# Irodov's 3.101 Help Required

1. Sep 2, 2015

### Shreyas Shree

1. The problem statement, all variables and given/known data
Find the capacitance of an isolated ball-shaped conductor of charge q
with permittivity E and outside radius R2.

2. Relevant equations

3. The attempt at a solution

I haven't understood the very first line. Otherwise everything else is fine.
From the first line, it says that we take a Gaussian surface between R1 and R2, and hence we have a charge q enclosed and we integrate this from R2 to R1. This is also fine. But my problem lies in the step where we take a Gaussian surface outside the capacitor and we integrate it from infinity to R2. But how is the charge enclosed q, should'nt it be zero?

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2. Sep 2, 2015

### ehild

You integrate with respect to r, he distance from the centre, from R2 to infinity. And you need to consider the charge enclosed by the Gaussian surface of radius r.

3. Sep 2, 2015

### Shreyas Shree

aaaahhh! nice! Thank you very much