Potential of Dipole I think I am close to completing it

[maherelharake]

Homework Statement

We have 2 points charges qa and qb. Qa =2 nC and is at the origin while
Qb =-10nC and is at (x0,z0)=(4,2)m.

a. Find the real potential for an arbitrary x and z.
b. Find the monopole/dipole moments.
c. Find the potential due to these moments and compare with the correct value for points along the x axis.

Homework Equations

The Attempt at a Solution

I have attached what I have done. I think I am close, but I can't get my part 'c' to check out, so I think something is wrong. Thanks.

 

[betel]

What doesn't check out? You calculated the potential due to the moments so now you should compare it with the exact value along the x axis.

 

[maherelharake]

When I was comparing the potential due to these moments with the correct value, I couldn't get them to match. Sorry, I should have explained that better.

 

[betel]

Of course the two will not match exactly. That's what multipole expansion is about. The question is by how much they will differ from the correct result.

 

[maherelharake]

Oh alright, I see that now. Thanks.