Two identical small spheres carry charge of $Q_1$ and $Q_2$ with $Q_1>>Q_2.$ The charges are $d$ distance apart. The force they exert on one another is $F_1.$ The spheres are made to touch one another and then separated to distance $d$ apart. The force they exert on one another now is $F_2.$ Then $F_1/F_2$ is
$\frac{{4{Q_1}}}{{{Q_2}}}$
$\frac{{{Q_1}}}{{4{Q_2}}}$
$\frac{{4{Q_2}}}{{{Q_1}}}$
$\frac{{{Q_2}}}{{4{Q_1}}}$
A hollow metal sphere of radius $5\, cm$ is charged so that the potential on its surface is $10\, V$. The potential at the centre of the sphere is.....$V$
A charge $q$ is placed at $O$ in the cavity in a spherical uncharge $d$ conductor. Point $S$ is outside the conductor. If the charge is displaced from $O$ towards $S$ still remaining with in the cavity,
The force on a charge situated on the axis of a dipole is $F$. If the charge is shifted to double the distance, the new force will be
A parallel plate capacitor is charged to a potential difference of $100\ V$ and disconnected from the source of emf. A slab of dielectric is then inserted between the plates. Which of the following three quantities change?
$(i)$ The potential difference
$(ii)$ The capacitance
$(iii)$ The charge on the plates
The work done required to put the four charges together at the corners of a square of side $a$ , as shown in the figure is