First slide

Average energy density of electromagnetic wave

Question

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

Moderate

Solution

$E_{rms} = 720 N / C$
Total average energy density = $\frac{1}{2} \in_{0} E^{2}$
$\begin{array}{rcl} = & \frac{1}{2} \in_{0} {[\sqrt{2} E_{rms}]}^{2} \\ = & \in_{0} {E_{rms}}^{2} \\ = & 8.8 x 10^{- 12} x {(720)}^{2} \\ = & 4.56 x 10^{- 6} {Jm}^{- 3} \end{array}$

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