Finding electric potential from a point charge using integration

[JamesL]

Here is the problem:

http://uploads.offtopic.com/files/physprob15.bmp

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The way we were taught in class so far to find the potential was using the integral of E dot dS. Which works itself down to (kq/r) evaluated from the r you are looking for to infinity.

In which case, V would equal 2.15059 v. But this answer is incorrect according to my homework service.

Any ideas?

 

[Tide]

You should have three different expressions for the electric field for the three regions. Did you account for that in your integration to infinity?

 

[JamesL]

You should have three different expressions for the electric field for the three regions. Did you account for that in your integration to infinity?

no, i don't think so. could you explain more?

 

[Tide]

In the region $1.4 < r < 4.2$ the electric field is determind only by the charge on the inner sphere. In $4.2 < r < 7$ the electric field is determined by both the charge on the inner sphere AND the inner surface of the shell. And outside the configuration $r > 7$ the electric field is due to the charge on the inner sphere and both the inner and outer surfaces of the shell.