Physics 1202: Lecture 38Today’s Agenda

• Announcements:

• Extra creditsExtra credits–Final-like problemsFinal-like problems–Team in class Team in class –Teams 12, 13 & 14Teams 12, 13 & 14

• HW 10 due this FridayHW 10 due this Friday• Review session this FridayReview session this Friday

–Q-center Q-center 5:30-7 pm in Gentry 131.

• Nuclear & Particle Physics

Nuclear Physics

The nucleus• Made of

– Z protons– N neutrons– Mass number A = Z + N

Z

A

• Size and shape– Almost spherical– Not always though

1.2 X 10-15 m=1.2 fm

The strong nuclear force• A force must exist to compensate electric repulsion• Nuclear force

– Always attractive– The same between protons or neutrons– 100 times stronger than electrostatic force– Short-range: about only 3 fm

The strong nuclear force• Neutron are essential

– Provide the “glue” to maintain protons together– Meed more n as A grows

Binding energy• The mass of an atom is less than its constituents• 12C (m=12 u): used to defined the atomic mass unit u• 6 protons + 6 neutrons + 6 electrons– Adding = 12.09893 u• “missing” mass = negative or binding energy

• The binding energy Eb

• Eb is very large– 12C: (m)c2 = (12.09893-12) u c2 = 92.2 MeV – roughly 107-8 times chemical energy (~ 1 eV) (remember atomic levels)

Binding energy per nucleon• Eb /A

– increases with A until 56Fe: fusion– decreases after: fission

Radioactivity• Discovered (1896) Henri Becquerel

– Uranium salt near photographic plates• Three types of radiation

– Alpha: alpha particle (4He nucleus)– Beta: 2 types of beta particles

» Electron ( - ) and positron ( + ) – Gamma: electromagnetic waves ( )

• Alpha: radium and radon• Beta:

– Neutrino (to account for energy conservation)• Gamma

Activity and half-life• In given interval t, a nucleus may decay

: decay constant

• Activity (absolute value) unit: Bq

• Decay law

• Half-life

5 steps methods• Draw and list quantitites

• Concepts and equations needed

• Solve in term of symbols

• Solve with numbers

• Checks values and units