Q. The $rms$ value of the electric field of the light coming from the Sun is $720\ N/C$. The average total energy density of the electromagnetic wave is
A) $6.37 \times 10^{- 9}J/m^{3}$
B) $81.35 \times 10^{- 12}J/m^{3}$
C) $3.3 \times 10^{- 3}J/m^{3}$
D) $4.58 \times 10^{- 6}J/m^{3}$
Explanation
(d) Average energy density
$v_{AV} = \frac{1}{2}\varepsilon_{0}E_{0}^{2} = \frac{1}{2}\varepsilon_{0}\left( \sqrt{2}E_{rms} \right)^{2} = \varepsilon_{0}E_{rms}^{2}$
$= 8.85 \times 10^{- 12} \times (720)^{2} = 4.58 \times 10^{- 6}J/m^{3}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.