CH 301 Chapter 1 notes v2

CH 301 Chapter 1 notes v2 - CH 301 Chapter 1 part 1...

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CH 301 Chapter 1 part 1: Electromagnetic Radiation . This is a form of energy which our eyes can only detect tiny portions of. White light is made up of many different colors. Light behaves as if it were a wave. For a wave, wavelength λ , speed c and frequency ν are linked by c = ν λ c = speed of light = 3 x 10 8 m/s In electromagnetic radiation, the strength or amplitude of the electrical and magnetic Fields field is varying, perpendicular to each other. Review the Electromagnetic Spectrum on page 3: extends in both directions, no limit Note visible light is but a tiny part. Class Example: What is the frequency of green light, wavelength 520 nm? In addition to being able to use c = νλ , you should be able to determine: For two different examples of electromagnetic radiation: Which has the longer (bigger) λ ? Which has the shorter (smaller) λ ? Which has the highest (greatest, largest) ν ? Which has the lowest (smallest) ν ? Know the Visible range: 400nm-700nm (0.4-0.7 μ m) Know the order: (inc. λ ): Gamma rays, X rays, UV, Visible, IR, Microwave, Radio Know the short λ (higher ν ) end of the visible is BLUER Know the long λ (lower ν ) end of the visible is REDDER Blackbody Radiation: ‘Blackbody’ - an object that can absorb or emit any wavelength of light with equal efficiency. Objects above absolute zero emit some E-M radiation. Blackbodies emit radiation over all wavelengths- relative amounts at each λ will vary with T. At room temperature most will be emitted in the IR, but consider the coil on an electric stove or a lightbulb wire. As T increases, the distribution of the intensities of emitted radiation will shift towards shorter λ , and the total amount of radiation emitted increases. See Figures on page 4. Relatively little radiation is given out at visible wavelengths, it is enough to make the object appear to go from dull red, to orange, to yellow and white hot. Note how peak wavelength λ max varies with T and how the area under the curve (total energy emitted) varies with T Wein’s Law: T λ max = 1/5 c 2 where c 2 = 1.44 x 10 -2 K.m Stephan-Boltzman law: Both were EXPERIMENTALLY DETERMINED Power emitted (W) = σ T 4 Surface area (m 2 ) where σ = 5.67 x 10 -8 Wm -2 K -4 Late ninteeth century theorists tried to set up models using classical mechanics failed – especially at short wavelengths (UV, gamma and X-rays) where the model predicted infinite values! (UV Catastrophe) The solution was the assumption that energy was QUANTIZED. Only specific amounts of energy can be absorbed or emitted by matter.
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Class Example: Stephan-Boltzman Law The back of a CPU chip gets warm. Calculate the ratio of the power being radiated from it when it is first turned on (ie. running at near room temp, 75ºF to when it has been running for a while and the heat sink maintains ~ 150ºF?) Examples: Wein’s Law The peak emission from a blackbody source is 1.4 μ m. What is its surface temperature?
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This note was uploaded on 09/02/2009 for the course CHE 301 taught by Professor Fatimafakhreddine during the Spring '08 term at University of Texas.

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CH 301 Chapter 1 notes v2 - CH 301 Chapter 1 part 1...

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