A solid sphere made out of metal having a bulk modulus of $2\times {10}^{11} {\mathrm{Nm}}^{-2}$ is charged uniformly to a surface charge density $\sigma =\sqrt{\alpha {\epsilon }_0} {\mathrm{Cm}}^{-2}$, where ${\epsilon }_0$ is the permittivity of free space and $\alpha$ (magnitude = $1$) is a positive constant of appropriate dimensions. If the volume strain produced in the sphere after charging is $x\mathit{\times }{10}^{-11}$, then the value of $x$ is