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As far as I know, Coulomb's law of electrostatic force is applicable on two different charges situated in same medium. But if two individual charges are in different media (say one charge on a iron surface another on a plastic surface), then how does one calculate the force between them?

And if I have wrong conception , please correct me.

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There are at least two (equivalent) ways:

1) Net surface charge would be induced at the interface between these two media as well as in the vicinities of the two point charges. You can then apply Coulomb's law to one of the point charge with the contributions from all other point and surface charges. This method requires integration.

2) You can use the method of images. If you google it, you will find many references on this method, however, usually applied to simple materials. The method is actually applicable to arbitrary materials. The essence is to introduce image charges and then match electromagnetic boundary conditions.

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As far as I know, Coulomb's law of electrostatic force is applicable on two different charges situated in same medium.

Only in fluids (gas, liquid). In solids, Coulomb's law cannot be directly applied because the solid contributes unknown amount of force itself (it maintains mechanical stress). One needs to carefully analyze forces in solids; usually people use stress tensor.

But if two individual charges are in different media (say one charge on a iron surface another on a plastic surface), then how does one calculate the force between them?

If the bodies with charges are separated by vacuum or a fluid (air, water), you can use Coulomb's formula with $\epsilon$ of the medium.

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