First slide

Electromagnetic waves and its nature

Question

In an electromagnetic wave in vacuum, the electric and magnetic fields are $40 π V / m$ and $0.4 \times 10^{- 7} T$ . The poynting vector is

Moderate

A

$4.4 W m^{- 2}$
B

$0.44 W m^{- 2}$
C

$5.65 W m^{- 2}$
D

$4.0 W m^{- 2}$

Solution

Poynting vector is $\vec{S} = \frac{\vec{E} \times \vec{B}}{μ_{0}}$
Magnitude of Poynting vector is
$S = \frac{E B}{μ_{0}} [\vec{E} a n d \vec{B} a r e p e r p e n d i c u l a r t o e a c h o t h e r] \Rightarrow S = \frac{40 π \times 0.4 \times 10^{- 7}}{4 π \times 10^{- 7}} = 4.0 W m^{- 2}$

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