Electromagnetic Waves - Result Question 15

15. If $\vec{E}$ and $\vec{B}$ represent electric and magnetic field vectors of the electromagnetic waves, then the direction of propagation of the waves will be along

[2002]

(a) $\vec{B} \times \vec{E}$

(c) $\vec{B}$

(b) $\vec{E}$

(d) $\vec{E} \times \vec{B}$

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Answer:

Correct Answer: 15. (d)

Solution:

  1. (d) Direction of propagation of electromagnetic waves is perpendicular to Electric field and Magnetic field. Hence, direction is given by

vector $\vec{S}=\vec{E} \times \vec{H}=\frac{\vec{E} \times \vec{B}}{\mu_0}$.

In electromagnetic waves, the rate of flow of energy crossing a unit area is described by Poynting vector. The magnitude of poynting vector

$ \begin{aligned} & |\overrightarrow{{}S}|=\frac{1}{\mu_0} EB \sin 90^{\circ}=\frac{E B}{\mu_0} \\ & =\frac{E^{2}}{\mu_0 c}(\because B=\frac{E}{c}) \end{aligned} $