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Physics Guru is an online education and competitive examinations preparation portal founded by Manish Verma, an IIT Madras alumnus, providing learning resources for competitive examinations.

The emphasis is on conceptual understanding, analytical thinking and problem solving rather than simply memorising formulas and standard methods.

The objective is to help students understand what they are learning and develop the ability to solve problems they have not seen before.

Capacitor Dielectric

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.