Two equal balls having equal positive charge ‘q’ coulombs are suspended by two insulating strings of equal length. What would be the effect on the force when a plastic sheet is inserted between the two? [CBSE AI 2014]

Force will decrease

Reason: Force between two charges each ‘q’ in vacuum is F_o = {1\over 4 \pi\varepsilon_0}{q^2\over r^2}

On inserting a plastic sheet (a dielectric K > 1)

Then F = {1\over 4 \pi\varepsilon_0K}{q^2\over r^2} i.e F = {F_o\over K}