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
Energy in Electromagnetic Waves
Practice!
| Practice 12.3.1 |
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| In a sinusoidal, traveling electromagnetic wave in a vacuum, the magnetic energy density… |
Poynting Vector and Intensity of an Electromagnetic Wave
| Practice 12.3.2 |
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The drawing shows a sinusoidal electromagnetic wave in a vacuum at one instant of time at points between x = 0 and x = λ. At this instant, at which of the values of x in the drawing does the instantaneous Poynting vector have its maximum magnitude? |
| Practice 12.3.3 |
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| A small source radiates an electromagnetic wave with a single frequency into vacuum, equally in all directions. As the wave propagates, its intensity: |
| Practice 12.3.4 |
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| What is the average magnitude of the Poynting vector 13 km from a radio transmitter broadcasting isotropically (equally in all directions) with an average power of 200 kW? |
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?
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