Dielectric Material : How Does The Electric Field Inside A Dielectric Decreases

In this article I will discuss briefly about dielectric material, dielectric constant, electric permittivity and how does coulomb’s law change in different dielectric mediums.

According to coulomb’s law, if two positive charges q0 and q are placed in an empty space and the distance between them is r, they will repel each other. The magnitude of the repulsive force is ,

The electric field E is the coulomb’s force on unit charges. That is , the coulomb’s force if q = 1.

If a dielectric is present between these two charges, the coulomb’s force and the electric field will be different. Let us consider an uniform electric field in an empty space created by two long oppositely charged plane.

Let us a put a dielectric material in the empty space. A dielectric material is often called an insulator. An insulator doesn’t allow current to pass. Because there are no free electron inside an insulator as they are non-metal element.

An atom has a positive nucleus and electron cloud ( electrons orbiting around the nucleus). If a negative charge is placed in an electric field, a coulomb’s force will be applied on it with a direction opposite to the electric field.

So, if an atom is placed inside an electric field, its electron cloud will feel an force opposite to the electric field. So, the electrons will be shift against the electric field. Therefore, the atom will be polarized.

Thus, there will be a net negative charge ẟ- in one side of the atom and a net positive charge ẟ+ on another side of the atom. So, when an insulator is put inside an electric field, all of its atom will be polarized.

Now there is a net negative charge on the left side of the insulator and a net positive charge on the right side of the insulator.

All other positive and negative charges inside the insulator cancel each other. These net negative and positive charges are called induced charge. This induced charge will create an electric field inside the dielectric. This electric field is called induced electric field. In the following figure, only induced charges are shown.

The new induced electric field Eᵢ and the external electric field E₀ are opposite in direction. So, the net electric field inside the insulator is ,

E = E₀ - Eᵢ …………(1)

From the equation (1) , we can see that the net electric field E inside a dielectric is always less than the electric field in empty space E₀.

So, E₀ > E

Now the dielectric constant K is the ratio between E₀ and E.

K = E₀/E .

As E₀ > E so, K>1. For empty space, E₀ = E. So, K = 1 for empty space.

So, if a charge q0 is put in an dielectric medium with a dielectric constant K, the electric field E will be 1/K times the electric field inside empty space E₀.

We define ε = ε0*k as the electric permittivity of the medium.

As the coulomb’s force F = E*q . So,

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