Review Topics

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MODERN PHYSICS III
Review Topics In phase / out of
phase Compressions and rarefactions
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Light atoms tend to combine and release energy as
they do so. Heavy atoms tend to split and
release energy as they do so. Uranium and
Plutonium are particularly useful in this regard,
and are the basis of nuclear fission.
Heavy nuclei break into lighter nuclei and energy
is released.
Light nuclei fuse into heavy nuclei and energy is
released.
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Three basic types of radioactive decay

• Alpha decay release of an alpha particle
  (helium-4 nucleus) from a nucleus
• One way for an atom to move from a heavier to a
  lighter atom to become more stable
• Beta decay release of a beta particle
  (electron)
• For a free neutron, decay into proton electron
  neutrino
• For a nucleus, conversion of a neutron to a
  proton
• Gamma decay release of a high energy wave
  (photon)
• From change in energy inside a nucleus
• From self-annihilation of a particle and its
  antiparticle

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Three basic types of radioactive decay

• Alpha decay release of an alpha particle
  (helium-4 nucleus) from a nucleus
• One way for an atom to move from a heavier to a
  lighter atom to become more stable
• Write an equation showing the change
• What is conserved?
• Beta decay release of a beta particle
  (electron)
• For a free neutron, decay into proton electron
  neutrino
• For a nucleus, conversion of a neutron to a
  proton
• Write an equation showing the change for each
  case
• What is conserved?
• Gamma decay release of a high energy wave
  (photon)
• From change in energy inside a nucleus
• From self-annihilation of a particle and its
  antiparticle
• Write an equation showing the change for the
  p(anti-p) case
• What is conserved?

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Lets look at a neutron sitting by itself in
space. After 15 minutes, the neutron has a 50
probability of decaying to a proton plus an
electron
Neutrons look like they have internal stuff
rather than just being a simple round blob
p
Decay products
Neutrino No charge Momentum Very tiny mass v
c

• Note that
• Charge is conserved
• Mass energy is (almost) conserved
• What is wrong with this picture?
• With the addition of the neutrino, momentum is
  conserved.

e-
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MODERN PHYSICS III
6 Quarks 6 leptons (electron, 3 neutrinos, two
others) Hadrons Baryons (3 quarks) and Mesons
(2) Plus their antiparticles Four Fundamental
forces Strong Force (gluons) Weak force (weird
particles) Electromagnetic force
(photons) Gravity (gravitons) - They
both have mass - They have opposite sign - If
they meet, they self-annihilate and release energy
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Note that charge is unitary (1, 0, -1) outside
the nucleon and fractional (/- 1/3 or /- 2/3)
inside it. Charge is quantized.
Light (not heavy)
Heavy
These dont live long
Here there be nucleons
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Which Fundamental Interaction/Force is
responsible for

• Friction?
• Electromagnetic.
• Nuclear Bonding?
• Residual Strong Nuclear.
• Orbiting Planets?
• Gravity.
• Which force carriers have not been observed?
• Gravitons (Gluons have been observed indirectly)

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More About Quarks

• They vary greatly in mass but are similar in
  charge

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More About Leptons

• They vary greatly in mass but are similar in
  charge

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Just Kidding

• Other name candidates included the
• "hold-on,"
• "duct-tape-it-on,"
• "tie-it-on!"

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The Muon and Tau

• These two heavier leptons decay into lighter
  leptons or quarks
• When they decay, three particles are produced
• One of the particles produced is always its
  corresponding neutrino
• The other particles could be a quark and its
  anti-quark or another lepton and its
  anti-neutrino

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Wave Theory of Light

• Christian Huygens (1629 1695) Light travels in
  wavelets
• Huygen's Wavelets

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Corpuscle Theory of Light Sir Issac Newton
(1642 1727)

• Newton believed that bodies emitted energy in
  particles or corpuscles that traveled in straight
  lines.
• 1666 Performed an experiment with a prism that
  showed that the suns light is white light
  consisting of all of the colors of the spectrum.

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Wave Theory of Light Thomas Young (1773
1829)-revisited

• 1801 Through use of the Double-Slit Experiment,
  the wave properties of light were first
  experimentally shown to exist.
• Experiment demonstrated that light undergoes
  interference and diffraction in much the same way
  that water and sound waves do.
• Used source of monochromatic light to eliminate
  the problems with phase differences associated
  with incoherent light.

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Young Double-Slit Experiment
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Wave Theory of Light James Clerk Maxwell (1831
1879)

• 1860 James Maxwell hypothesized that electric
  fields changing in time would create magnetic
  fields and vice-versa.
• These fields travel together in space as waves.
• Electromagnetic Wave

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Lets talk about photons.
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So if light IS a wave, and if light strikes a
surface, how will it impart energy? - Based on
its intensity (amplitude) - The energy will be
gradually absorbed as the whole material heats
up
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• What actually happens is different
• Electrons are ejected from a metal surface as
  soon as certain frequencies of light strike it
• Frequencies of light lower than a threshold
  frequency will not eject electrons, regardless of
  the intensity
• The ejected electrons have an upper bound of
  energy
• All of this ends up looking suspiciously like
  light is striking the metal in discrete packets,
  not a diffuse wave
• Einstein got the Nobel prize for figuring out how
  this works
• (I have heard it said that he really got it for
  Relativity, but no one could figure out how it
  worked so they gave it to him for something
  people could understand)

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• Photons
• Massless particles that carry no charge
• They carry energy
• They have momentum
• The energy of a photon is proportional to its
  frequency
• The energy of a beam of light is given by the
  intensity of the beam times the energy of the
  photons in the beam
• Photon energy E hf
  hc/l

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• How do we get photons?
• In most (all?) cases they are created by a change
  in the energy of some piece of matter
• One source is the decay of atomic particles
• Another source is the annihilation of a particle
  with its antiparticle
• A typical source of light is the change in the
  energy of an electron in orbit around an atom

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We like to think of electrons as being in pretty
orbits. We like to think of electrons as
particles, but they also act like waves and spend
part of the time inside the nucleus! Electrons
act like waves that move in specific resonant
frequencies. There is a fundamental state
(ground state) that is close to the atom. As
energy is added to the electron, it is added in
discrete chunks too little energy cannot be
absorbed, too much energy and some of it goes
into increasing the harmonic of the electron
and some is thrown away. Energy is quantized at
the atomic level.