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LearningGoal7.3Icandescribethesubnuclearpropertiesandevolutionofatomicnuclei.
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Icandescribehowthefourfundamentalinteractionsareinvolvedintheevolutionofatomicnucleithroughthe
actionofstars.
Icandescribenuclearfissionandfusion,explainingwhylightnucleireleaseenergyinfusionandheavynuclei
releaseenergyinfission.
Icananalyzebalancednucleardecayequations.
Iunderstandthatneutronsandprotonsarecompositeparticles,consistingoffractionallychargedquarksbound
bythestrongnuclearforce,andthatnuclearbondingisduetotheresidualstrongnuclearforce.
Icantracetheimpactofphysicsonhistory,andviceversa,throughWorldWarIIandtheColdWar,andcanspeak
topublicpolicyaroundnuclearenergyandwarfare.
PartI:ParticlesandInteractions
IntheStandardModelofParticlePhysics,physicistscanexplainallobservedsubatomicphenomenawithaverysmall
numberofinteractionsandbasicparticles.
Thefourfundamentalinteractionsarelistedinorderofstrength,andincluderanges,below.
1. Iftheelectromagneticforceissomuchstrongerthangravity,andbothareinfiniteinrange,whyisitthatInotice
thegravitationalpulloftheEarthbutnotitselectromagneticpull?
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Inordertothinkaboutthebasicparticlesthatmakeupallmatterintheuniverse,wewillfirsttalkaboutthechemical
elements,manyofwhicharelikelyfamiliartoyoualready.
Whenwelookattheuniverseasawhole,thetwosimplestelements,hydrogen(consistingofoneproton,one
electron)andhelium(twoprotons,twoneutrons,andtwoelectrons)arebyfarthemostabundant.Thediagram
belowdemonstratestheabundanceoftheseelementsmeasuredinpartsper10,000.Thismeansthat,outofevery
10,000atomsintheuniverseasawhole,7,500arehydrogenand2,300arehelium,butonly5aresulfuratoms.
2. Fillinthe#ofprotonslist.Thefirst2aredoneforyou.
Number of Protons
1
2
3a. Thechartaboveshowstheelementsinorderofhowmuchthereisintheuniverse.Highlightthese“common
elements”onthechartbelow
b. Aretheremorenucleiwithevenoroddnumbersofprotonsinthislist?
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4. Thetablesbelowusesthesizeofeachblocktorepresenttheabundanceoftheelement.Onetableisforthe
abundanceintheEarth’scrust,andtheotherisfortheuniverseasawhole.Explainwhichiswhich.Proposea
physicsexplanationforwhytheyaredifferent?
PartII:NuclearFUSION
5. Shownhereisaschematicdrawingofonemechanism
–ofmany–bywhichscientistsbelievethatmore
complexelementsaregeneratedfromsimpler
elements.Timeispassingfromtoptobottom.
a. Atthestartarefourprotons–whichofthe
followingbestdescribesasingleproton?
Itisahydrogenatom.
Itisanionofhydrogen–ahydrogennucleus.
Itisanisotopeofhelium.
b. Inorderfortwoprotonstocollidewithone
anotherand‘touch’,whichforcemustthey
overcome?
Electricalrepulsion
Gravitationalattraction
c. Whentwoprotonsareincloseproximity,theelectricalpotentialenergyis(circleone):HIGHLOW
Thismeansthat,inordertotouch,thekineticenergyoftheprotonsmustbe(circleone):HIGHLOW
Thismeansthattheambienttemperaturemustbe(circleone):HIGHLOW
Ingeneral,whenforcesarerepulsive,ittakesworkto(circleone):BRINGTHEMTOGETHER
SPLITTHEMAPART
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e. Asweseeinthefirststep(bluebox),whenthe
protonsfusetogether,oneofthem“turnsinto”a
neutronthroughtheso-calledweaknuclear
interaction.Theresultantproton-neutralpaircan
bebestdescribedas:
anisotopeofhydrogen
anisotopeofhelium
Scientistsnoticedsomethingverystrangehere.How
manyotherparticlesareformedinthisstep?__
Oneofthenewparticlesbehavesexactlylikean
electron(samemass,allthesamepropertiesEXCEPT
it’spositivelycharged.)Theycalledthisnewparticlean
anti-electron,orapositron.
