Question

The r.m.s value of electric field of the light coming from sun is $720N/C$. The average energy density of electromagnetic wave is

• A. $3.3\times {10}^{-3}{J/m}^3$
• B. $4.58\times {10}^{-6}{J/m}^3$
• C. $6.37\times {10}^{-9}{J/m}^3$
• D. $81.35\times {10}^{-12}{J/m}^3$

Answer 1

The correct option is B $4.58\times {10}^{-6}{J/m}^3$

In EM waves,
Electric energy density = Magnetic energy density

$\therefore$ Total energy density $=2\times$ Electric energy density
$=2\left[\frac{1}{2}{ϵ}_0{E}^2\right]=2\times \frac{1}{2}\times 8.854\times {10}^{-12}\times {720}^2=4.58\times {10}^{-6}J/m^3$