Q. When two charges are kept in air medium, at certain distance apart, the force between them is . When they are kept in a dielectric medium at the same distance of separation, the force between them becomes . Then the dielectric constant of the medium is

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Solution:

According to Coulomb's law, force between two charges at a certain distance apart in air medium is given as,

where, is the permittivity of free space.
When these charges are kept in a dielectric medium, then force between them is given as
.....(ii)
where, is the dielectric constant.
Given,