If on the concentric hollow spheres of radii $r$ and $R( > r)$ the charge $Q$ is distributed such that their surface densities are same then the potential at their common centre is
$\frac{{Q\left( {{R^2} + {r^2}} \right)}}{{4\pi \varepsilon _0\left( {R + r} \right)}}$
$\frac{{QR}}{{R + r}}$
Zero
$\frac{{Q(R+r)}}{{4\pi \varepsilon _0\left( {{R^2} + {r^2}} \right)}}$
A charged particle $'q'$ is shot from a large distance with speed $v$ towards a fixed charged particle $Q$. It apporaches $Q$ upto a closet distance $r$ and then returns. If $q$ were given a speed $'2v$', the closest distance of approach would be
A parallel plate capacitor has plates with area $A$ and separation $d$ . A battery charges the plates to a potential difference $V_0$. The battery is then disconnected and a dielectric slab of thickness $d $ is introduced. The ratio of energy stored in the capacitor before and after the slab is introduced is
A negative charged particle is released from rest in a uniform electric field. The electric potential energy of charge
Two point charges placed at a distance $r$ in air experience a certain force. Then the distance at which they will experience the same force in a medium of dielectric constant $K$ is
The resultant capacitance between $A$ and $B$ in the fig. is.....$\mu F$