The energy associated with electric field

(4) Average energy density of magnetic field,

$u_{B}=\frac{B_{0}^{2}}{4 \mu_{0}}$

Average energy density of electric field,

$\mathrm{u}_{\mathrm{E}}=\frac{\varepsilon_{0} \mathrm{E}_{0}^{2}}{4}$

Now, $\mathrm{E}_{0}=\mathrm{CB}_{0}$ and $\mathrm{C}^{2}=\frac{1}{\mu_{0} \in_{0}}$

$\mathrm{u}_{\mathrm{E}}=\frac{\varepsilon_{0}}{4} \times \mathrm{C}^{2} \mathrm{~B}_{0}^{2}=\frac{\varepsilon_{0}}{4} \times \frac{1}{\mu_{0} \varepsilon_{0}} \times \mathrm{B}_{0}^{2}=\frac{\mathrm{B}_{0}^{2}}{4 \mu_{0}}=\mathrm{u}_{\mathrm{B}}$

$\therefore \mathrm{u}_{\mathrm{E}}=\mathrm{u}_{\mathrm{B}}$

Since energy density of electric and magnetic field is same, so energy associated with equal volume will be equal i.e. $u_{E}=u_{B}$