1
$\begingroup$

I came across this question :

A thin metallic spherical shell contains a charge $Q$ on it. A point charge $q$ is placed at the Centre of the shell and another charge $q1$ is placed outside it. All three charges are positive. What is the force on the charge at the Centre?

Answer options are: - (a) towards left (b) right (c) up (d) zero

According to solution : - its (d) zero

Now I am confused as I know that field inside the shell is zero but there will be field from q1 which will have force F towards left right. So searching for an answer , I came across a point that shell acts as a faraday cage, thus blocking the field outside it.

So I would like you to explain the shielding effect of shell and also if this works only for conducting shells or does it work for non-conducting shells too?

Improve this question
$\endgroup$
3
  • $\begingroup$ Welcome to Physics Stack exchange! It seems there is a mismatch between your question body and actual title. In the question body, you've accepted that it does and you are confused about the case with placing a charge inside it with an external charge outside. I have now edited your question to improve searchability and also that more people will get help from the question you've asked. One personal suggestion I have is to not use a lot of acronyms in the posts so as to make the question more appealing to future readers. Again, a very welcome! :) $\endgroup$
    – Brian
    Commented Apr 27, 2021 at 18:53
  • $\begingroup$ @buraian thanks mate I really appreciate the help I am very newbie here. $\endgroup$
    – HelpAyoungPhysicist
    Commented Apr 27, 2021 at 18:54
  • 1
    $\begingroup$ Check the videos on Khan academy on this , it may help you see here $\endgroup$
    – Brian
    Commented Apr 27, 2021 at 19:04

1 Answer 1

Reset to default
2
$\begingroup$

I'm going to use the following Corollary (For proof see Section 3.3 Electricity and Magnetism by Purcell)

In the space inside a hollow conductor of any shape whatsoever, if that space itself is empty of charge, the electric field is zero.

Now suppose if the charge $q$ weren't present in the shell, then the field inside this shell would be zero, due to the uniqueness theorem as give above. This fact is independent of whatever is going on with charge $q_1$ or $Q$. If we now reintroduce $q$ at the center of the shell, this induces a total charge $-q$ on the surface of the cavity. This charge is uniformly distributed over the surface because $q$ is located at the center. This charge, therefore, doesn't change the fact that the field is zero at the center of the cavity. The force on $q$ is therefore zero.

P.S. this would not work for a non-conducting shells.

Improve this answer
$\endgroup$
1
  • $\begingroup$ from what i have learned is that due the symmetry and inverse square law the net electric field for any point inside the shell comes out to be 0. So if the point was not at Centre but a diff point inside the shell then would it change anything? I mean the force acting on the charge wud still be zero? $\endgroup$
    – HelpAyoungPhysicist
    Commented Apr 27, 2021 at 18:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.