Question

how does the force between two point charges change,if dielectric constant of the medium in which they are kept, increases?

Answer 1

Electrostatic force depends on magnitude of charges, distance between the charges and material between them.

The dielectric constant does not change because, it is a constant. But the relative permittivity can vary and the force is inversely proportional to it.

So, as permittivity of medium increases, the magnitude of force decreases

Answer 2

According to Coulomb's law the force of interaction between two stationary point like charges Q1 and Q2 separated by distance r in space, is given by

F=(1/4 pi eo) Q1Q2/r^2………….(1).

Here,eo is permittivity of space.

If the charges are placed in a dielectric medium having permittivity e, then the force between them will be

F'=(1/4pi e)Q1Q2/r^2……………….(2)

Here, e is permittivity of the dielectric medium.

The ratio of permittivity of medium to permittivity of space is called relative permittivity er =(e/eo).

er is also called dielectric constant of the medium and is denoted by K.

Now, in equation (2) if we replace e by er.eo, then it can be seen clearly that the force between the charges in medium is (1/er) or (1/K) times the force between them in space.

So greater the dielectric constant less the force between the charges.

The discussion given above is applicable to linear dielectric medium in which the polarization P is proportional to the externally applied electric field,E. If electric field is made stronger, the non linear terms in the relation between P and E become important and effective dielectric constant increases and then force between the charges placed in such medium decreases