# Interaction Energy

1. Mar 5, 2009

### latentcorpse

Hi. in my notes the interaction energy is simply stated as being

$W=\int dV \rho(\vec{r}) \phi_{ext}(\vec{r})$
however no explanation is given as to where this comes from.
is it simply a fact or should i be able toderive this from somewhere?

2. Mar 5, 2009

### lanedance

I think this essentially assumes you initially have an external potential phi, then this is the energy required to move the given charge distribution ito that potential from infinity