A lead bullet moving with velocity ' V ' strikes a wall and stops. If $75 \%$ of its energy is converted into heat, then the increase in temperature is ( $\mathrm{s}=$ specific heat of lead, $\mathrm{J}=$ mechanical equivalent of heat)

In photoelectric effect, the photocurrent

The average force applied on the wall of a closed container depends as $\mathrm{T}^{\mathrm{x}}$ where T is the temperature of an ideal gas. The value of $x$ is

Four electric charges $+\mathrm{q},+\mathrm{q},-\mathrm{q}$ and -q are placed in order at the corners of a square of side 2 L. The electric potential at point midway between the two positive charges is