Lecture17_10 - Physics19 GreatIdeasofPhysics...

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Click to edit Master subtitle style Physics 19 Great Ideas of Physics Lecture 17: The Quantum
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Classical Physics starts to unravel. .. The discovery of radioactivity and early studies of atomic  structure (cathode rays, Rutherford’s experiment) raised  many new questions: What causes radioactivity, and where does the enormous energy  released in radioactive decays come from? How can stable atoms exist? How does the nucleus stay in one piece? Why don’t the electrons spiral in? These were hints that classical physics wasn’t as complete  as some hoped Relativity was developed about the same time (1905-1916)
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Click to edit Master subtitle style Thermal (“Black-body”)  Radiation Enter the quantum…
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Thermal Emission of Light Thermal “jiggling” of the  electric charges in any  object causes light to be  radiated All objects radiate a  spectrum of light  dependent on their  temperature For room temperature 
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The “Ultraviolet Catastrophe” Unfortunately, the laws of  classical physics give a  totally incorrect prediction  for the spectrum of thermal  radiation Newtonian mechanics Boltzmann’s statistical  mechanics Maxwell’s equations Classical physics predicts  that every object should 
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Max Planck (1858-1947) Quiet, conservative German  theoretician, who unwittingly  started a revolution in  physics Worked on the thermal  radiation problem hoping to  find some equation that fit  the spectrum and work  backwards to understand 
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Planck’s Hypothesis In 1900, Planck found an  empirical function that fit the  data, then tried to understand it To get the right answer he  assumed the oscillating charges  could only absorb and emit  radiation of a particular  frequency in chunks of size: E = h f In other words, an oscillator with  frequency f could only have  energies which were an integral  multiple of h*  f. Energy is not absorbed 
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Review of Planck’s Model The emission of thermal  radiation can be modeled as due to oscillating charges Electrons in atoms In classical physics, an oscillator  of a given frequency can have  any energy In 1901, Max Planck obtained a  correct thermal spectrum by  assuming that the energy of an  oscillator with frequency f can  only be an integer multiple of hf h = Planck’s constant, a very  Classical Oscillator Planck’s Oscillator Energy (all energies are possible) Energy E = hf E = hf E = hf (energy is “quantized”)
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Planck’s Constant Like the speed of light, G, etc, Planck’s  constant is one of the most fundamental  quantities in nature Whenever Planck’s constant appears, it signals  that quantum physics is going on Planck’s constant h is small, but not zero: h = 6.626   10-34 Joule-seconds    =  0.0000000000000000000000000000000006626 J s
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Lecture17_10 - Physics19 GreatIdeasofPhysics...

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