From key@physics.utoronto.ca Tue Feb 26 13:31:51 2008 Date: Tue, 26 Feb 2008 13:31:51 -0500 From: Tony Key To: dexi.chen@utoronto.ca Subject: Planetary Model and Black Body Radiation Great question, Amy. I expect my next lecture will make this clearer, but here is an explanation in case you can't wait! Briefly, Planck was the physicist who first solved the black body problem by postulating for the very first time that energy was quantized - i.e. that not all energies were available to atoms or molecules of a substance. This idea was confirmed for light by Einstein, and then used by Bohr in the Planetary Model to predict the energies of the lines observed in atomic spectra - the difference in energies between two states being equal to Planck's constant multiplied by the frequency of the emitted light. The black body radiation spectrum shows the distribution of the electromagnetic radiation that is given off by the vibrations (accelerations) of the charge particles (electrons and ions) that exist in all materials. As the material is heated, the spectrum moves to the higher frequency end of the spectrum ( from reddish to orange, to yellow, and so on). Most of the spectrum, of course, is in the infrared or ultraviolet parts of the spectrum which our eyes don't register. Maxwell told us that accelerated charged particles emitted such radiation, and Planck tried to explain the observed black body spectrum using Maxwell's equations. Planck - and many other physicists - were unable to explain the spectrum if they assumed, as all classical physicists would expect, that all possible energies up to a maximum (given by the heat energy available) were possible for the motion of the particles of the material. However, when Planck fiddled with his calculations he found he could reproduce the spectrum perfectly if he assumed that the particles could have discrete (not continuous) energies separated by multiples of a very small quantity. This quantity is equivalent to the product of a constant (that we now call Planck's constant) multiplied by a frequency. i.e. E = hf. This was shown to be true for particles of light by Einstein. Bohr then had all he needed to explain the observations on the basis of the Planetary Model. Hope that makes it clear, thanks for your question. TonyK dexi.chen@utoronto.ca wrote: > Hi, Professor Key, > > On one of our lecture slides for today it showed the black body spectrum, > but I don't really understand how this relates to either planetary model > or Bohr's model. Can you please help me explain about it? > > Also, will the lecture audio recordings be posted on the course website > as well? > > Thank you very much! > > Amy > [ Part 2, Text/X-VCARD (charset: UTF-8 "Internet-standard ] [ Unicode") (Name: "key.vcf") 9 lines. ] [ Unable to print this part. ]