The rms value of the electric field of the light coming from the sun is 720 NC-1. The average total energy density of the electromagnetic wave is:
Answer & explanation
Correct answer: option 2
Total average energy density of electromagnetic wave is
u = $\frac{1}{2} \varepsilon_0 \mathrm{E}_{\mathrm{rms}}^2+\frac{1}{2 \mu_0} \mathrm{~B}_{\mathrm{rms}}^2$
$=\frac{1}{2} \varepsilon_0 \mathrm{E}_{\mathrm{rms}}^2+\frac{1}{2 \mu_0}\left(\frac{\mathrm{E}_{\mathrm{rms}}^2}{\mathrm{c}^2}\right) \quad\left(∵ \mathrm{B}_{\mathrm{rms}}=\frac{\mathrm{E}_{\mathrm{rms}}}{\mathrm{c}}\right)$
$=\frac{1}{2} \varepsilon_0 \mathrm{E}_{\mathrm{rms}}^2+\frac{1}{2 \mu_0} \mathrm{E}_{\mathrm{rms}}^2 \varepsilon_0 \mu_0$
$=\frac{1}{2} \varepsilon_0 \mathrm{E}_{\mathrm{rms}}^2+\frac{1}{2} \varepsilon_0 \mathrm{E}_{\mathrm{rms}}^2=\varepsilon_0 \mathrm{E}_{\mathrm{rms}}^2$
$=\left(8.85 \times 10^{-12}\right) \times(720)^2=4.58 \times 10^{-6} \mathrm{Jm}^{-3}$