剑桥AS物理课程知识点总结.docx

剑桥AS物理课程知识点总结.docx

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剑桥AS物理课程知识点总结

I

Displacementisthedistancetravelledinaparticulardirection.

Velocityisdefinedbythewordequation

Thegradientofadisplacement-timegraphisequaltovelocity:

Distanceandspeedarescalarquantities.Ascalarquantityhasonlymagnitude.

Displacementandvelocityarevectorquantities.Avectorquantityhasbothmagnitudeanddirection.

Vectorquantitiesmaybecombinedbyvectoradditiontofindtheirresultant.

II

Accelerationisequaltotherateofchangeofvelocity.

Accelerationisavectorquantity.

Thegradientofavelocity-timegraphisequaltoacceleration:

Theareaunderavelocity-timegraphisequaltodisplacement（ordistancetravelled）.

Theequationsofmotion（forconstantaccelerationinastraightline）are:

Vectorssuchasforcescanberesolvedintocomponents.Componentsatrightanglestooneanothercanbetreatedindependentlyofoneanother.Foravelocityvatanangleθtothex-direction,thecomponentsare:

x-direction:

vcosθy-direction:

vsinθ

Forprojectiles,thehorizontalandverticalcomponentsofvelocitycanbetreatedindependently.Intheabsenceofairresistance,thehorizontalcomponentofvelocityisconstantwhiletheverticalcomponentvelocitydownwardsincreasesatarateof9.81ms-2.

III

Anobjectwillremainatrestorinastateofuniformmotionunlessitisactedonbyanexternalforce.ThisisNewton’sfirstlawofmotion.

Forabodyofconstantmass,theaccelerationisdirectlyproportionaltotheresultantforceappliedtoit.ResultantforceF,massmandaccelerationaarerelatedbytheequation:

resultantforce=mass×accelerationorF=ma

ThisisaformofNewton’ssecondlawofmotion.

Whentwobodiesinteract,theforcedtheyexertoneachotherareequalinmagnitudeandoppositeindirection.ThisisNewton’sthirdlawofmotion.

Theaccelerationproducedbyaforceisinthesamedirectionastheforce.Wheretherearetwoormoreforces,wemustdeterminetheresultantforce.

Anewton（N）istheforcerequiredtogiveamassof1kganaccelerationof1ms-2inthedirectionoftheforce.

Thegreaterthemassofanobject,themoreitresistschangesinitsmotion.Massisameasureoftheobject’sinertia.

Theweightofanobjectisaresultofthepullofgravityonit:

weight=mass×accelerationoffreefall（W=mg）

weight=mass×gravitationalfieldstrength

Anobjectfallingfreelyundergravityhasaconstantaccelerationprovidedthegravitationalfieldstrengthisconstant.However,fluidresistance（suchasairresistance）reducesitsacceleration.Terminalvelocityisreachedwhenthefluidresistanceisequaltotheweightoftheobject.

IV

Forcesarevectorquantitiesthatcanbeaddedbymeansofavectortriangle.Theirresultantcanbedeterminedusingtrigonometryorbyscaledrawing.

Vectorssuchasforcescanberesolvedintocomponents.Componentsatrightanglestooneanothercanbetreatedindependentlyofoneanother.ForaforceFatanangleθtothex-direction,thecomponentsare:

x-direction:

Fcosθ

y-direction:

Fsinθ

Themomentofaforce=force×perpendiculardistanceofthepivotfromthelineofactionoftheforce.

Theprincipleofmomentsstatethatforanyobjectthatisinequilibrium,thesumoftheclockwisemomentsaboutanypointprovidedbytheforcesactingontheobjectequalsthesumoftheanticlockwisemomentsaboutthatsamepoint.

Acoupleisapairofequal,parallelbutoppositeforceswhoseeffectistoproduceaturningeffectonabodywithoutgivingitlinearacceleration.

torqueofacouple=oneoftheforces×perpendiculardistancebetweentheforces

Foranobjecttobeinequilibrium,theresultantforceactingontheobjectmustbezeroandtheresultantmomentmustbezero.

V

TheworkdoneWwhenaforceFmovesthroughadisplacementsinthedirectionoftheforce:

W=FsorW=Fscosθ

whereθistheanglebetweentheforceandthedisplacement.

Ajouleisdefinedastheworkdone（orenergytransferred）whenaforceof1Nmovesadistanceof1minthedirectionoftheforce.

TheworkdoneWbyagasatpressurepwhenitexpands:

W=pΔV

whereΔVistheincreaseinitsvolume.

Whenanobjectofmassmrisesthroughaheighth,itsgravitationalpotentialenergyEpincreasesbyanamount:

Ep=mgh

ThekineticenergyEkofabodyofmassmmovingatspeedvis:

Ek=1/2mv2

Theprincipleofconservationofenergystatesthatforaclosedsystem,energycanbetransformedtootherformsbutthetotalamountofenergyremainsconstant.

Theefficiencyofadeviceorsystemisdeterminedusingtheequation:

efficiency=（usefuloutputenergy）/（totalinputenergy）×100%

Poweristherateatwhichworkisdone（orenergyistransferred）:

P=W/tandP=Fv

Awattisdefinedasarateoftransferofenergyofonejoulepersecond.

