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physics question

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Lifer
Consider a conductor which contains a cavity. Suppose that there is a point charge
q within the cavity. Prove that the net charge on the wall of the cavity is -q.
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Obviously the net charge has to be -q because if you created a guassian surface inside of the conductor, the flux through that surface would have to be zero and that's only possible if the charge on the inner wall of the conductor is -q.

If the net charge on the conductor is Q. what is the net charge on its outer surface?
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This is the part that is confusing me. How did the conductor develop a net charge? And would that answer to the question be q regardless of the total net charge of the conductor?
 
It's just asking if the conductor were to have a net charge already, what would the charge on the surface be. And the answer would be 2Q. This is because the inner surface would have a charge of -Q like you said before so the outter surface must have charge 2Q so that the net charge is Q.

The first instance the outter surface would have a charge Q while the inner surface would have charge -Q so that the net charge of the conductor is 0
 
Originally posted by: Mo0o
It's just asking if the conductor were to have a net charge already, what would the charge on the surface be. And the answer would be 2Q. This is because the inner surface would have a charge of -Q like you said before so the outter surface must have charge 2Q so that the net charge is Q.

The first instance the outter surface would have a charge Q while the inner surface would have charge -Q so that the net charge of the conductor is 0
Click to expand...

Q? q
 
If q != Q, then it's just Q + q. You need the net charge on the conductor (which resides at its surfaces) to be +Q, so if you have -q at the inner surface, then -q + (Q + q) = Q.
 
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