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 : $$4\pi \epsilon_{\circ}$$ = $$ \frac{1}{9\times 10^{9}}$$ SI Units}]

When two identical conducting spheres are brought into contact, the total charge is shared equally between them. The total charge is $$2.1 + (-0.1) = 2.0$$ nC. So each sphere gets $$\frac{2.0}{2} = 1.0$$ nC $$= 1.0 \times 10^{-9}$$ C.

After separation by distance $$r = 0.5$$ m, the electrostatic force between them is $$F = \frac{kq_1 q_2}{r^2} = \frac{9 \times 10^9 \times (1.0 \times 10^{-9})^2}{(0.5)^2}$$.

$$F = \frac{9 \times 10^9 \times 10^{-18}}{0.25} = \frac{9 \times 10^{-9}}{0.25} = 36 \times 10^{-9}$$ N.

Therefore, the electrostatic force is $$36 \times 10^{-9}$$ N.