plate capacitor with +q and –q charges.

Let Eo be the electric field between the plates in air. If a dielectric slab is introduced in the space between them, the dielectric slab gets polarised. Suppose +qi and –qi be the induced surface charges on the face of dielectric opposite to the plates of capacitor (Fig. ). 

Fig. Polarization Of Dielectric Material

These induced charges produce their own field Ei which opposes the electric field Eo. 

So, the resultant field, E < Eo. 

But the direction of E is in the direction of Eo.

∴ E = Eo + (–Ei)

(∵ Ei is opposite to the direction of Eo)