# Electric Energy Density

$μ_{e}=21 ϵE_{2}$ See Chapter 9 Capacitor notes. This is derived from the energy stored of a capacitor, given by

$U=21 CV_{2}=21 dϵA (Ed)_{2}$

Energy density is then obtained just dividing $U$ by the volume, $V=Ad$ so everything simplifies down to $μ_{e}=21 ϵE_{2}$