Whentheylookedcarefully,theyfoundthatforevery
particle,thereisa“backwards”particlethatisidentical
butoppositelycharged.Researchwhatthesenew
particlesarecalled.Whathappenswhentwooppositemeet?
Theothernewparticleformediscalledaneutrino.Itisvery,verysmall.Comparedtoaneutrino,anelectronlooks
huge.Neutrinosaresosmallforalongtimewethoughttheyhadnomass(likephotons),butnowweknowtheyhave
masswejusthaven’tfiguredoutgoodwaystomeasurethem.65billionneutrinospassthroughevery1cmofEarth
everysecond.Whydon’tyounoticealltheneutrinoshittingyou?
f. Asecondfusionthenoccurs(redbox),andagammarayisemitted.Agammarayisaphoton,justlikethephotons
givenoffinBohr’sexperiment,butitisnotvisible.Doyouexpectittobehigherorlowerfrequencythanvisible
light?
Thefusionwithanadditionalprotonmakesanucleusconsistingoftwoprotonsandoneneutrons.Thisisbest
describedas(circleone):
anisotopeofhydrogen
anisotopeofhelium
g. Afinalfusioninteractionbringstogethertwoseparatesetsofinteractions,formingasinglenucleuswithtwo
protonsandtwoneutrons.Thisfinalproductisbestdescribedas(circleone):
anisotopeofhydrogen
anisotopeofhelium
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7. Whentalkingabouthydrogenfusioninapreviousproblem,wediscussedtheneedforhightemperaturesto
overcometherepulsionofprotons.Becausetheheliumnucleiintheimageaboveeachhavetwoprotons,the
electricrepulsionforceis_____________timesstronger.(fillintheblank)
Thismeansforthefusionofheliumnucleirequires(circleone)HIGHERLOWERtemperaturethanthefusionof
hydrogennuclei.
Infact,thetemperaturesneededtofusehydrogentogethertomakeheliumareintherangeoftensofmillionsof
degrees.Suchtemperatureareoftenfoundinstars.Wouldfusingheliumtakemoreorlesskineticenergy?
Iftimepermits:
8. Moremassivenucleiareformed
throughfusioninthecentersof
redsupergiantstars,uptoa
maximummassassociatedwith
theironnucleus.Themost
commonreactionsareshownat
right.
Ifthereactionsshownarethe
mostcommon,wouldyou
expectmoreheavynucleiinthe
universetohaveaneven
numberofprotonsoranodd
numberofprotons?Why?
Inthefinalreactionshownabove,twosiliconnucleifusetomakeiron.Becauseeachsiliconatomhas______
protons(writein),thismeanstheelectricalrepulsionforcebetweenthemis________timeshigherthanthe
repulsionbetweentwoprotonsandthereforethetemperaturerequiredforthisreactiontooccuris(circleone)
VERYHIGHVERYLOW.Thisfusionofsiliconintoironis,infact,thelastactofamassivestarabouttoexplodein
amassivesupernova.Alltheironyou’veeverencounteredinyourlife,includingtheatomsofitinyourownblood,
wereformedinthefinalfractionofasecondbeforeastarexploded.
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PartIII:NuclearFISSIONDay2
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InquiryPacket:NuclearEvolution
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9.Forthesemoremassiveelements,nuclearenergyisreleasedwhentheysplitapartRATHERthanwhentheyfuse
together.Thisiscallednuclearfission.Onepossiblefissiondrawingforuranium(theheaviestnaturallyoccurring
element)isshownbelow.
Identifyalloftheparticlesinvolvedinthisprocessbeforedoingthesimulation.
