Hey dear,
As you would be knowing that force for two charge bodies is given by
F=q1*q2/4πε0*r².
Now when a dielectric medium is inserted between them, ε0 changes its value by 'k' times.
Therefore the force now becomes
F=q1*q2/4πkε0 r²
Hence the force now is 1/k times the force without dielectric.
Hope this helps
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