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Could someone remind me of the formula for potential function for a system consisting of 2 coaxial tubes maintained at a fixed potential difference and the in between medium has uniform conductivity $c$? (I have forgotten how to calculate these things, have been a long time since high school!)

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Formula:$$V(r)=V_0\frac{(r-r_1)(r_1r_2)}{(r_2-r_1)(r_1r)}$$

How to derive: I found the boundary value of current by integration, substituted in ohm's law, and integrated once more.

Remember that current varies inversely to r.

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I've not cancelled the $r_1$ on purpose, it makes the origin of the equation more apparent. –  Manishearth Feb 28 '12 at 16:40
    
And you can easily see that pd is $\propto \Delta\frac{1}{r}$ –  Manishearth Feb 28 '12 at 16:47
    
Thank you, Manishearth. –  fossil Feb 28 '12 at 17:46

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