Interaction Energy: What is W=\int dV $\rho(\vec{r}) \phi_{ext}(\vec{r})$?

[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?

 

[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