Chapter28 - Chapter 28 Quantum Physics Thermal Radiation An...

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Chapter 28 Quantum Physics
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Thermal Radiation ± An object at any temperature is known to emit thermal radiation ± Characteristics depend on the temperature and surface properties ± The thermal radiation consists of a continuous distribution of wavelengths from all portions of the em spectrum
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Thermal Radiation, cont ± At room temperature, the wavelengths of the thermal radiation are mainly in the infrared region ± As the surface temperature increases, the wavelength decreases ± It will glow red and eventually white ± The basic problem was in understanding the observed distribution in the radiation emitted by a black body ± Classical physics didn’t adequately describe the observed distribution
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Color and Temperature The temperature of a Pahoehoe lava flow can be estimated by its color. It is about 1,000-1,200 C. Much of a person's energy is radiated away in the form of infrared energy (~10 μ m in wavelength)
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Blackbody Radiation ± A black body is an ideal system that absorbs all radiation incident on it ± The electromagnetic radiation emitted by a black body is called blackbody radiation
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Blackbody Approximation ± A good approximation of a black body is a small hole leading to the inside of a hollow object ± The nature of the radiation leaving the cavity through the hole depends only on the temperature of the cavity walls
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Blackbody Experiment Results ± The total power of the emitted radiation increases with temperature ± Stefan’s Law ± P = σ A e T 4 ± For a blackbody, e = 1 ± The peak of the wavelength distribution shifts to shorter wavelengths as the temperature increases ± Wien’s displacement law ± λ max T = 2.898 x 10 -3 m . K
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Stefan’s Law – Details ± P = σ A e T 4 ± P is the power ± σ is the Stefan-Boltzmann constant ± σ = 5.670 x 10 -8 W / m 2. K 4 ± Was studied in Chapter 17
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Wien’s Displacement Law ± λ max T = 2.898 x 10 -3 m . K ± λ max is the wavelength at which the curve peaks ± T is the absolute temperature ( in K ) ± The wavelength λ max is inversely proportional to the absolute temperature ± As the temperature increases, the peak is “displaced” to shorter wavelengths
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Example ± The radius of our Sun is 6.96x10 8 m, and its total power output is 3.85x10 26 W. (a) Assuming that the Sun’s surface emits as a blackbody, calculate its surface temperature. (b) Using the results of part (a), find λ max for the Sun.
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Intensity of Blackbody Radiation, Summary ± The intensity increases with increasing temperature ± The amount of radiation emitted increases with increasing temperature ± The area under the curve ± The peak wavelength decreases with increasing temperature
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Ultraviolet Catastrophe ± At short wavelengths, there was a major disagreement between classical theory and experimental results for black body radiation ± This mismatch became known as the ultraviolet catastrophe ± You would have infinite energy as the wavelength approaches zero
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Planck’s Theory of Blackbody Radiation ± In 1900, Planck developed a structural model for blackbody radiation that leads to an
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This note was uploaded on 06/08/2009 for the course PHYS 2c taught by Professor All during the Spring '08 term at UC Riverside.

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Chapter28 - Chapter 28 Quantum Physics Thermal Radiation An...

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