Question

A dielectric slab of thickness $d$ is inserted in an isolated charged parallel plate capacitor whose negative plate is at $x=0$ and positive plate is at $x=3d$. The slab is equidistant from the plates. As $x$ goes from $0$ to $3d$,

• A. The magnitude of the electric field remains the same.
• B. The direction of the electric field changes continuously.
• C. The electric potential increases continuously.
• D. The electric potential increases at first, then decreases and again increases.

Answer 1

The correct option is C The electric potential increases continuously.


In regions outside the dielectric $($ regions $\text{I}$ and $\text{III})$, the electric field $E_0$ is directed from positive to negative.

In region $\text{II}$, due to induced charges, there is an electric field $E_k$ which is opposite to $E_0$, resulting in a net electric field $E_0-E_k$ in the direction of $E_0(\text{since}{E}_0>E_k)$.

Hence, the magnitude of the electric field is not the same. However, its direction is from positive to negative plate.
Hence, options $\left(a\right)$ and $\left(b\right)$ are incorrect.

Since $E_0$ is directed from the positive to negative plate, and we know that potential increases opposite to the direction of the electric field, electric potential will increase continuously as we go from $x=0$ to $x=3d$.

Hence, option $\left(c\right)$ is correct.

Key concept: The electric field due to induced charges is opposite in direction to the original field but lesser in magnitude.