physics

PhysicsNotes:EMspectrum:rangeofEMwavesinorderofincreasingfrequency Characteristics:EMwavesaretransversewaves,cantravelviavacuumatsamespeed3.0x10 ⁸m/sThelongerthewavelength,thelowerthefrequency,thelowertheenergy,viceversa.ThedirectionofoscillationisPERPENDICULARtothedirectionofwavetravel(transversewaves).

Utility:RADIOtelescopesstudytheradiofrequencyportionoftheelectromagneticspectrumemittedbyastronomicalobjects,suchasstars.RadiowavesproducedbyoscillatingelectriccurrentsinatransmittingantennaispickedbyradioandTVaerialsatthereceivingend. MICROWAVESofaparticularfrequencyarereadilyabsorbedbywatermolecules.Almostallfoodcontainswater,sofoodcanbeheatedquicklyinamicrowaveoven.Aradarsendsoutmicrowaves,whichgetreflectedbyotherobjects.Whenthesereflectedsignalsaredetected,theradaroperatorwillgeta'picture'ofthesurroundings.Whenusingasatellitephonenetworktomakeanoverseascall,microwavesignalsaresentouttospacebyalargeaerialdishtosatellitesorbitingtheearth.Thesesatellitesthenrelaythesignalstothereceivingdishoverseas. AdvancedsecuritysystemsusepassiveINFRARED(PIR) motiondetectors.Thesesensors"see"theinfraredenergyemittedbyanintruder'sbodyheat.Whenanintruderwalksintothefieldofviewofthedetector,thesensordetectsasharpincreaseininfraredenergy.Thetransmitterintheremotecontrolhandsetsendsoutpulsesofinfraredlightwhentheuserpressesabutton.Eachbuttonhasauniquepatternofinfraredpulses. Areceiverinthedevicerecognizesthispattern,causingthedevicetorespondaccordingly.Infraredcamerasdetectinfraredenergy(heat)andconvertsitintoanelectronicsignal.Thesignalsareprocessedtoproduceathermal imageonavideomonitor. Lightwavelength:

Intelecommunication, digital signalsaresentthroughopticalfibresusingshortpulsesofLIGHT.Inmedicine,lasersareusedtoseal bloodvessels.Inindustry,lasersareusedtoweldmetalstogether. ProlongedexposuretoUVraysincreasesyourriskofdevelopingskincancer.ThisisbecauseUVrayscarriesenoughenergytoremoveelectronsfromatomsormolecules,therebyionizingthem.LivingcellsandtissuecanbedamagedbyUVradiation.UVcounterfeitcurrencydetectorsareusedtodetectspecialinksinthebillsthatonlyappearundertheproperwavelengthofUVlight.SunbedsworkbyexposingtheskintoUVradiation,similartothatfoundinsunshine.Thisreleasesachemicalwhichcausestheskintotan.UVsterilizerusesUVlightthatkillsmicroorganismsthatgothroughitbyalteringanddamagingtheirDNA. LivingcellsandtissuecanbedamagedbyX-rays.ThisisbecauseX-rayscarryenoughenergytoremoveelectronsfromatomsormolecules,therebyionizingthem.OverexposuretoX-rays

