# Deriving electromagnetic fieldfor point charge

1. ### nos

33
Hi all,

I was going through the derivation for the electromagnetic field of point charges by Griffith(Introduction to electrodynamics page 437).

I'm missing minus sign somewhere:

The book says that:$\nabla(\vec{n}\cdot\vec{v})=\vec{a}(\vec{n}\cdot \nabla tr)+\vec{v}-\vec{v}(\vec{v}\cdot\nabla tr)-\vec{n}\times(\vec{a}\times \nabla tr)+\vec{v}\times (\vec{v}\times \nabla tr)$

Using the rule for triple cross products gives:

$\vec{v}+(v^{2}-\vec{n}\cdot\vec{a})\nabla tr$

However is should be:

$\vec{v}+(-v^{2}+\vec{n}\cdot\vec{a})\nabla tr$

I'm sure Im missing something.

Thanks

2. ### nos

33
oh I've got it. Forgot the minus sign in the triple cross product rule :\$