# Homework Help: Jackson 3.7 Three point charges (q,−2q,q) are located in ...

1. Aug 20, 2016

### Pual Black

1. The problem statement, all variables and given/known data
Hi,
The question is
"
Three point charges (q,−2q,q) are located in a straight line with separation a and with the middle charge (−2q) at the origin of a grounded conducting spherical shell of radius b ..."

I have found a solution for this problem but there is one step that I don't understand. I have uploaded the pdf file.
Last page the term $-\frac{1}{r}$ vanishes and at the bottom of the page the
$\frac{1}{b}-\frac{1}{r}$ vanishes too. Can you tell me why?

#### Attached Files:

• ###### F07_hw04a.pdf
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2. Aug 20, 2016

### Pual Black

at l=0 the term vanishes.
But what about the other values of l

Last edited: Aug 20, 2016