Draw a labelled diagram of Van de Graff generator. State its working principle to show how by introducing a small charged sphere into larger sphere, large amount of charge can be transferred to the outer sphere. State the use of this machine and also point out its limitations.
OR
(a) Deduce the expression for the torque acting on a dipole of dipole moment $\overrightarrow{\mathit{p}}$ in the presence of a uniform electric field $\overrightarrow{\mathrm{E}}$.
(b) Consider two hollow concentric spheres, ${\mathrm{S}}_1$ and ${\mathit{S}}_2$, enclosing charges $2\mathit{Q}$ and $4\mathit{Q}$ respectively as shown in the figure.
(i) Find out the ratio of the electric flux through them.
(ii) how will the electric flux through the sphere ${\mathit{S}}_1$ change if a medium of dielectric constant ' ${\mathit{\epsilon }}_{\mathit{r}}^{\prime }$ ', ' ${\mathit{\epsilon }}_{\mathit{r}}$ ', is introduced in the space inside ${\mathit{S}}_1$ in place of air?
Deduce the necessary expression.