VI

Linearmomentumistheproductofmassandvelocity:

momentum=mass×velocityorp=mv

Theprincipleofconservationofmomentum:

Foraclosedsystem,inanydirectionthetotalmomentumbeforeaninteraction（e.g.collision）isequaltothetotalmomentumaftertheinteraction.

Inallinteractionsorcollisions,momentumandtotalenergyareconserved.

Kineticenergyisconservedinaperfectlyelasticcollision;relativespeedisunchangedinaperfectlyelasticcollision.

Inaninelasticcollision,kineticenergyisnotconserved.Itistransferredintootherformsofenergy（e.g.heatorsound）.Mostcollisionsareinelastic.

Newton’sfirstlawofmotion:

Anobjectwillremainatrestorkeeptravelingatconstantvelocityunlessitisactedonbyaresultantforce.

Newton’ssecondlawofmotion:

Theresultantforceactingonabodyisequaltotherateofchangeofitsmomentum:

resultantforce=rateofchangeofmomentumor

Newton’sthirdlawofmotion:

Whentwobodiesinteract,theforcestheyexertoneachotheraeequalandopposite.

TheequationF=maisaspecialcaseofNewton’ssecondlawofmotionwhenmassmremainsconstant.

VII

Densityisdefinedasthemassperunitvolumeofasubstance:

Pressureisdefinedasthenormalforceactingperunitcross-sectionalarea:

Pressureinafluidincreaseswithdepth:

ρ=mgh

Hooke’slawstatethattheextensionofamaterialisdirectlyproportionaltotheappliedforce.Foraspringorawire,F=kx,wherekistheforceconstant.TheforceconstanthasunitsofNm-1.

Stressisdefinedas

Strainisdefinedas

Todescribethebehaviorofamaterialundertensileandcompressiveforces,wehavetodrawagraphofstressagainststrain.ThegradientoftheinitiallinearsectionofthegraphisequaltotheYoungmodulus.TheYoungmodulusisanindicationofthestiffnessofthematerial.

TheYoungmodulusEisgivenby

TheunitoftheYoungmodulusispascal（Pa）orNm-2

Theareaunderaforce-extensiongraphisequaltotheworkdonebytheforce.

ForaspringorawireobeyingHooke’slaw,theelasticpotentialenergyEisgivenby

VIII

Anelectricfieldisafieldofforce,createdbyelectriccharges,andcanberepresentedbyelectricfieldlines.

Thestrengthofthefieldistheforceactingperunitpositivechargeatapointinthefield,

Inauniformfield（e.g.betweentwoparallelchargedplates）,theforceonachargeisthesameatallpoints;thestrengthofthefieldisgivenby

.

Anelectricchargemovinginitiallyatright-anglestoauniformelectricfieldfollowsaparabolicpath.

IX

Electriccurrentistherateofflowofcharge.Inametalthisisduetotheflowofelectrons.Inanelectrolyte,theflowofpositiveandnegativeionsproducesthecurrent.

Thedirectionofconventionalcurrentisfrompositivetonegative;thedirectionofelectronflowisfromnegativetopositive.

TheSIunitofchargeiscoulomb（C）.Onecoulombisthechargewhichpassesapointwhenacurrentof1Aflowfor1s.

charge=current×time（ΔQ=IΔt）

Theelementarychargee=1.9×1019C.

ThecurrentIinaconductorofcross-sectionalareaAdependsonthemeandriftvelocityvofthechargecarriersandtheirnumberdensityn.

I=nAvq

Thetermpotentialdifference（p.d.）isusedwhenchargesloseenergyinacomponent.Itisdefinedastheenergytransferredperunitcharge.

or

Thetermelectromotiveforce（e.m.f.）isusedwhenchargesgainelectricalenergyfromabatteryorsimilardevice.Itisalsodefinedastheenergytransferredperunitcharge.

or

Avoltisajoulepercoulomb.Thatis,1V=1JC-1.

Poweristherateofenergytransfer.Inelectricalterms,poweristheproductofvoltageandcurrent.Thatis,P=VI.

Resistanceisdefinedastheratioofvoltagetocurrent.Thatis:

Theresistanceofacomponentis1ohmwhenapotentialdifferenceof1voltisproducedperampere.

ForaresistanceR,thepowerdissipatedisgivenby:

P=I2Ror

EnergytransferredinacircuitinatimeΔtisgivenby:

W=IVΔt

X

Kirchhoff’sfirstlawstatesthatthesumofthecurrentsenteringanypointinacircuitisequaltothesumofthecurrentsleavingthatpoint.

Kirchhoff’ssecondlawstatesthatthesumofthee.m.f.saroundanyloopinacircuitisequaltothesumofthep.d.saroundtheloop.

Thecombinedresistanceofresistorsinseriesisgivenbytheformula:

R=R1+R2+…

Thecombinedresistanceofresistorsinparallelisgivenbytheformula:

Ammetershavealowresistanceandareconnectedinseriesinacircuit.

Voltmetershaveahighresistanceandareconnectedinparallelinacircuit.

XI

AconductorobeysOhm’slawifthecurrentinitisdirectlyproportionaltothepotentialdifferenceacrossitsends.

Ohmiccomponentsincludeawireatconstanttemperatureandaresister.

Non-ohmiccomponentsincludeafilamentlambandalight-emittingdiode.

Asemiconductordiodeallowscurrentinonedirectio