The energy associated with electric field is $\left(U_{E}\right)$ and with magnetic fields is $\left(U_{B}\right)$ for an electromagnetic wave in free space. Then :
Correct Option: , 4
(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. $u_{E}=u_{B}$