# PHYS 2212 Module 12.3

## Energy Carried by Electromagnetic Waves

12.3 Energy Carried by Electromagnetic Waves

### Learning Objectives

By the end of this section, you will be able to:

• Express the time-averaged energy density of electromagnetic waves in terms of their electric and magnetic field amplitudes
• Calculate the Poynting vector and the energy intensity of electromagnetic waves
• Explain how the energy of an electromagnetic wave depends on its amplitude, whereas the energy of a photon is proportional to its frequency

Practice!

### Poynting Vector and Intensity of an Electromagnetic Wave

Discuss!

What is the approximate magnitude of the electric field on a page if you are reading a book outdoors under the shade of a tree and happen to leave the book on the ground where the Sun shines on it? The intensity of sunlight reaching the surface of the Earth is about 1000 W/m2.

A fusion reactor focuses 2×1013 watts of x-rays on this deuterium capsule for 5 nanoseconds.

(a) What is the average energy density of the radiation?

(b) What is the electric field amplitude of the radiation?