Consider a charged parallel plate capacitor without dielectric. An electric field exists directed from the positive plate toward the negative plate.
When a dielectric slab is brought in, its molecular response depends on its nature:
- If the molecules are non-polar, the external field displaces their electron clouds toward the positive plate, inducing dipole moments.
- If the molecules are already polar, the existing dipoles rotate to align with the field, with the negative end of each dipole orienting toward the positive plate and the positive end toward the negative plate.
After this reorientation, the dielectric can be visualized as a chain of aligned dipoles. A layer of bound negative charge accumulates adjacent to the positive plate, and a layer of bound positive charge accumulates adjacent to the negative plate.
These bound charges produce an internal electric field within the dielectric that opposes the original field. The superposition of the two fields results in a reduced net electric field inside the dielectric region.
Because the net field is reduced, the potential difference between the plates is reduced.
For a given charge on the plates, a lower potential difference corresponds to a higher capacitance. Therefore, the introduction of the dielectric slab increases the capacitance.