The energy stored in the electric field produced by a metal sphere is $4.5 J$. If the sphere contains $4 µC$ charge, its radius will be : [Take : $\frac{1}{4 \pi \in_0} = 9 \times 10^9 N - m^2/C^2$]

Question

$\begin{array}{1 1}(1)\; 20\; mm \\ (2)\; 32\; mm \\(3)\; 28\; mm\\(4)\; 16\; mm \end{array} $