In the shown figure the conductor is uncharged and a charge q is
placed inside a spherical cavity at a distance a from the
centre(C).Point $P$ and a charge $+Q$ are shown.
I want to find electric field due to induced charges on the inner
surface of cavity at point $P$.
I feel the $q$ charge inside the shell will induce $-q$ charge on the inner surface of the shell.So the inner surface of the shell will induce an electric field of magnitude $\dfrac{q}{4 \pi \epsilon_o c^2}$ at point $P$.Is my assumption correct?
Also,I want to find the electric potential due to charges on the inner surface of the cavity and $q$ at $P$.
I think it should be $\dfrac{k(-q)}{c}+\dfrac{k(q)}{x}$ where $x$ is distance between charge $q$ and point $P$.But the answer given in my textbook is $0$.I've no idea why!Its seems very strange.Can someone clarify?
Best Answer
For problems of this kind you need Gauss' law, saying that the electric flux through a closed surface is proportional to the charges contained inside that surface. Then you also need that inside of a conductor, there cannot be any electric field.
Taking these things together give you a route towards your first question. Please think about it yourself before continue to read.
For the second part it is not clear to me whether the blob of conducting material is uncharged before or after the point charge is put into the cavity. From the solution of your problem I think that the conductor is has been grounded shortly after the point charge was implemented.
You can again use Gauss' law to derive the result. Alternatively one can use that a spherical distribution of charges act to things on the outside like if the charges were concentrated in the middle.