大学物理演示实验英文版.docx
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大学物理演示实验英文版.docx
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大学物理演示实验英文版
BuildaRaftPoweredbySurfaceTension
Abstract
Haveyoueverwonderedwhyawaterstridercanwalkonwater?
Orhowdetergentcancleanyourdishes?
Ifyouobservecarefully,youcanfinddozensofsimilarlyinterestingphenomenathatarealllinkedtothesurfacetensionofwater.Thisscienceprojectwillhelpyouunderstandandmeasurethepropertiesofwatersurfacetension.
Objective
Inthisscienceproject,youwill:
1.Developanunderstandingofwatersurfacetension,
2.Understandchangesinwatersurfacetensionundervariousconditions,
3.Makemeasurementstoestimatethenetwatersurfacetension.
Introduction
Ifyou'veeverblownupaballoon,youknowit'sprettyeasytoblowupwithoutbecomingoutofbreathwhenit'smadeofsoft,stretchyrubber.That'sbecausetheballoonofferslittleresistancetobecomingstretchedoutasitisblownup.Butforaballoonmadeofthickerorstifferrubber,moreenergyisrequiredtoblowitup.Theballoonoffersmoreresistancetostretching.Thinkofaballoonasamodelforsurfacetension.How"stretchy"theballoonmaterialisdetermineshowmuchresistance(surfacetension)mustbeovercomebytheenergyofyourblowinginordertoinflatetheballoon.Surfacetensionisdefinedastheenergyrequiredtoincreasethesurfaceareabyaunitamount.
Liquidsalsoexperiencesurfacetension.Themolecules(smallindividualparticles)oftheliquidexperienceintermolecularattractions,whichsimplymeansthemoleculesarepullingandpushingawayfromeachother,justlikemagnetsbothattractandrepeleachother.Inthecaseofawatermoleculesurroundedonallsidesbyotherwatermolecules,everypullingforceisbalancedbyapushingforce.Thenet(whichmeansoverall)effectisnochange.Butatthesurface,whereairandwatermeet,thatisn'ttrue.Thewatermoleculesatthesurfaceexperiencemorepullsdownwardtowardtheotherwatermoleculesbelowthemthanupwardtowardtheair.Thisisthesurfacetensionofthewater.Youcanseeitatworkwhenyoufillaglass.Evenifthewaterisattherimoftheglass,youcanaddjustafewmoredropssothatthewaterisslightlytallerthantherim.Don'tbelieveit?
Lookatthepicturebelowandthentryityourself!
Figure1.Becauseofsurfacetension,thetopofthisfullglassofwatercurvesoutward.Ifmorewaterisadded,itwilleventuallyspilloverthesideoftheglass.Notetheedgeoftheglass,andhowfarthewatercurvesoverit.
Inthisscienceproject,you'llmakeasmallraftfromaclearplasticsheet(atransparency),andseeifyoucanpropelitbytakingadvantageofthesurfacetensionofwater.
Terms,Concepts,andQuestionstoStartBackgroundResearch
∙Resistance
∙Surfacetension
∙Molecule
∙Velocity
∙Kineticenergy
∙Mass
∙Acceleration
Bibliography
Thesewebsitesaregoodintroductionstosurfacetension:
∙MassachusettsInstituteofTechnology.(n.d.).IntroductiontoSurfaceTension.RetrievedAugust12,2010,fromweb.mit.edu/nnf/education/wettability/intro.html
∙Exploratorium.(n.d.).StickyWater.RetrievedAugust12,2010,from.exploratorium.edu/ronh/bubbles/sticky_water.html
Thiswebsiteoffersanexplanationofvelocity:
∙Physics4Kids.(n.d.).Velocity,Speed,andMotion...OhMy!
RetrievedAugust12,2010,from.physics4kids./files/motion_velocity.html
Thiswebsiteexplainskineticenergy:
∙Zobel,E.A.(n.d.).KineticEnergy.RetrievedAugust12,2010,fromzonalandeducation./mstm/physics/mechanics/energy/kineticEnergy/kineticEnergy.html
MaterialsandEquipment
∙Transparencies,likethoseusedonanoverheadprojector(1box);availableatyourlocalofficesupplystore
∙Marker
∙Ruler,metric
∙Scissors
∙Kitchensponges(1package)
∙Tape
∙Labnotebook
∙Liquidlaundrydetergent
∙Eyedropper;availableatyourlocalpharmacy
∙Stopwatch,accurateto0.01second(sec)
∙Scale,onethatcandetectsmallmassesingrams(g).Agoodkitchenscalewillwork.Alternatively,tryapostalscale.Youmightbeabletousetheoneatyourlocalpostoffice,oryourschoolmighthaveatriple-beambalance.
∙Liquidhandsoap
∙Toothpaste
∙Toothpicks(1box)
∙Water
∙Largebasinorsinktoholdthewaterandyourtransparencyraftinearlytrials
oItshouldbelargeenoughthattheraftcantravelashortdistancewithinit.
oMakesureitcanbeeasilyemptied,asyouwillbefillingandre-fillingitwithfreshwateroften.
