1

Atatomiclevel,ferromagneticmaterialsexhibitlocalmagneticpolarizationeffects–organizedin“domains”.Eachdomainishighlymagnetized,butbecauseitsdirectionisrandom,thebulkeffectisthattheseadduptozero.So,thematerialisnotmagnetized.

2

Whenanexternalmagneticfieldispresentinthearea nearaferromagneticmaterial,allthedomainsalign,leadingtoamagnetizedmaterial.Thiswillattract theferromagneticfieldtothesolenoid.

3

Whentheferromagnet movesintothecenterofthesolenoid,itwillstopmovingifwedon’tturnthecurrentoff.Thisisbecausethemotionoftheferromagnet intandemwiththesurroundingmagneticfieldinducessomethingcallededdycurrents,whichinfactopposethemotionoftheferromagnet throughthecoil.So….

4

Weneedtoturnofthemagneticfield!– Wedothisbyturningoffthecurrent!Momentumcontinuestopropeltheprojectilethrough thecoils.

5

Notethetwoswitchesonthecircuit;theyaremeanttoisolateonepartofthecircuitfromtheother; notethatthecapacitorsparticipateinbothcircuitsections.

6

Notethetwoswitchesonthecircuit;theyaremeanttoisolateonepartofthecircuitfromtheother; notethatthecapacitorsparticipateinbothcircuitsections.

7

Whenwe“close”switch1, wecreateapathforcurrenttoflowthroughthecircuit.Infact,whathappensisthatthesourcewilldepositenough+chargeonthetopcapacitorplate(anddepletethecorrespondingamountfromthebottomplate)sothateventually,thecapacitorswillhaveapotentialdropacrosstheirterminalsequaltothesourcevoltage(useKVLtoprovethatthishastobetrue).Theseriesresistorisslowingdownthecurrent– keepingitatreasonablelevelssoitdoesn’tburnallyourdevices.

8

So,inasimplifiedcircuit,ifthesourcesuddenlygoesfrom0Vto5V(V1above,“simulating”theswitchturningon),thecapacitorC1willslowly(redline)becomechargedsothattheimbalanceofchargecreatesavoltagedropacrossthecapacitorequaltothevoltageV1.R1istheretokeepthecurrenttoafinitevalue– soitactuallyslowsdowntheprocessofchargedepositionontheplates.

9

Ifyounow“open”switch1(thuscuttingoffcurrentflowinthatbranch)and“close”switch2(thusallowingcurrenttoflowthroughthatbranch),thenyoumight notlooklikeyouhaveasourceofcurrentbutyoudo.Rememberallthechargeyoustoredinyourcapacitorthatdidn’thaveanywheretogo?Nowitdoes.KVLnowwantsthevoltageacrossthecapacitortogotozero,socurrentwillflowintothebranchontheright.

10

Currentwillnowflowthroughtheinductor– whichjusthappenstobethecoilyouareusinginyourcoilgun!Thiscoildoesnothavealargeresistance,butitdoeshavesomeresistance. Nevertheless,theRCtimeconstantinthiscaseismuchsmaller,sothishastheeffectofproducingaverylargecurrent overaverysmalltimeperiod.

11

Notehow thevoltageacrossthecapacitordischarges throughtheresistorRcoil – yes,thisisnotaninductorbutremember,theinductorhassomeresistance.ThisresistanceisrepresentedbyRcoil inthecircuitabove.

12

So,comingbackto thecoil– itwillsuddenlybesubjectedtoaveryhighcurrent,whichwilllastforenoughtimetopulltheprojectileintoitscore.Remember,oncethere,weneedthecurrenttoturnoff. Whichitdoes– naturally,becausethecapacitoritselfonlyprovidesthecurrentforasmallamountoftime.Momentumdoestherest.Yourprojectilewillcontinuetotraveldownthetubeuntilitrunsoutofkineticenergy(dueto?).

13

14