Question

Please can someone tell me monopole factor for system of multipoles, extension of a dipole

Answer 1

The monopole term is just what you get by treating the source as a point charge, where the value of that charge is equal to the net charge in the system, so you'd have
\[\frac{ Qnet }{ 4 \pi \epsilon _{0}r }\] for the monopole contribution to the potential at r.
So, for example, if there were equal amounts of positive and negative charge, the monopole term would be zero and the first term in the multipole expansion would (probably) be the dipole term.

Answer 2

Thanks! Could you also help me in question regarding the sphere and the cavity