Chapter30[1]

# Chapter30[1] - CHAPTER 30 Ideal blackbody: absorbs all...

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CHAPTER 30 Ideal blackbody: absorbs all light that is incident on it. My grandparents’ coal furnace. Objects that absorb well also emit (radiate) well. The color of the emitted ratiation will change as the object is heated. Which is hotter? Red hot or blue hot? Distribution of frequencies does not depend on the material from which the black body is made! AS THE TEMPERATURE GOES UP, THE PEAK SHIFTS! Wien’s Displacement Law: 10 1 1 -1 peak f (5.88x10 s K )T SI unit Hz = s   The temperature of an object is directly proportional to the frequency of radiation it emits. BLACKBODY RADIATION: Anyone who has ever watched the glowing embers of a campfire has seen blackbody radiation (BBR, for short). When an object is heated to a high enough temperature, it begins to glow. A metal, for example, can be heated until it glows red-hot. Some metals can be heated even more until they glow blue-hot. What we call a “blackbody” is an object that can both absorb and emit electromagnetic energy perfectly. A glowing blackbody does not, however, emit only one wavelength, but rather a range of wavelengths, with a peak wavelength corresponding to a given temperature. Look at the figures above. They are graphs of the intensity of EM waves emitted by a body at some temperature versus the frequency of those EM waves. Notice that there is a "peak" of frequencies in the visible part of the EM spectrum. This maximum frequency for a given temperature can be calculated from the Wien’s Displacement Law above. If we are looking for wavelenght, the easy relationship is

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. Remember that we measure T in Kelvins and wavelength in meters. One more point before we leave this discussion of BBR: remember the definition of power is energy per time and the definition of intensity is power per area. For a blackbody the power it emits at some temperature is (approximately) (T in Kelvins as above). THE ULTRAVIOLET CATASTROPHE The classical theory predicts that at high frequencies, the blackbody emits an infinite amount of energy. This is not a reasonable result. Planck said, “Energy is quantized!” Imagine you can only drive 10 mph, 20 mph, 30 mph… n E nhf n 0,1,2,. ..  h is called PLANCK’S CONSTANT. h = 6.63 x 10 -34 J · s This is one of the fundamental constants of nature! (h, c, e, m e )
Example: m = 1.2 kg, k = 35 N/m, v max = 0.95 m/s. a) What are the frequency of oscillation and total energy of this system?    2 2 max max max 35N/m k1 k 1 2f f 0 . 8 6H z m2 m 2 1 . 2 k g 11 E mv 1.2kg 0.95m/s 0.54J 22        b) How big is one quantum of energy in this system?

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## This note was uploaded on 10/13/2011 for the course PHY 102 taught by Professor Alexandrakis during the Fall '06 term at FIU.

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Chapter30[1] - CHAPTER 30 Ideal blackbody: absorbs all...

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