Question

A 300f capacitor separated by a distance of 4.0mm was charged to a potential difference of 250 V. What is the energy density in its region in terms of jouled/m³

Answer 1

The electric field between plates

$$E=\frac{V}{d}=\frac{250}{0.004}=6.25*10^4\:\rm V/m$$

The energy density

$$w=\frac{\epsilon_0E^2}{2}\\ =\frac{8.85*10^{-12}*(6.25*10^4)^2}{2}=0.017\:\rm J$$