# A question in electrostatics

1. Sep 18, 2008

### loui1410

Hello,

I have my first exam tomorrow in Electricity & Magnetism and I really need to know whether I solved this question correct or not, and I do not own the answer key to the book which I took the question from. Thanks in advance!

Note: English is not my mother-tongue and neither do I study physics in English. That is, I have translated the following question. Excuse me for any errors in the use of scientific terms.

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A spherical conductor, whose radius is 10cm, carries a charge of +8*10^-8 C, and another spherical conductor, whose radius is 6cm, is placed inside the first one and shares with it the same centre. The inner sphere is grounded.
1- Calculate the charge that is on the inner sphere.
2- Calculate the potential that is on surface of the outer sphere.
3- Calculate the potential and the strength of the electric field at 4cm distance from the centre of the two spheres.
4- Calculate the potential and the strength of the electric field at a point distanced 20cm from the centre of the two spheres.
5- Calculate the required work in order to transfer a charge of -5*10^-6 from the surface of the outer sphere to the surface of the inner sphere.

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I have studied the required material in a really short period of time, so I am a bit confused at the moment and might ask really stupid questions, and my following attempts for solving the question may be too dumb :P

1- I there is no charge on the inner sphere because it is grounded.

2- V = k*q/r
V = 9*10^9 * (8*10^-8)/(0.1)
V = 7200 V

Is that correct? :S

3- I am confused about this one. If my answer to section 1 was correct, aren't the potential and the strength of the electric field supposed to be zero as well?

4- V = k*q/r
V = 9*10^9 * (8*10^-8)/(0.2)
V = 3600 V

E = V/r = 18000 V/m

5- W = q(V-V') = -5*10^-6 * (7200-0) = -0.036 J