The ratio of the relative strengths of the gravitational force and the electromagnetic force between two charged particles is

Four charges are arranged at the corners of a square $ABCD$,as shown in the adjoining figure. The force on the charge kept at the centre $O$ is

Two identical simple pendulums each of length $L = 5 \, cm$ are suspended from the same support. When the bobs are given an equal charge of $q = 2 \, \mu C$ each, the distance between the bobs becomes $d = 6 \, cm$. Find the mass $m$ of each bob. (Take $g = 10 \, m/s^2$ and $k = 9 \times 10^9 \, N \cdot m^2/C^2$). (in $ \, kg$)

Two identical conducting spheres,having charges of opposite sign,attract each other with a force of $0.108 \ N$ when separated by $0.5 \ m$. The spheres are connected by a conducting wire,which is then removed,and thereafter,they repel each other with a force of $0.036 \ N$. The initial charges on the spheres are

Two charges are placed as shown in the figure. Where should a third charge be placed so that it remains at rest?

Find the value of $q$ so that the net electrostatic force on the $10 \mu C$ charge placed at a vertex of the square is zero. The charges are arranged as shown in the figure.