# Homework Help: Griffiths eqn 4.5

1. Mar 15, 2008

### ehrenfest

[SOLVED] Griffiths eqn 4.5

1. The problem statement, all variables and given/known data
This question refers to Griffiths E and M book.
$$\vec{F} = (\vec{p} \cdot \nabla)\vec{E}$$
Can someone please explain what the expression in parenthesis is? I have never seen that before? Is it a vector or a scalar or an operator or what? And what is the operation between what is parenthesis and E? I think Griffiths really botched this. Obviously Griffiths wants to use this notation to write "more compactly" the equation before the previous equation, but he cannot just make up notation like that!

2. Relevant equations

3. The attempt at a solution

Last edited: Mar 15, 2008
2. Mar 15, 2008

### kdv

I don't find anything ambiguous about it.

$$\vec{p} \cdot \nabla = p_x \partial_x + p_y \partial_y + p_z \partial_z$$
so this is a scalar operator. Then you apply it to each of the components of E to get each of the components of F.

3. Mar 15, 2008

### ehrenfest

I see what you mean.