Question

The rms value of the electric field of the light coming from sun is 720 N/C. The average energy density of the EM 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

$4.58\times {10}^{-6}J/m^3$

Here, $u_E=\frac{1}{2}{ϵ}_0{E}_{rms}^2=\frac{1}{2}\times 8.85\times {10}^{-12}\times {720}^2=2.29\times {10}^{-6}J/m^3$

Hence, average total energy density = $2u_E=4.58\times {10}^{-6}J/m^3$