Nuclear Physics

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Nuclear Physics

• E mc2

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Outline

• Theory of Special Relativity
• Postulates
• E mc2
• The Atom
• What makes up the atom?
• What holds the atom together?
• Quantum Physics

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The theory of Special Relativity

• Galilean relativity
• To car A, car B is going 100 20 80m/s.
• Special relativity Light moves at v c for all
  observers

v 100 m/s
b
v 20 m/s
a
v .9 c
v c
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Consequences of Relativity

• Due to Einstein 1905
• Length contraction Objects moving near the speed
  of light appear shorter
• Time dilation Clocks moving near the speed of
  light appear to be running slow.
• No object having mass can move at c.
• Rest energy E mc2

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E mc2

• All laws of nature must hold, regardless of how
  fast or slow the observer is moving.
• Energy conservation is a law of nature.
• Einstein predicted that, in order for Energy
  Conservation to hold, there must be a certain
  amount of energy associated with mass, called the
  Rest Energy mc2.

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What does it mean?

• It means that all matter can theoretically be
  converted into energy.
• Example
• My mass is 70 kg.
• My rest energy is E (70 kg) (3 x 108 m/s)2
• E 6.3 x 1018 J 6 x 1015 Btu!
• My rest energy is about 5 of the energy consumed
  per year in the US!
• Matter is equivalent to energy.

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Atoms the constituents of matter

• An atom
• Protons and electrons are charged oppositely,
  having 1.6 x 10-19 C
• The type of atom is determined by Z the number
  of protons.

Electrons (-e) in orbit
Central nucleus of Protons (e) and neutrons
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The atomic nucleus

• m electron 9.11 x 10-31 kg
• E (9.11 x 10-31 kg) c2 8.2 x 10-14J
• Convert to units of eV.
• m electron .511 MeV
• m proton 938.26 MeV
• m neutron 939.55 MeV.
• The nucleus has the most mass, and therefore the
  most available energy.

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Isotopes

• The kind of atom is determined by Z
• The number of neutrons, N, can vary.
• Atoms having the same Z but different N are
  called ISOTOPES.

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Chart of Nuclear Isotopes
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Nuclear stability

• Not all isotopes are equal
• Range of stability
• Z lt 40 is Z?N
• Z gt 40, N gt Z
• Isotopes which are NOT stable experience
  RADIOACTIVE DECAY.

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Nuclear Decay

• Alpha decay The nucleus expels two protons and
  two neutrons an alpha particle.
• Beta decay The nucleus expels one electron (a
  beta particle), and a neutron in the nucleus
  turns into a proton!
• Gamma decay The nucleus expels a high energy
  photon a gamma particle.
• Fission A large nucleus, like Uranium, splits
  into two smaller nuclei, releasing several
  neutrons.

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A HOT Nucleus.

• Many kinds of decay, but all of them release a
  lot of energy!
• For comparison
• The energy of a visible light photon is 1-3 eV
• The energy released per nuclear decay is anywhere
  from 100 keV to 100 MeV in fission.
• The energy comes from lost nuclear mass!

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Neutron decay

• A neutron will spontaneously turn into a proton,
  electron and a neutrino (anti-).
• The mass of the products is .78 MeV less than the
  mass of the neutron! That energy is released as
  heat.