increasestheriskofdevelopmentalproblemsandcancer.Theriskofradiationisgreatertoafetus,sopregnantpatientsareadvisedtoavoidexposuretoX-rays.WhenabeamofX-raysstrikeacrystal,thecrystallineatoms"spread"theX-raysintomanyspecificdirections.Thepatternofthe"spread"canbestudiedtounderstandthecrystalstructure.AftertheX-rayspassthroughthebaggage,theyarepickedupbyadetector.The"picture"formedonthemonitorisusedbyairportstafftodetermineiftherearedangerousitemsinthebaggage.Onaphotographicfilm,darkareasshowwheretheX-rayshavepassedthroughsoftbodytissues.Whiteareasshowwheredensermaterial,suchasbones,haveabsorbedtheX-rays.AnimageofvaryingdensitieswillshowuponfilmaccordingtotheamountofX-raysthatreacheacharea. BecauseGAMMArayskillbacteriaandcells,theyhavebeenusedtotreatcertaintypesofcancer.Inacontrolledprocedure,multipleconcentratedbeamsofgammaraysarefocusedontoatumortokillthecancercells.Medicalinstrumentsaresealedinanair-tightbagandthenplacedinaverylargefieldofradiationthatcanpenetratethebagtokillbacteria.Manufacturersusegammaraystocheckforcracksorflawsintheirproducts.SimilartoX-rays(butmorepowerful),gammaraystakepicturesoftheinsideofproductstoshowflawsthatarenotvisiblefromtheoutside. StaticElectricityLikechargesrepel,unlikechargesattractSIunitforamt.ofcharge:Coulomb,CEboniterodrubbedwithduster,dusterbecomesnegativelychargedbyfrictionaselectronsflowfromrodtoduster.Uncharged:equalamountofpositiveandnegativechargeChargemetalballbyinduction:polyethylenestriprubbedwithwoollencloth,broughtclosetoball,earthballmomentarilytoinducechargesbeforeearthing,removepolyethylenestripNegativechargedelectronsaretransferredfromacetaterodtocloth,clothhasmorenegativechargethanpositivecharge.Paperisattractedtorodaselectronsinpaperareattractedtorod,leavingpositivechargefurtherfromrod,andforcesofattractionbetweenunlikechargesisgreaterthanforcesofrepulsionoflikecharges.Excesschargesinthemetalspherewillbeneutralisedwhenearthed.Insulator:plastic/perspex/polythene/rubber/ebonite/glassPaintdropletsarepositivelychargedandspreadoutaslikechargesrepel.Astationarynegativechargeinanelectricfieldexperiencesanelectricforcehorizontallytotherightasthedirectionoftheelectricfieldishorizontallytotheleft.

ElectronsmovefromfueltopipeSparksuddendischargetoignitethefuelElectronsflowalongthecablefromearthtoaircrafttoneutralisepositivecharges Current of ElectricityCellhasemfof2v,meanscellsupplies2joulesofenergypercoulombofchargeitdelivers.TofindRoflampQ:

CharacteristicV–IgraphofaconductorthatobeysOhm’slaw:

Resistanceincreasingascurrentincreases: Q=ItI=V/RVcanbep.d.,emfV=W/QR=pl/AA=π( d/2)²Conventionalcurrentflowsfrompositivetonegativeterminalofbattery.emfvsp.d.:emfisW/Qaroundacompletecircuit,p.d.IsW/QviathecomponentAnohmicconductorisaconductorinwhichthecurrentpassingthroughitisdirectlyproportionaltothepotentialdifferenceacrossit,providedthatphysicalconditions(suchastemperature)remainconstant.

Ohmicconductor:currentpassingviametallicconductorisdirectlyproportionaltothep.d.acrossit,providedthatphysicalconditions(suchastemperature)remainconstantsothatresistanceremainsconstant.Whenthetemperatureofametallicconductorincreases,themetalionsintheconductorvibratemorevigorously.Themovingelectrons(chargecarriers)collidewiththemetalionsmorefrequently,slowingdowntheirdriftandloweringthecurrent,increasingelectricalresistance.Asthecurrentflowingthroughthefilamentincreases,theheatingeffectofthecurrentcausesthetemperatureofthefilamenttoincrease,andthereforetheresistanceofthefilamentincreases.Thelonger-lastingbulbtypehasafilamentoflargermassandlargercross-sectionalarea,thusithasthelargerheatcapacity.Asaresult,ittakesalongertimetofuse. LEDisasemiconductordiodethatproduceslight.Asemiconductordiodeisadevicethatallowscurrenttoflowinonedirectiononly.AsLEDdoesnotallowcurrenttoflowintheotherdirection,itmustbeconnectedinthecorrectdirectionforittolightup.ThelongerlegoftheLEDconnectstothepositiveterminalandtheshorterlegconnectstothenegativeterminalofthebattery. Filamentlamps(non-ohmic)havelowenergyefficiency.Thehighertheirpotentialdifference,thehighertheirresistance,thebrightertheyare.

Thermistor:ResistancevarieswithtemperatureNegativetemperaturecoefficientthermistorhasresistancedecreaseastemperatureincreases.

1/Rt=1/R1+1/R2+1/R3+...Rs=R1+R2+R3+…