Question

A force $F$ acts between sodium and chlorine ions of salt (sodium chloride) when put $1cm$ apart in the air. The permittivity of air and dielectric constant of water are ${\epsilon }_0$ and $K$ respectively. When a piece of salt is put in water electrical force acting between sodium and chlorine ion $1cm$ apart is

• A. $F$
• B. $FK/{\epsilon }_0$
• C. $F/K{\epsilon }_0$
• D. $F/K$

Answer 1

Answer: (D)

Force in air $F_a=\frac{1}{4\pi {\epsilon }_0}\frac{q_1{q}_2}{r^2}$
Force in dielectric medium $F_m=\frac{1}{4\pi {\epsilon }_{0}K}\frac{q_1{q}_2}{r^2}$
or $F_m=\frac{1}{K}\left(\frac{1}{4\pi {\epsilon }_0}\frac{q_1{q}_2}{e^2}\right)$
$=\frac{F}{K}(\because r$ is constant in both conditions $)$