1Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review1Hour Exam 3 Review• Hour Exam 3: Wednesday, Apr. 18 In-class• Twenty multiple-choice questions• Will cover: Basic Quantum MechanicsUses of Quantum MechanicsThe nucleus, radioactive decay• This is Lectures 22-31, Chap. 13-15.5 in text• You should bring– Your student ID– 1 page notes, written double sided– Calculator– Pencil for marking answer sheetMon. Apr 16, 2007 Phy107 Hour Exam 3 Review2From last time…• Radioactive decay– Unstable nuclei change to a more stableconfiguration by emitting a particle– Heavy nuclei can emit alpha particle(helium nucleus)– Lighter atoms can emit beta particle (electron)Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review3Beta decay• Nucleus emits an electron(negative charge)• Must be balancedby a positive chargeappearing in the nucleus.This occurs as a neutronchanging into a protonMon. Apr 16, 2007 Phy107 Hour Exam 3 Review4How can this be?• Nucleons have aninternal structure.• Made up of differenttypes of quarks.• In this sense,neutrons andprotons not sodifferent.Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review5Neutron decay• Can be clearer tovisualize thisdiagramatically.• One of the downquarks changed to anup quark.• We identify this newcombination of quarksas a proton.Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review6Example of beta decay•14C (radioactive form of carbon) decays bybeta decay (electron emission).• Carbon has 6 electrons, so six protons.•14C has (14-6)=8 neutrons.Now have a new element (one more proton)Element with 7 protons in nucleus is Nitrogen ! 614C "714N + e#Beta decaynumber of nucleons stays fixed, butone of the neutrons changes into a protonone additional proton -> different element!2Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review7Radioactive carbon•14C naturally present in the atmosphereas a result of transmutation of 14N.Cosmic ray proton shatters nucleusof atmospheric gas atom.This produces neutrons.Neutron knocks protonout of 14N nucleus.14N becomes 14C after losing neutronMon. Apr 16, 2007 Phy107 Hour Exam 3 Review8Natural atmospheric abundance•14C is produced at a particular rate fromtransmutation of 14N in upper atmosphere.•14C is radioactive with half-life of 5,730 yrs– Decays at rate set by half-life• Balance of these results in an equilibriumratio in the atmosphere• One radioactive 14C atom for every ~ 1012non-radiocative 12C atom. ! 14C12C= 1.3 "10#12Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review914C to 12C ratio•14C has a half-life of ~6,000 years,continually decaying back into 14N.• Steady-state achieved in atmosphere,with 14C:12C ratio ~ 1:1 trillion (1 part in 1012)As long asbiologicalmaterial alive,atmosphericcarbon mixingested (as CO2),ratio stays fixed.After death, no exchangewith atmosphere. Ratiostarts to change as 14C decaysMon. Apr 16, 2007 Phy107 Hour Exam 3 Review10Carbon-dating questionThe 14C:12C ratio in a fossil bone is found to be 1/8that of the ratio in the bone of a living animal.The half-life of 14C is 5,730 years.What is the approximate age of the fossil?A. 7,640 yearsB. 17,200 yearsC. 22,900 yearsD. 45,800 yearsSince the ratio has been reducedby a factor of 8, three half-liveshave passed.3 x 5,730 years = 17,190 yearsMon. Apr 16, 2007 Phy107 Hour Exam 3 Review11Other carbon decays• Lightest isotopes of carbon are observed to emit aparticle like an electron, but has a positive charge!•This is the antiparticle of the electron.•Called the positron.Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review12Antimatter• Every particle now known to have an antiparticle.• Even antimatter has been generated.Matter and antimatterannihilate when inclose proximity.Photons are created sothat energy isconserved.3Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review13What is going on?•14C has more neutronsthan the most stableform 12C.– So it decays by electronemission, changingneutron into a proton.• Other isotopes of carbonhave fewer neutrons– Decays by emittingpositron, changingproton into neutron.Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review14Decay question20Na decays in to 20Ne, a particle is emitted?What particle is it?Na atomic number = 11Ne atomic number = 10A. AlphaB. Electron betaC. Positron betaD. Gamma20Na has 11 protons, 9 neutrons20Ne has 10 protons, 10 neutronsSo one a proton (+ charge ) changed to aneutron (0 charge) in this decay.A positive particle had to be emitted.Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review15Gamma decay• So far– Alpha decay: alpha particle emitted from nucleus– Beta decay: electron or positron emitted• Both can leave the nucleus in excited state– Just like a hydrogen atom can be in an excited state– Hydrogen emits photon as it drops to lower state.Nucleus also emits photon asit drops to ground stateThis is gamma radiationBut energies much larger,so extremely high energyphotons.Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review16Radioactive decay summary• Alpha decay– Heavy nucleus emits alpha particle(2 protons + 2 neutrons)• Beta decay– Beta—: Nucleus emits electron (neutron becomes proton)– Beta+: Nucleus emits positron (proton becomes neutron)• Gamma decay– Nucleus emits high-energy photon and drops from excitedstate to lower energy state.Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review17Review: Quantum Mechanics• Quantization of light– Light comes in discrete clumps (photons)– Light shows both particle and wave-like properties– Photon energy E = hf=hc/λ• Matter waves– Matter shows both particle and wave-like properties–– Evidence for wave properties isinterference and diffraction ! deBroglie wavelength = Planck's constantmomentumMon. Apr 16, 2007 Phy107 Hour Exam 3 Review18Photoelectric effect summary• Light is made up of photons, individual‘particles’, each with energy hf• One photon collides with one electron- knocks it out of metal.• If photon doesn’t have enough energy,cannot knock electron out.• Intensity ( = # photons / sec)doesn’t change this.! =hc"=1240 eV # nm"4Mon. Apr 16, 2007 Phy107 Hour Exam 3 Review19Photoelectric effect questionA scientist is trying to eject electrons from a metal byshining a light on it, but none are coming out. Toeject electrons, she should change the light by…A. decreasing the frequencyB. increasing the frequencyC. increasing the intensityD. increasing the wavelengthMinimum frequency(maximum wavelength)required to eject electronMon. Apr 16, 2007 Phy107 Hour Exam 3 Review20Compton
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