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Modern Physics

Modern Physics. Atom. Nucleons – subatomic particles in the atom’s nucleus (protons and neutrons ). Ion – An atom with a net electric charge which is due to the loss or gain of electrons. Strong nuclear force – Nucleons attractive binding force. Photoelectric Effect - Link.

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Modern Physics

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  1. Modern Physics

  2. Atom Nucleons – subatomic particles in the atom’s nucleus (protons and neutrons). Ion – An atom with a net electric charge which is due to the loss or gain of electrons. Strong nuclear force – Nucleons attractive binding force.

  3. Photoelectric Effect - Link The energy of a photon is directly proportional to the photon’s frequency. Planck’ constant (h) – A fundamental constant of quantum theory that determines the scale of the micro world. E = h f E = photon energy h = Planck’s constant f = frequency

  4. 38.3The Photoelectric Effect Einstein found support for his quantum theory of light in the photoelectric effect. The photoelectric effect is the ejection of electrons from certain metals when light falls upon them. These metals are said to be photosensitive. PHET SIMULATION LINK

  5. 38.8Quantum Physics Physicists became convinced that the Newtonian laws that work so well for large objects do not apply to the microworld of the atom. The study of the motion of particles in the microworld of atoms and nuclei is called quantum mechanics. The branch of physics that is the general study of the microworld of photons, atoms, and nuclei is simply called quantum physics.

  6. Alpha Decay • alpha particle: a “doubly • ionized helium atom” or simply a “helium nucleus” • written a,a ,or He +2 4 2 4 2 • Z > 82 for alpha decay possible • have relatively slow speeds (0.1 c) • can be stopped by a few cm of air or • an ordinary sheet of paper • natural a’s have energy between 4 and 10 MeV • half-lives from 10-6 s to 1010yr (link)

  7. Beta Decay (link) 0 -1 0 -1 • electron, written e or b • or positron, written e or b 0 +1 0 +1 • occurs primarily in light nuclei • penetrates many meters of air • or thin sheets of metal • high speed (approach speed of light) or

  8. Gamma Decay • high energy photons • (electromagnetic radiation) • written g • penetrates 2 km air or 30 cm lead • short lifetimes • energy range of keV to MeV • have short wavelength (high frequency) (link)

  9. 39.3Radiation Penetrating Power Alpha particles penetrate least and can be stopped by a few sheets of paper; beta particles by a sheet of aluminum; gamma rays by a thick layer of lead.

  10. N(t) = # radioactive nuclei • present at time t • N0 = number initially present • = the disintegration constant, which is equal to (ln 2)/T1/2, where T1/2is the half-life of the decaying nucleus

  11. 39.8Carbon Dating The radioactive carbon isotopes in the skeleton diminish by one half every 5730 years. The red arrows symbolize relative amounts of carbon-14.

  12. 40.1Nuclear Fission In a typical example of nuclear fission, one neutron starts the fission of the uranium atom and three more neutrons are produced when the uranium fissions.

  13. 40.1Nuclear Fission Critical Mass The critical mass is the amount of mass for which each fission event produces, on the average, one additional fission event. A subcritical mass is one in which the chain reaction dies out. A supercritical mass is one in which the chain reaction builds up explosively.

  14. 40.3The Nuclear Fission Reactor Components of a Fission Reactor • A reactor contains three main components: • the nuclear fuel combined with a moderator, • the control rods, and • water.

  15. 40.8Controlling Nuclear Fusion In the fusion reactions of hydrogen isotopes, most of the energy released is carried by the lighter-weight particles, protons and neutrons, which fly off at high speeds.

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