Electromagnetic Wave Energy: Density, Flux, and Intensity

All flashcards

What is electromagnetic energy density?

Energy per unit volume stored in the fields: $u=\frac{1}{2}\varepsilon_0 E^2+\frac{B^2}{2\mu_0}$. In a vacuum wave, the electric and magnetic contributions are equal.

How can an electromagnetic wave transfer energy to matter?

Its fields exert forces on charged particles, and those forces can do work on the particles.

What does energy flux describe for an electromagnetic wave?

Energy transferred per unit area per unit time. For a wave traveling at speed $c$, its magnitude is $S=uc$.

What does the Poynting vector describe?

The direction and rate of electromagnetic energy flow; in vacuum, $\vec{S}=\frac{1}{\mu_0}\vec{E}\times\vec{B}$.

How is average intensity related to electric-field amplitude?

For a sinusoidal wave in vacuum, $I=\frac{1}{2}c\varepsilon_0 E_0^2$; intensity is proportional to the square of the amplitude.

Quiz questions

1. What does energy flux measure?Answer: Energy transferred per unit area per unit time.
2. For an electromagnetic wave traveling at speed $c$, what is the magnitude of its energy flux in terms of energy density $u$?Answer: $S=uc$
3. In an electromagnetic wave, which direction does the Poynting vector point?Answer: Along the direction of energy transport and wave propagation.
4. For a sinusoidal electromagnetic wave, how does peak intensity compare with average intensity?Answer: Peak intensity is twice the average intensity.
5. A source radiates uniformly in all directions. If the distance from it doubles, how does the intensity change?Answer: It falls to one-quarter of its original value.