The compensating winding in a dc machine

Show via the construction of a suitable Gaussian surface, that the capacitance of a spherical capacitor consisting of two concentric shells of radii $$a$$ and $$b$$ is given by $$C = 4\pi {\varepsilon _0}{{ab} \over {\left( {b - a} \right)}}$$ where $${\varepsilon _0}$$ is the free space permittivity.

An electron with velocity $$'u'$$ is placed in an electric field, $$E$$ and magnetic field, $$B.$$ The force experienced by the electron is given by

A current $${\rm I}$$ in the short conducting element shown in Fig. produces a flux density $${B_1}$$ at point $$1.$$ Determine the magnitude and the direction of the flux density vector at point $$2$$