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Question : 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?

Doubt by Safir 

Solution : 

Let F = Original Force between the two charges.
F' = Force between the same two charges when the plastic sheet is inserted between the two charges.
K = Dielectric Constant of Plastic Sheet

We Know
K = F/F'
F'=F/K

So the force between the two charges will reduce.