1. May 12, 2012

### ellocomateo

Hello,
I want to derive the electric field vector from a quadrupole potential.
I am starting from
faculty.physics.tamu.edu/pope/em603.pdf (6.32)
and I am considering E=-∇$\phi$
with $\phi$=1/2 Qijxixj/r5
I am very unsure if I carry out the thing properly, I would appreciate your comments:

Ek=1/2 ∂k(Qij xixj/r5)
=Qijkxixj/r5 - 5xkxixj/r7
=Qijkinj-5nkninj)/r4

with ni=xi/r

Now, what is the meaning of δkinj ?! Can it be anyhow simplified?