John Travoltage
Physics
Purpose
Todeterminethelikelihoodofgettinga“carpetshock”(basedoncontrollablevariables),andto
avoidorproducesuchshocksunderaseriesofchallengingconditions
Apparatus
computerPhETsimulation,“JohnTravoltage”(availableathttp://phet.colorado.edu)
Discussion
Whenyouwalkacrossacarpetandtouchametaldoorknob,youmightfeelasmallshock.The
frictionbetweenthedifferentmaterialsinyourfootwareandthecarpetgivesyouastaticcharge.
(Physicistscallitatriboelectric charge,meaningachargefromfrictionalcontact.)Themetal
doorknobisaconductor,sotheexcesschargeonyourbodyjumpstotheknobrapidly,creatingthe
spark.Yournervesaresensitivetoelectricalimpulses,soyoufeeltheshock.
Inthesimulation,JohnTravoltage’sfootcanberubbedacrossacarpetsothathisbodycollects
charge.Hisarmcanberotatedsothatasparkwilljumpfromhisfingertothedoorknob.
Procedure
Step 1: Startthecomputerandallowittocompleteitsstartupprocess.
Step 2: LaunchthePhETsimulation,“JohnTravoltage.”Ifyouneedassistance,askyourinstructorfor
help.Gotohttp://phet.colorado.edu/en/simulation/travoltageandclick“RunNow.”
Step 3:RubJohnTravoltage’sfootbackandforthagainstthecarpetuntilasparkjumpsfromhisfingertothe
doorknob.
HowdoesJohnTravoltagereactwhenthesparkjumps?(hint:turnthevolumeup)Isthereanythinghesays?
____________________________________________________________________________________
____________________________________________________________________________________
Step 4: Explorethesimulation.
a. MoveJohnTravoltage’shandclosetothedoorknob,thenrubhisfootonthecarpetagain.Compared
totheprocessyouobservedinStep3,whathappensdifferentlythistime?
____________________________________________________________________________________
____________________________________________________________________________________
b.
HowcanyougetthemaximumchargebuiltuponJohnTravoltagewithoutproducingaspark?
DescribewhatyouneedtodowithJohn’sfoot.
____________________________________________________________________________________
____________________________________________________________________________________
18 | P a g e
c.
Underwhatcircumstances—ifany—willasparkjumpfromJohnTravoltage’sfoot tothedoorknob?
Whydoyousupposethisis(intermsofthephysicsinvolved)?(Hint:thinkaboutwhatJohnis
wearingonhisfootandwhatwouldpreventthechargefrommovingfromhisfoottothedoorknob.)
____________________________________________________________________________________
____________________________________________________________________________________
Step 5: Meetthechallenges.
a. RotateJohnTravoltage’sarmsothathisfingerispointingdirectly at thedoorknob.Carefullyrubhis
footagainstthecarpetwithoutproducingaspark.Whatisthegreatestnumberofchargesyoucan
getontoJohnTravoltage’sbody?(Estimatehowmanysmallsphericalchargescanbecollectedonhis
bodybefore adischargeoccurs?)
____________________________________________________________________________________
____________________________________________________________________________________
b.
Produce thelongestsparkpossible:thesparkisbetweenJohn’shandpointingdirectlyaway from
theknobandthedoorknob,itself.(Hint:Thecorrectprocedurerequireslessthanaminuteto
complete.)Describewhatyoudid?
____________________________________________________________________________________
____________________________________________________________________________________
Summing Up
1.
Underwhichconditionsisasparkmost likely tojumpbetweenJohnTravoltageandthedoorknob?
Describeintermsoftheamountofchargeonhisbodyandthedistancebetweenhishandandthe
doorknob.
____________________________________________________________________________________
____________________________________________________________________________________
2.
Considertwovariablequantities:theamountofchargeQ onJohnTravoltage’sbodyandthedistance
d fromhisfingertothedoorknob.
Whatcanyouconcludeabouttheamountofchargeandthelikelihoodofasparkjumping?(ieasyou
increasetheamountofchargedoesthatincreaseordecreasethelikelihoodofasparkjumping?)
____________________________________________________________________________________
____________________________________________________________________________________
WhatcanyouconcludeaboutthedistanceJohn’sfingerisfromthedoorknobandthelikelihoodofa
sparkjumping?(ieasyouincreasethedistancefromthedoorknobdoesthisincreaseordecrease
thelikelihoodofasparkjumping?)
____________________________________________________________________________________
____________________________________________________________________________________
WhenJohnTravoltage’shandisturnedawayfromtheknob,nosparkwillstart.ButinStep5,you
sustained asparkfromthatconfiguration.Whatdoesthissayabouttheconductivityofair?(in
otherwordsdoesairconductelectricity?)
____________________________________________________________________________________
____________________________________________________________________________________
3.
19 | P a g e