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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