(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Consider a thin spherical shell having surface charge density σ, and radius a as shown in the diagram (see attachment).

Find, by integration over the sphere, the potential at a point P, a distance r fromthe centre of the sphere, for r > a. Using this result also find the electric filed at the same point. Hence verify that for points outside the sphere the charge acts as if it were all concentrated at the centre of the shell.

NB: Do not use Gauss' Law in answering this question!

(Hint: consider the spherical shell to be built up from infinitesimal rings of charge, centred on their axis of symmetry through OP):

2. Relevant equations

n/a

3. The attempt at a solution

I thought that calculating the electric field first might be easier, as we've done roughly the same problem with only one ring of charge; this gave the result E = (Q*r)/((4*Pi*Epsilon)*(a^2+r^2)^1.5).

What I've done so far for the above problem is to set the surface charge density as Q/4*Pi*a^2.

The electric field as I've written it down so far is:

dE = (2*σ*dS*cos(theta))/(4*Pi*Epsilon*(a^2+r^2)).

I've simplified this equation quite a bit, writing it as a surface integral, pulling the constants out of the integral, etc. until I ended up with this:

E = Q/(8*Pi^2*Epsilon)*integral((r*dS)/(a^2*(a^2+r^2)^0.5)).

Now, what I want to know is whether I'm on the right path at all, or if I've missed out something; also, if this is correct, how do I set up the surface integral? There's two parts to it, and one is obviously the rings and their radius going from 0 to a, but what's the other bit?

Any help on this would be very much appreciated.

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# Homework Help: Electromagnetism II - Potential and Electrical Field of a Spherical Shell

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