Simple electrical field problem

AI Thread Summary
The discussion revolves around calculating the charge q of a metal sphere surrounded by a concentric metal shell with given charges. The tutor initially suggested that q equals -55 micro coulombs, but the user disagreed, arguing that q should be -25 micro coulombs due to the lack of an electric field inside the shell. Other participants confirmed that the charge on the outer surface does not affect the inner sphere, supporting the user's reasoning. Ultimately, the user decided to dismiss the tutor based on the clarification received. The conversation highlights the principles of electrostatics related to conductors and charge distribution.
cyberstudent
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A metal sphere of radius R with a charge q is surrounded by a concentric metal sphere as shown in Figure P.23. The outer surface of the the sperical shell has a charge of + 30.0*10^-6C and the inner surface of the shell has a charge of +25.0*10^-6C.
(a) Find q
(b) Sketch qualitative graphs of (i) the radial electric field component Er and (ii) the electric potential V as functions of r.

I hired an online tutor who told me that q = -(25+30) micro coulombs
= -55 micro coulombs.

I disagree.

Shouldn't q=-25 micro coulombs? There is no electric field inside the shell, since it is a conductor. The charge residing on the outer surface of the shell therefore should not have any effect on the sphere inside the concentric shell.

Should I fire my tutor or can I have confidence in his answers?

P.S. Diagram attached
 

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cyberstudent said:
A metal sphere of radius R with a charge q is surrounded by a concentric metal sphere as shown in Figure P.23. The outer surface of the the sperical shell has a charge of + 30.0*10^-6C and the inner surface of the shell has a charge of +25.0*10^-6C.
(a) Find q
(b) Sketch qualitative graphs of (i) the radial electric field component Er and (ii) the electric potential V as functions of r.

I hired an online tutor who told me that q = -(25+30) micro coulombs
= -55 micro coulombs.

I disagree.

Shouldn't q=-25 micro coulombs? There is no electric field inside the shell, since it is a conductor. The charge residing on the outer surface of the shell therefore should not have any effect on the sphere inside the concentric shell.

Should I fire my tutor or can I have confidence in his answers?

P.S. Diagram attached
I can't see your diagram yet, but it sounds like you are correct. The charge on the outermost surface has no effect. The net charge from the inner surface of the outer shell and everything inside is zero.
 
Thanks

I can't see your diagram yet, but it sounds like you are correct. The charge on the outermost surface has no effect. The net charge from the inner surface of the outer shell and everything inside is zero.

Great. Thanks for the confirmation. He is fired. :biggrin:
 

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