A block of mass m having charge q, is hinged by a spring of spring constant k in a vertical electrostatic field E. The spring extension in equilibrium will be

$\frac{mg}{k}$ $\frac{qE}{k}$ $\frac{mg+qE}{k}$ $\frac{mg-qE}{k}$

$\frac{mg-qE}{k}$

From the Free Body Diagram qE + kx0 = mg $x_0=\frac{mg-qE}{k}$ ∴ (D)