∙Bathtubforlatertrials
∙Volunteer
∙8-inch(in.)x10-in.pictureframe,plasticorwood(theframewillgetwet)
∙Disposableplasticcups,small(4)
∙Ziptiesortwistties
ExperimentalProcedure
ExploringtheExistenceofSurfaceTension
1.Thinkofashapeforyourraftanddrawtheshapeontothetransparency.
a.Theraftshouldbesymmetric.
b.Theraftshouldbesmallenoughthatitcantravelashortdistanceinyourbasinorsink,butstillbelargeenoughtoholdthespongepiece.
c.Youwillneedtocutaspaceinthebackoftheraftwherethespongewillbe.Itshouldbeslightlylargerthanthesizeyou'llbecuttingyoursponge(whichwillbeinstep3).
d.Thelengthoftheraftisquiteflexible,butwesuggestaninitialsizeof3–5cmlong,andawidththatisapproximatelyhalfofthelengthyouchoose.Recordthesizeinyourlabnotebook.
e.SeeFigure2foranexampleoftheraftdesign.
Figure2.Exampleofaraftdesign.Notethewayinwhichthesmallpieceofspongeisattachedtothetransparencytopreventitfrommovingandsinking.
2.Cutouttheraft.
3.Cutthespongeintosmall,identicallysizedpieces.Thespongeshouldfitintothespacecutinthebackoftheraft.
4.Runatoothpickhorizontallythroughoneofthesmallspongepiecessoenoughofthetwoendsofthetoothpickcanrestonthetransparency,thenattachthetoothpickendswithtapesothatthespongeisattachedtothesmallspaceatthebackoftheraft.SeeFigure2,above.Recordthesizeoftheraftandthespongeinyourlabnotebook.
5.Fillabasinorsinkwithtapwater.
6.Puttheraftontothewatersurfaceandletitfloat.
7.Usinganeyedropper,putadropofdetergentontothespongeattheendoftheraft.Ifonedropisn'tenough,putoneortwomore.Theraftdoesn'trequiremuchdetergenttostartmoving.
8.Observethemotion.Recordallobservationsinyourlabnotebook.
9.Repeatstep7withthesameraftandsponge,continuingtoadddetergentadropperfulatatime,inordertobecomefamiliarwiththeeffectsofthechangeinsurfacetensionthatdetergentcreates.
a.Note:
Afterafewtrials,thewaterwillhavetoomuchdetergentinitforitssurfacetensiontochangeifmoreisadded.Whenthishappens,youwillneedtoreplaceyourwaterwithnewwater.Thiswillbefairlyoften.
10.Fromyourbackgroundresearch,youshouldknowthatdetergentsdecreasethesurfacetensionofwater.Howcanthishelptoexplainyourresults?
ExploringOtherSubstancesasSurface-Tension“Motors”forYourRaft
1.Fillupthebasinorsinkwithfreshwater.
2.Replacethespongeontheraftwithafreshsponge.
3.Puttheraftontothewatersurfaceandletitfloat.
4.Putadropofsoapontothespongeattheendoftheraft.Youcouldcarefullypumponedrop,orunscrewthepumplidandletonedropfallfromthetubethatisattachedtothepumpontothesponge.
5.Observethemotionandcompareittothefirstraft,whichuseddetergent.Recordyourobservationsinadatatable,liketheonebelow.
SubstanceUsed
DidtheRaftMove?
Yes/No
FastestSpeed?
Yes/No
GoodorBadMotor?
HowDidItAffectWater'sSurfaceTension?
Detergent(firstraft)
Toothpaste
Vegetableoil
Tablesalt
6.Repeatsteps1–5.Makesurethatyouusethesameraftforeverytest,andthatyoureplacethewaterwithnewwaterforeverydifferentsubstanceyoutest.Youcantryallsortsofsubstances:
vegetableoil,cornstarch,salt,andevenvinegar!
Tryoutanythingthatyouthinkmightmovetheraft.
a.Besureyouhavethreetrialsforeachsubstance.Thiswillensureyourresultsareaccurateandrepeatable.Recordallresultsinyourdatatable.
7.Whichsubstancesworkedasa"motor"?
Whichonesdidn't?
Whichsubstancewasthebestmotor?
Canyourelateyourfindingsbacktosurfacetension?
ExploringtheEffectofShapeontheMotionofYourRaft
1.Nowtestdifferentraftshapes.Designanothershapeforyourraftanddrawtheshapeontothetransparency.
a.Youmightwanttotryanasymmetricshape.
b.Youmightwanttoexperimentwithdifferentsizes,ordifferentbasicshapes.
2.Cutouttheraft,leavingaspaceforasmallpieceofsponge,andinsertthespongeattheendoftheraft.Again,recordallmeasurementsinyourlabnotebook.
3.Preparefreshwaterinthebasinorsink.
4.Puttheraftontothewatersurfaceandletitfloat.
5.Putonedropofthesubstancethatworkedbestintheprevioussectionontothespongeattheendoftheraft.
6.Observethemotionandcompareittotheotherrafts.Usethesamesubstanceforyoursurface-tensionmotorforeachtrial.Recordyourobservationsinadatatable,suchastheonebelow.
a.Besureyouhavethreetrialsforeachsubstance.Thiswillensureyourresultsareaccurateandrepeatable.Recordallresultsinyourdatatable.
RaftShape
DidtheRaftMove?
Yes/No
FastestSpeed?
Yes/No
WhyDidtheRaft'sShapeCauseThisMotion?
Largesquare
Smallsquare
Largecircle
Smallcircle
MeasuringNetSurfaceTension
1.PrepareanewraftwithashapesimilartotheoneinFigure3below.Youwillbetestingtheraftwithinan8-in.x10-in.pictureframe,sotheraftsizeissuggestedtobe3–5cmlong,andawidthapproximatelyhalfofthislength.
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