A2 Physics 物理 出国英语Nuclear Energy Part 1 - Mass and energy.pptVIP

MassandenergyAlbertEinstein’sfamousequationE=mc2givestheuniversalrelationshipbetweenenergyandmass.Ifapieceofmaterialofmass5.0kgcouldbecompletelyconvertedintoenergy,howmuchenergywouldbereleased?energy(J)=mass(kg)×[speedoflight(ms–1)]2energy(J)=5.0×(3.0×108)2energyreleased(J)=4.5×1017JThisisacolossalamountofenergy–threetimestheamountthatarrivesattheEarthfromtheSuneverysecond.

Energy–massconversionTheconversionofsmallmassesintolargequantitiesofenergyisthebasisofnuclearpowerandthenuclearbomb.ItisalsothesourceoftheSun’senergy.Energycanalsobeconvertedintomass.Forexample,agammaphotoncanbeconvertedintoanelectronandapositron.γe–e+Thisiscalledpairproduction.

Massandenergycalculations

Atomicmassunit1u=1.66×10–27kgThemassofanatomisoftenexpressedinatomicmassunits(u),toavoidusingverytinyfractionsofthekilogram.Oneatomicmassunitisequaltoone-twelfthofthemassofacarbon-12atom:Themassandrelativechargeofeachoftheconstituentparticlesoftheatomisshowninthetablebelow:1.00731.00875.49×10–4protonneutronelectronparticlemass/urelativecharge+10–1

MassdefectThemassofanucleusisalwayslessthanthetotalmassofthenucleonsfromwhichitismade.Thedifferencebetweenthemassofanucleusandthetotalmassofitsnucleonsiscalledthemassdefectofthenucleus.massdefect=(massofnucleons)–(massofnucleus)Fromthedefinitionofanatomicmassunit,onecarbonatomhasamassof12u.Butwhatisthecombinedmassofthesixprotonsandsixneutronsthatmakeupacarbon-12nucleus?totalmass=(6×1.0073)+(6×1.0087)=12.096u(massdefectcanbegiveninkgoratomicmassunits)

BindingenergyThebindingenergyofanucleusisdefinedastheenergyrequiredtoseparateallofthenucleonsinanucleus.Thisistheenergyneededtoprovideth