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) $\vec{E} \times \vec{B}$
Show Answer
Answer:
Correct Answer: 15. (d)
Solution:
- (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} $