Let’svisitasimulationofthisat
https://trinket.io/glowscript/f61cab61ca
Inthissimulation,asampleofuraniumthatismostly238U(showninblue)hasbeenrefinedtoincludegreateramounts
oftheisotope235U(showninred),whichisunstabletofissionwhenstruckbyaslowneutron.Theuraniumatomsare
partofacompoundalongwithoxygenatoms(green).Eachtimeyouhit“RUN”thesystemisrandomized.
10. Inwhatsensedoesthissimulationshowa“runaway”or“chain”reaction?Whatismeantbythisterm?
11. Iftherewerefewer235Unuclei(red)–perhapsbecauseyourrefiningplantisn’tasgoodasthoseinothercountries
–wouldyouexpectthereactiontobeasefficient?Explain.
12. Infusion,istheelectricrepulsionstrongerorweakerthanthestrongforce?Howaboutinfission?Explain.
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Anotherwaytovisualizethisisusingthediagrambelow.Forelementslessmassivethaniron,yougetenergyout
whenyoufusethemtogether.Thisishowstarsshine.Forelementsmoremassivethaniron,yougetenergyoutwhen
theysplitapart.Thisishownuclearpowerplantsandatombombswork.Anyelementmoremassivethanironhadto
befabricatedbyovercomingtheCoulombbarrierbyahugeinfluxofenergy,typicalofsupernovaexplosions.
13. Basedonthischart,wouldyouexpectmoreenergytobereleasedbythefusionofhydrogenintohelium,orbythe
fissionofuraniumintokryptonandbarium?Explainwhy.
14. Therearetwobasictypesofnuclearweapons:atomicbombsthatusefissionandthermonuclearbombsthatusea
smallamountoffissiontogettohightemperatures,thenusefusiontogeneratetheirenergy.Assumingtheyare
thesamesize,whichismorepowerful(Usethechart).
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PartIV:Sub-nuclearStructure
Infact,protonsandneutronsarenotfundamentalparticles.TheStandardModelofParticlePhysicstreatsthemas
compositeparticles,madeupofthreequarksboundtogetherbythestrongnuclearforce:
15. Ifthismodelisaccurate,whatMUSTtheelectricchargesoftheupanddownquarksbe,expressedintermsofthe
fundamentalchargee?Explainhowyouknow.Rememberthattheprotonhasatotalchargeof+eandthe
neutronhasatotalchargeofzero.
upquarkelectriccharge: downquarkelectriccharge:
Thestrongnuclearforcehasitsownkindof“charge’–notelectriccharge,butcolorcharge.There
areTHREEcolorchargesinthestrongforce,comparedtojustthetwo(plusandminus)inthe
electricforce.ThethreecolorsareRED,GREEN,andBLUE.(Infact,theyarejustconvenient
ideas–nothingisreallyredorblueoranything.)Everyparticleeffectedbythestrongforceisa
colororcombinationofcolors.Whatcolordoyougetbycombiningredblueandgreen?
Inthismodel,protonsandneutronsdon’tattractoneanotherthroughthestrongforce–onlyquarksdothat–in
fact,theprotonandneutronattracteachotherthroughapolarization-typeeffectcalledtheresidualstrongforce.
Labeltheseasprotonsorneutrons.Istheredupquarkontheleftattractedorrepelledfromtheblueupquarkin
theneighboringparticle?
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16. Theconversionofprotonintoaneutron,whichoccursinthefirststepofthefusionprocessintheSun,actuallyis
theconversionofasinglequark,a(circleone)UPDOWNquarkintoa(circleone)UPDOWNquark.
TheworkofNobelPrizewinnerMurrayGell-MannandothersledustotheSTANDARDMODELOFPARTICLEPHYSICS,
inwhichALLparticlephysicscanbeconsolidatedintoasingletheory–fourinteractions,andjustasmallnumber
ofparticles.Thisexplaineverythingintheuniverse,toveryhighprecision,exceptgravity.Wehaven’tfiguredout
whygravityworksinthismodel.Whatisthemostfamiliarlepton?Howaboutboson(rightbox)?