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Module 3 : MAGNETIC FIELD

Lecture 20 : Magnetism in Matter

	Poynting Vector
	Electromagnetic waves, like any other wave, can transport energy. The power through a unit area in a direction normal to the area is given by Poynting vector , given by
	$\begin{displaymath}\vec S = \frac{1}{\mu_0}\vec E\times\vec B\end{displaymath}$
	As $\vec E, \vec B$ and $\vec k$ form a right handed triad, the direction of $\vec S$ is along the direction of propagation. In SI units $\vec S$ ismeasured in watt/m . The magnitude of $\vec S$ for the electromagnetic wave travelling in vacuum is given by
	$\begin{displaymath}S = \frac{EB}{\mu_0} = \frac{E^2}{c\mu_0}\end{displaymath}$
	where we have used the relationship between and in free space. For harmonic waves, we have
	$\begin{displaymath}S = \frac{E_0^2}{c\mu_0}\sin^2(kx-\omega t)\end{displaymath}$
	The average power transmitted per unit area, defined as the intensity is given by substituting the value 1/2 for the average of the square of sine or cosine function
	$\begin{displaymath}I = \frac{E_0^2}{2c\mu_0}\end{displaymath}$
	Example-27
	Exercise-5