Q89 / 90 · JEE Main 2021 (25 Feb Shift 2) · Physics

Two identical conducting spheres with negligible volume have $2.1 nC$ and $- 0.1 nC$ charges, respectively. They are brought into contact and then separated by a distance of $0.5 m$ . The electrostatic force acting between the spheres is ___ $\times 10^{- 9} N$ .
[Given : $4 π ε_{0} = \frac{1}{9 \times 10^{9}}$ SI unit]

Correct answer: 36

Solution

$q = \frac{(2.1 - 0.1)}{2} nC = 1 nC$

$f = \frac{9 \times 10^{9} \times 10^{- 18}}{{(0.5)}^{2}} = 36 \times 10^{- 9}$

Hence, the correct answer is 36.