Question

What is the force between two small charged spheres having charges of $2\times {10}^{-7}$C and $3\times {10}^{-7}$C placed 30 cm apart in air?

Answer 1

Charge on the first sphere, $q_1=2\times {10}^{-7}$ C

Charge on the second sphere, $q_2=3\times {10}^{-7}$ C

Distance between the spheres, $r=30cm=0.3m$

$F=\frac{q_1{q}_2}{4\pi {\epsilon }_0{r}^2}$

Where, ${\epsilon }_0=$ Permittivity of free space

$\frac{1}{4\pi {\epsilon }_0}=9\times {10}^{9}N{m}^2{C}^{-2}$

$F=\frac{9\times {10}^9\times 2\times {10}^{-7}\times 3\times {10}^{-7}}{(0.3{)}^2}=6\times {10}^{-3}N$

Hence, force between the two small charged spheres is $6\times {10}^{-3}$N. The charges are of same nature. Hence, force between them will be repulsive.