Question

The rms value of the electric field of the light coming from the Sun is $720\text{N/C}$. The average total energy density of the electromagnetic wave is -

• A. $4.6\times {10}^{-6}{\text{J/m}}^3$
• B. $6.4\times {10}^{-9}{\text{J/m}}^3$
• C. $1.3\times {10}^{-6}{\text{J/m}}^3$
• D. $3.3\times {10}^{-3}{\text{J/m}}^3$

Answer 1

The correct option is A $4.6\times {10}^{-6}{\text{J/m}}^3$

We know that, the average total energy density of the electromagnetic wave is,

$u=\frac{1}{2}{ϵ}_0{E}_0^2=\frac{1}{2}{ϵ}_0(\sqrt{2}{E}_{\text{rms}}{)}^2$

$\Rightarrow u=\frac{1}{2}\times 8.85\times {10}^{-12}\times (\sqrt{2}\times 720{)}^2$

$\Rightarrow u=4.6\times {10}^{-6}{\text{J/m}}^3$

Hence, option $\left(A\right)$ is the correct answer.