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
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5 N
By Coulomb's law, in a medium of dielectric constant \( K \) the permittivity becomes \( K\varepsilon_0 \), so the force between the same charges at the same distance is reduced \( K \) times.
\( \displaystyle F' = \frac{1}{4\pi K\varepsilon_0}\,\frac{q_1 q_2}{r^{2}} = \frac{F}{K} \)
\( \displaystyle F' = \frac{20\,\mathrm{N}}{4} = 5\,\mathrm{N} \)