A particle of mass 0.5 g and charge $10 \mu \mathrm{C}$ is subjected to a uniform electric field of $8 \mathrm{NC}^{-1}$. If the particle is initially at rest, the velocity of the particle after a time of 5 s is

125 identical charged small spheres coalesce to form a big charged sphere. If the electric potential on each small sphere is 60 mV , then the electric potential on the bigger sphere formed is

Two particles of charges 4 nC and $Q$ are kept in air with a separation of 10 cm between them. If the electrostatic potential energy of the system is $1.8 \mu \mathrm{~J}$, then $Q=$