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8.Electromagnetic waves
medium
A $27\, mW$ lager beam has a cross -sectional area of $10\, mm^2$. The magnitude of the maximum electric field in this electromagnetic wave is given by:........$kV/m$ [Given permittivity of space ${ \in _0} = 9 \times {10^{ - 12}}\, SI\, units$, speed of light $c = 3 \times 10^8\, m/s$]
A
$2$
B
$0.7$
C
$1$
D
$1.4$
(JEE MAIN-2019)
Solution
Intensity of $EM$ wave is given by
${\mathrm{I}=\frac{\text { Power }}{\text { Area }}=\frac{1}{2} \varepsilon_{0} \mathrm{E}_{0}^{2} \mathrm{C}}$
${=\frac{27 \times 10^{-3}}{10 \times 10^{-6}}=\frac{1}{2} \times 9 \times 10^{-2} \times \mathrm{E}^{2} \times 3 \times 10^{8}}$
${\mathrm{E}=\sqrt{2} \times 10^{3}\, \mathrm{kV} / \mathrm{m}}$
${=1.4\, \mathrm{kV} / \mathrm{m}}$
Standard 12
Physics
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