Question:
Two identical conducting spheres with negligible volume have $2.1 \mathrm{nC}$ and $-0.1 \mathrm{nC}$ charges, respectively. They are brought into contact and then separated by a distance of $0.5 \mathrm{~m}$. The electrostatic force acting between the spheres is__________ $\times 10^{-9} \mathrm{~N}$.
[Given : $4 \pi \varepsilon_{0}=\frac{1}{9 \times 10^{9}}$ SI unit]
Solution:
$\mathrm{q}=\frac{(2.1-0.1)}{2} \mathrm{nC}=1 \mathrm{nC}$
$f=\frac{9 \times 10^{9} \times 10^{-18}}{(0.5)^{2}}=36 \times 10^{-9}$
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