1. Nuclear Physics E = mc 2

2. Outline Theory of Special Relativity Postulates E = mc 2 The Atom What makes up the atom? What holds the atom together? Quantum Physics

3. 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

4. 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 = mc 2

5. E = mc 2 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” = mc 2.

6. 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 10 8 m/s) 2 E = 6.3 x 10 18 J = 6 x 10 15 Btu! My rest energy is about 5% of the energy consumed per year in the US! Matter is equivalent to energy.

7. 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

8. The atomic nucleus m electron = 9.11 x 10 -31 kg E = (9.11 x 10 -31 kg) c 2 = 8.2 x 10 -14 J 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.

9. 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.

10. Chart of Nuclear Isotopes

11. Nuclear stability Not all isotopes are equal Range of stability Z < 40 is Z  N Z > 40, N > Z Isotopes which are NOT stable experience RADIOACTIVE DECAY.

12. 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.

13. 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!

14. 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.