# Electric Potential: Two Spherical Shells

Bryon

## Homework Statement

A point charge q = -8 µC is surrounded by two thick, conducting spherical shells of inner and outer radii a1 = 0.3 m, a2 = 0.4 m, a3 = 0.7 m, and a4 = 0.8 m respectively. The inner shell is uncharged; the outer shell has a net charge Q = -10 µC. At this point in the problem, the potential at infinity is unspecified.

## Homework Equations

Vb - Va = ∫E∙dl = kQ(1/b - 1/a)

## The Attempt at a Solution

Va4 - Vinf[/SUB] = -202275V OK
Va3 - Va4 = 0V OK
Va2 - Va3 = -77057.14286V OK
Va1 - Va2 = 0V OK
V(0.15) - Va1 = -239733.3333V

The final part of the problem asks this:

If now you are given V(inf) = 1.4 x 10^5 V, find the potential at r = 0.15 m.

I tried to do it like this:

-237933.333V + 1.4x10^5 = -99733.3333V which is incorrect. Do I have to go and find each one with respect to v(inf) = 1.4x10^5? Or do I need to find the total charge of the outermost shell and then find the potential from there to 0.15m?

## Answers and Replies

Staff Emeritus
Homework Helper
Gold Member
-237933.333V + 1.4x105 V = V(0.15) - Va1 + V, which isn't what you want.

It appears to me that what's being asked for is V(0.15) ‒ V .

You need (V(0.15) ‒ Va1) + (Va1 ‒ Va2) + (Va2 ‒ Va3) + (Va3 ‒ Va4) + (Va4 ‒ V).

I believe that your first answer, Va4 - Vinf[/sub] = -202275V, assumed that the potential at infinity was unspecified, so you used zero.