The force of repulsion between two point charges is $F$, when they are $d$ distance apart. If the point charges are replaced by conducting spheres each of radius $r$ and the charges remain same. The separation between the centre of sphere is $d$, then force of repulsion between them is

A small spherically symmetric charge $q$ is placed at one vertex of a cube as shown. The flux through the faces $ABCD$ and $HGEF$ are, respectively,

Charges $Q, 2Q$ and $4Q$ are uniformly distributed in three dielectric solid spheres $1, 2$ and $3$ of radii $\frac{R}{2}, R$ and $2R$ respectively, as shown in the figure. If magnitudes of the electric fields at point P at a distance R from the centre of spheres $1, 2$ and $3$ are $E_1\mathit{,} E_2$ and $E_3$ respectively, then