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chapter27

# chapter27 - Chapter 27 Quantum Physics Need for Quantum...

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Chapter 27 Quantum Physics

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Need for Quantum Physics Problems remained from classical mechanics that relativity didn’t explain Blackbody Radiation The electromagnetic radiation emitted by a heated object Photoelectric Effect Emission of electrons by an illuminated metal Spectral Lines Emission of sharp spectral lines by gas atoms in an electric discharge tube
Development of Quantum Physics 1900 to 1930 Development of ideas of quantum mechanics Also called wave mechanics Highly successful in explaining the behavior of atoms, molecules, and nuclei Involved a large number of physicists Planck introduced basic ideas Mathematical developments and interpretations involved such people as Einstein, Bohr, Schrödinger, de Broglie, Heisenberg, Born and Dirac

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Blackbody Radiation An object at any temperature emits electromagnetic radiation Sometimes called thermal radiation Stefan’s Law describes the total power radiated The spectrum of the radiation depends on the temperature and properties of the object
Blackbody Radiation Classical View Thermal radiation originates from accelerated charged particles Problem in explaining the observed energy distribution Opening in a cavity is a good approximation

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Blackbody Radiation Graph Experimental data for distribution of energy in blackbody radiation As the temperature increases, the total amount of energy increases Shown by the area under the curve As the temperature increases, the peak of the distribution shifts to shorter wavelengths
Wien’s Displacement Law The wavelength of the peak of the blackbody distribution was found to follow Wein’s Displacement Law λ max T = 0.2898 x 10 -2 m • K λ max is the wavelength at which the curve peaks T is the absolute temperature of the object emitting the radiation

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The Ultraviolet Catastrophe Classical theory did not match the experimental data At long wavelengths, the match is good At short wavelengths, classical theory predicted infinite energy At short wavelengths, experiment showed no energy This contradiction is called the ultraviolet catastrophe
Planck’s Resolution Planck hypothesized that the blackbody radiation was produced by resonators Resonators were submicroscopic charged oscillators The resonators could only have discrete energies E n = n h ƒ n is called the quantum number ƒ is the frequency of vibration h is Planck’s constant , 6.626 x 10 -34 J s Key point is quantized energy states

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Max Planck 1858 1947 Introduced a “quantum of action,” h Awarded Nobel Prize in 1918 for discovering the quantized nature of energy
Photoelectric Effect When light is incident on certain metallic surfaces, electrons are emitted from the surface This is called the photoelectric effect The emitted electrons are called photoelectrons The effect was first discovered by Hertz

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