10 -15 m Physics 116 World’s largest popcorn ball Session 38 Nuclei Dec 2, 2011 www.thepopcornfactory.com/ R. J. Wilkes Email: ph116@u.washington.edu
Announcements • � Final exam is NOT droppable! Everyone must take the final. • � Usual format. YOU must bring bubble sheet and pencil and calculator. • � Exam will be designed for 1 hr but you can take 2 hrs. • � Final will be about 1/3 on material since exam 3, remainder covers all material discussed during the course • � Textbook sections skipped in previous exams will also be skipped for the final. ONLY material covered in class (and not labelled “cultural supplement” or equivalent) will be on the exam. • � As usual we will have practice questions posted Weds next week, and go over the practice questions in class next Friday.
Lecture Schedule (to end of term) !"#$%& 3
A bit more on atomic physics • � Pauli exclusion principle (Wolfgang Pauli, Austria, c. 1925) – � Only one electron in an atom can have any possible quantum state (specific set of quantum numbers n, l, m l and m s ) – � Once a particular quantum state is occupied, other electrons are excluded from that state – � Exclusion principle applies to all fermions (particles with half-integer spin angular momentum, in units of � ): protons in nuclei obey same rules Quantum rules for atomic electrons: n = 1, 2, 3… principal quantum number, energy level (n=1=“ground state”) l = 0, 1, 2… (n-1) orbital angular momentum , in units of � (total of n states possible) Hydrogen atom’s electron levels: m l = -l … 0 … +l “magnetic quantum number”, z- component of L (total 2l+1 states) 2 in n=1 (l=0) m s = -1/2, +1/2 spin angular momentum, in units 8 in n=2 (l=0,1) of � (only 2 states possible) 18 in n=3 (l=0,1,2) 4
Periodic Table today: organizes elements by properties and atomic weight Yesterday Z and A increase as you go across and down. Elements in same column have similar properties. http://www.molres.org/MRI_DownloadPT.html “Noble gases”: Z refuse to mix with (# of protons) other elements! A (they have (total n+p) electrons filliing all allowed states) “Rare earth” elements go Uranium is heaviest natural element in here (white numbers = (Parenthesis means unstable element) 5 synthetic elements)
Nuclear (please, not “nuke-ular”!) physics FYI: physics terminology • � “Chemistry” = study of molecules as systems; size scale ~10 -9 m • � Atomic physics = study of atoms as systems – lasers, low temperature states of matter; size scale ~10 -10 m • � Nuclear physics = study of atomic nuclei as systems – nuclear reactions, radioactivity, properties of isotopes; size scale 10 -15 m • � Particle (“High Energy”) physics = study of “elementary” particles as systems; size scale << 10 -15 m (=1 femtometer [fm] or fermi) – � recall: EP’s = things made when you smash nuclei with high energy projectiles; not the same as “fundamental” particles like quarks – � Why “high energy”? Recall de Broglie: ! = h / p � • � high momentum (= high energy) means short wavelength – � Use HE particles like microscopes, to study structure of particles • � Energy scale corresponds to size scale (eV = electron volt) – � Atoms ~ few eV, nuclei ~ few million eV, quarks ~ billions of eV 6
What holds nuclei together? • � Recall basics of nuclear structure: – � Nucleus = Z protons (Z=“atomic number”: 1=hydrogen, 2=helium…) and N neutrons; Z + N = A (atomic mass number) – � Protons have + charge: they repel each other via 1/r 2 Coulomb’s Law • � Remember nuclear size scale: incredibly small r = huge force! A = atomic mass number = total number of protons + neutrons Z = atomic number , # of protons (determines which element it is) N = A – Z =number of neutrons (determines which isotope it is) Isotopes have same Z but different A’s. Example: 6 C 12 = “carbon-12”, has 6 protons (like all carbon) and 6 neutrons (unlike “carbon-14”, which has 8 neutrons) – � Nuclear “glue” holds protons and neutrons into nuclei • � “strong nuclear force” is carried by particle called (what else?) gluon – � But strong force has limited range: not infinite like electromagnetic force • � Nucleus is like a compressed spring, held together by a rope – � Cut the rope, and stored “binding energy” is released • � Energy taken up to bind nucleus is reflected in mass E=mc 2 + conservation of energy 7
Radioactivity • � H. Becquerel (1896): discovered radioactivity – � Sample of mineral containing uranium fogged photographic plate • � Marie Walenska-Curie (“Madame Curie”, c. 1900): with P. Curie, made many important early discoveries (Nobel Prize 1903) – � “Radiation” from radioactive elements takes 3 main forms • � Alpha rays are positively charged, heavy (actually: helium nuclei) – � Cannot penetrate much matter (piece of paper stops them) • � Beta rays are negatively* charged, very low mass (electrons) – � More penetrating than alphas, but stopped by a thin lead sheet • � Gamma rays are uncharged, massless (high energy photons) – � Able to penetrate even thick lead sheets – � Later (1932) another form was identified: neutrons • � Uncharged, so not subject to electrical forces: can percolate through shielding for a long distance (dangerous: hard to stop!) – � Neutrons decay to proton + electron + neutrino, mean life ~ 15 min – � All these particles can be produced by radioactive decay of nuclei * Nuclei can also emit anti-electrons = positrons, + charge 8
Binding energy curve • � The whole is not equal to the sum of its parts ! – � Mass of helium nucleus (2p+2n) is less than 2*M proton + 2*M neutron • � Difference = binding energy (stored energy) • � Fuse p’s and n’s to make He, and energy is released: nuclear fusion – � Mass of uranium nucleus is more than sum of its p’s and n’s • � Break uranium nucleus into lighter nuclei and E is released: nuclear fission – � Iron nucleus has minimum binding energy per particle: most stable Energy of Energy of proton an isolated bound inside nucleus proton of mass number A Energy is released when heavy nucleus is broken up, or light nuclei are fused Uranium Atomic mass number, A 9
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