Two point charges $q_1$ and $q_2$ are '$l$' distance apart. If one of the charges is doubled and the distance between them is halved. The magnitude of the force becomes 'n' times, where 'n' is

In a parallel plate air capacitor of plate separation '$d$', a dielectric slab of thickness '$t$' is introduced between the plates. The capacitance becomes one-third of the original value. The dielectric constant of the slab will be

A parallel plate air capacitor having area of each plate '$A$' and the distance between the plates '$d$' has uniform electric field E in the space between the plates. The energy stored in the capacitor is ($\epsilon_0$ = permittivity of free space.)

In meter bridge experiment, two resistances X and Y in the two gaps give a null point dividing the wire in the ratio $2 : 3$. When each resistance is increased by $30\,\Omega$, the null point divides the wire in the ratio $5 : 6$. The resistance X and Y are respectively