A dielectric slab

When a dielectric slab of dielectric constant K is introduced between the plates of the capacitor, the net electric field in the dielectric becomes

Where,

E₀ is the electric field when there is vacuum between the plates.

K is the dielectric constant of the dielectric

The net electric field is due to charges +Q, -Q and due to induced charges +Q’,-Q’in opposite direction).

So, the net electric field becomes

Where,

Q’=induced charge due to dielectric

Q=charge on the capacitor in vacuum

K=dielectric constant

Since dielectric constant K>1

Q’<Q

Therefore, on inserting a dielectric slab between plates of capacitor the induced charge Q’ is less than Q.