The rms value of the electric field of light coming from the sun is 720 N/C. The average total energy density of the electromagnetic wave is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 4.58 x 10-6 J/m3
In an electromagnetic wave the electric and magnetic fields carry equal energy. The average energy density of each is \( \frac{1}{2}\varepsilon_0 E_{\mathrm{rms}}^{2} \), so the total is
\( \displaystyle u_{\mathrm{av}} = \varepsilon_0 E_{\mathrm{rms}}^{2} \)
\( \displaystyle u_{\mathrm{av}} = 8.85\times10^{-12}\,\mathrm{C^{2}\,N^{-1}\,m^{-2}} \times (720\,\mathrm{N\,C^{-1}})^{2} \)
\( \displaystyle u_{\mathrm{av}} = 8.85\times10^{-12} \times 5.18\times10^{5}\,\mathrm{J\,m^{-3}} \)
\( \displaystyle u_{\mathrm{av}} \approx 4.58\times10^{-6}\,\mathrm{J\,m^{-3}} \)