Nuclear Physics
E = mc2
Outline Theory of Special Relativity
Postulates E = mc2
The Atom What makes up the atom? What holds the atom together?
Quantum Physics
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
b v = 100 m/s
a v = 20 m/s
v = .9 c v = c
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
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.
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.
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
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.
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.
2311
210
11 HHH
X NAZ
Chart of Nuclear Isotopes
Nuclear stability Not all isotopes are equal Range of stability
Z < 40 is ZN Z > 40, N > Z
Isotopes which are NOT stable experience RADIOACTIVE DECAY.
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.
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!
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.
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