Physics 27: 1-6 Early Quantum Mechanics
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 27: Sections 1 to 6
- 27: 1 J. J. Thomson determined "cathode" rays were low-mass particles with a negative electrical charge by placing them in a magnetic field, which warped the path taken by the rays. While he could not determine mass or charge directly, he was able to determine the charge to mass ratio. Robert Millikan's oil-drop experiment determined the charge on the electron, allowing computation of electron mass and proving that atoms weren't the smallest particles.
- 27: 2 Wien's law shows the relationship between maximum intensity and temperature. The law describes a continuum that in fact is not observed: at high frequencies/short wavelengths, power output should continue to increase, but instead, intensity falls off. Planck introduced the concept of quantized energy to explain this "ultraviolet catastrophe". There is a limit to the smallest amount of energy that a particle or photon can carry.
- 27: 3 Einstein proposed that light emitted by a molecular oscillator decreases the molecules vibrational energy in integral amounts or quanta. His theory is supported by the photoelectric effect, which occurs when electrons are emitted from a metal surface as light shines on it. According to the wave theory, the maximum kinetic energy of the ejected electrons should depend entirely on light intensity and not at all on light frequency. According to the photon theory, maximum kinetic energy is not changed by intensity; instead, the maximum kinetic energy depends directly on the frequency of the light, and if the frequency is too little, no electron will receive enough energy to be ejected, regardless of intensity. Experiments carried out by Robert Millikan in 1913 and 1914 supported Einstein's photon theory.
- 27: 4 Because photons travel at the speed of light, they must be treated as relativistic particles when calculating their momentum and energy.
- 27: 5 Other phenomenal support the photon theory. In the Compton effect, scattered light has a longer wavelength then incident light. The lower frequency of light indicates that the photon loses energy, which is transferred to other particles during the collision.
- 27: 6 Photons interact with atoms and electrons as a pass-through matter in four ways:
- A photon colliding with an electron may dislodge it from its atom; the photon disappears (the photoelectric effect).
- The photon may be absorbed by electron, which moves to a higher or excited energy state, still bound to its atom.
- The photon may be scattered and lose energy (the Compton effect).
- A photon can be converted entirely to matter, producing an electron-positron pair.
|Electron charge to mass ratio
||Particles of mass m with electrical charge e will have a velocity v on a circular orbital path of radius r when passing through a magnetic field of B.|
||The wavelength at which the most radiation occurs depends on the absolute temperature of the blackbody
Read the following weblecture before chat: Early Quantum Mechanics
Use the Blackbody simulation below to explore what happens when you change the temperature of your blackbody.
- You can change the vertical scale by clicking on the Zoom/Unzoom magnifying glasses in the upper left. You can change the horizontal scale by clicking on the Zoom/Unzoom magnifying glasses in the lower right. [You may need to scale the intensity way down and the wavelength way up to see the wave function at low temperatures!]
- Change the temperature. At which wavelength does maximum intensity occur for the Earth (construed as a black body)? for your kitchen oven? For the filament of a lightbulb? For the sun? for a Type A bright star (9000K)?
- What color does each object appear?
Physics simulation Java Applets are the product of the PHET Interactive Simulations project at the University of Colorado, Boulder.
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