Class20 - Radiation The third form of heat transfer happens...

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2/22/12 Oregon State University PH 202, Lecture 20 1 Radiation The third form of heat transfer happens via electromagnetic waves that carry energy. Here are examples of electromagnetic waves. (Notice that the more energetic the wave, the shorter its wavelength.) Gamma rays shorter wavelength = higher energy X-rays : Ultraviolet : Visible light : Infrared : Microwaves : Radio waves longer wavelength = lower energy
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2/22/12 Oregon State University PH 202, Lecture 20 2 What causes these waves? One typical cause is a charged particle that is accelerating . But virtually all bodies are made up of charged particles. So any body whose particles are “bouncing around” at all—so that they have kinetic energy sufficient to collide with other particles—are accelerating and will therefore emit electromagnetic radiation. And we’d expect that the more energy these particles are “bouncing around” with, the more energy their accelerations would emit via radiation, no? This is exactly what we observe experimentally. And what indicator compares the average kinetic energies of the particles of any two bodies (and thus should also predict their relative rates of radiation)? Temperature . Sure enough : Any body that has a temperature (and that’s every body) emits energy via EM waves— radiation .
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2/22/12 Oregon State University PH 202, Lecture 20 3 Experimentation has revealed the numerical nature of the tempera- ture dependence—the Stefan-Boltzmann law of radiation: Q emitted = e σ
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This note was uploaded on 03/10/2012 for the course PH 202 taught by Professor Staff during the Winter '08 term at Oregon State.

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Class20 - Radiation The third form of heat transfer happens...

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