First slide

Intensity of electromagnetic wave

Question

A plane electromagnetic wave is propagating through free space. If amplitude of electric field is 50 v/m, the intensity of the wave is

Moderate

A

$6 .8 W / m^{2}$
B

$12 .5 W / m^{2}$
C

$3 .3 W / m^{2}$
D

$2 .1 W / m^{2}$

Solution

Average energy density $\bar{u} = \frac{1}{2} ε_{0} E_{m}^{2}$
$∴$ Average intensity $I = \bar{u} c = \frac{1}{2} ε_{0} {cE}_{m}^{2}$
$\Rightarrow I = \frac{1}{2} \times 8 .85 \times 10^{- 12} \times 3 \times 10^{8} \times {(50)}^{2} W / m^{2} = 3 .3 W / m^{2}$

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