Two charges in air experience a force of 20 N. If the space between them is filled with a medium of dielectric constant K = 4, the new force will be

5 N

The correct answer is option (2) : 5 N

We know, $K =\frac{ε}{ε_0}$

and in case of medium, $F'=\frac{1}{4\pi ε}\left(\frac{Q_1Q_2}{r^2}\right), ε=Kε_0$

$⇒F'=\frac{1}{4\pi Kε_0}\left(\frac{Q_1Q_2}{r^2}\right)⇒F'=\frac{F}{4}=\frac{20}{4}=5\, N$