3/17/2015 FourForcesBOUNDARIESOFSCIENCECRN52926201402

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CourseContent ... WeekSix FourForcesH

FUNDAMENTALFORCES

Therearefourfundamentalforceswithinallatoms,thatdictateinteractionsbetweenindividualparticles,andthelargescalebehaviorofallmatterthroughouttheUniverse.Theyarethestrongandweaknuclearforcestheelectromagneticforceandgravitation.

GRAVITY

GravitationisaforceofattractionthatactsbetweeneachandeveryparticleintheUniverse.Itisalwaysattractive,neverrepulsive.Itpullsmattertogether,causesyoutohaveaweight,applestofallfromtrees,keepstheMooninitsorbitaroundtheEarth,theplanetsconfinedintheirorbitsaroundtheSun,andbindstogethergalaxiesinclusters.

THEELECTROMAGNETICFORCE

Theelectromagneticforcedeterminesthewaysinwhichelectricallychargedparticlesinteractwitheachotherandalsowithmagneticfields.Thisforcecanbeattractiveorrepulsive.Likecharges(twopositiveortwonegativecharges)repeleachotherunlikechargesattract.Theelectromagneticforcebinds[negativelycharged]electronsintotheirorbitalshells,aroundthepositivelychargednucleusofanatom.Thisforceholdstheatomstogether.Theelectromagneticforcecontrolsthebehaviorofchargedparticlesandplasmas(aplasmaisamixtureofequalnumbersofpositiveionsandnegativeelectrons)as,forexample,insolarprominences,coronalloops,flares,andotherkindsofsolaractivity.Theelectromagneticforcealsogovernstheemissionandabsorptionoflightandotherformsofelectromagneticradiation.Lightisemittedwhenachargedparticleisaccelerated(forexample,whenanelectronpassesclosetoanion,orinteractswithamagneticfield)orwhenanelectrondropsdownfromahighertoalowerenergylevelofanatom(fromanoutertoaninner'orbit'aroundtheatomicnucleus).

THESTRONGNUCLEARFORCE

Thestrongnuclearforcebindstogethertheprotonsandneutronsthatcompriseanatomicnucleusandpreventsthemutualrepulsionbetweenpositivelychargedprotonsfromcausingthemtoflyapart.Thestrongnuclearinteractionbetweenindividualprotonsandneutronsisbelievedtobearemnantofanunderlyingstrongforce(whichiscalledthe'colorforce')thatbindsquarkstogetheringroupsofthreetomakeprotonsandneutrons.Becausethestrongforcebindsnuclearparticlessotightlytogether,hugeamountsofenergyarereleasedwhenlightweightnucleiarefusedtogether(fusionreaction)orheavynucleiarebrokenapart(fissionreaction)..Thestrongnuclearforceinteractionistheunderlyingsourceofthevastquantitiesofenergythatareliberatedbythenuclearreactionsthatpowerthestars.

THEWEAKNUCLEARFORCE

Theweaknuclearforcecausestheradioactivedecayofcertainparticularatomicnuclei.Inparticular,thisforcegovernstheprocesscalledbetadecaywherebyaneutronbreaksupspontaneouslyintoaproton,andelectronandanantineutrino.Ifaneutronwithinanatomicnucleusdecaysinthisway,thenucleusemitsanelectron(otherwiseknownasabetaparticle)andtheneutrontransformsintoaproton.Thisincreases(byone)thenumberofprotonsinthatnucleus,therebychangingitsatomicnumberandtransformingitintothenucleusofadifferentchemicalelement.Theweakforceisresponsibleforsynthesizingdifferentchemicalelementsinstarsandinsupernovaexplosions,throughprocessesinvolvingthecaptureanddecayofneutrons.Whenconfinedwithinastable(nonradioactive)atomicnucleus,aneutronisastable,longlivedparticle.Onceremovedfromanatomicnucleus,afreeneutronwillundergobetadecay,typicallyinabouttwentyminutes.Thereverseprocessofbetadecayoccursinthecollapsingcoresofsupernovae,whereprotonsandneutronsarefusedtogethertocreatethevastnumbersofneutronsthatpopulatetheendproductofthecollapseaneutronstar.

FourForces

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