Two metal spheres, one of radius $$\frac{R}{2}$$ and the other of radius $$2 \mathrm{R}$$ respectively have the same surface charge density They are brought in contact and separated. The ratio of their new surface charge densities is
When $$10^{19}$$ electrons are removed from a neutral metal plate, the electric charge on it is
Two point charges $$20 \mu \mathrm{C}$$ and $$-10 \mu \mathrm{C}$$ are separated by a distance of $$1 \mathrm{~m}$$ in air. At what point on the line joining the two charges, the electric potential is zero.
A spherical metal ball of density '$$\rho$$' and radius '$$r$$' is immersed in a liquid of density '$$\sigma$$'. When an electric field is applied in the upward direction the metal ball remains just suspended in the liquid. Then the expression for the charge on the metal ball is :