Chpte 5: Cuent, esistnce nd lectomotive Foce Chge cie motion in conducto in two pts Constnt Acceletion F m q ndomizing Collisions (momentum, enegy) >esulting Motion http://phys3p.sl.psu.edu/phys_nim/m/ndom_wlk.vi Avege motion Dift elocity v d ~10-4 m/s Typicl speeds ~ 10 6 m/s p1c5: 1
Cuent Flow net motion of chges Chge cies chge q speed v d v d Cuent te t which chge psses the e dq C units: 1 1Amp 1A dt 1S v d Negtive chge cies move in opposite diection of conventionl cuent. p1c5:
Connection with micoscopic pictue: q v d v dt dq nqv d A dt Cuent Density: J A nqv d J nqv d J n q v + n q v + 1 1 d1 d d A dq chge tht psses though A nume tht pss though A chge on ech (n A v d dt) q, n nume density nume/volume (woks fo negtive chge cies, multiple types of chge cies s well) p1c5: 3
xmple: 18 guge coppe wie (~1.0 mm in dimete) -constnt cuent of A -n 8.5x10 8 m -3 (popety of coppe) find J, v d p1c5: 4
Cuent s esponse to n pplied electic field J J( ) σ σ conductivity 1 ρ esistivity J σ m units A m A m Ω m p1c5: 5
J slope 1/ρ J J line esponse Ohm s Lw nonline esponse diode nonline esponse diection dependence! ρ depends upon mteil Tempetue f ρ does not depend on, the mteil is sid to oey Ohm s Lw p1c5: 6
Fo cylindicl conducto ρ J ρ J ρ A J ρ A esistnce see lso exmple 8-3 e: ltente geometies xmple: 50 mete length of 18 guge coppe wie (~1.0 mm in dimete) constnt cuent of A ρ 1.7x10-8 Ω. m find,, p1c5: 7
Tempetue Dependence of ρ ρ ρ ρ T Metllic Conducto T Semiconducto T Supeconducto Fo smll chnges in tempetue: ρ ρ [ 1 + α ( T T )] α T o o tempetue coefficient of esistivity [ 1 + α ( T T )] T o o p1c5: 8
esisto Colo Codes Colo nume colo nge lck 0 none ±0% own 1 silve ±10% ed gold ±5% onge 3 yellow 4 vlue: n 1 n x10 n 3 ±x% geen 5 lue 6 violet 7 gy 8 white 9 10x10 ±5% p1c5: 9
lectomotive Foce nd Cicuits Stedy cuent equies complete cicuit pth cnnot e only esistnce cnnot e only potentil dops in diection of cuent flow lectomotive Foce (MF) povides incese in potentil convets some extenl fom of enegy into electicl enegy Single emf nd single esisto: + - p1c5: 10
Mesuements oltmetes mesue otentil Diffeence (o voltge) coss device y eing plced in pllel with the device. Ammetes mesue cuent though device y eing plced in seies with the device. A p1c5: 11
el Souces nd ntenl esistnce del emf detemined y how enegy is conveted into electicl enegy ntenl esistnce unvoidle intenl losses ging tteies > incesing intenl esistnce p1c5: 1
Open Cicuit 0 0 A Shot Cicuit 0 A p1c5: 13
Complete Cicuit + + A Chging Bttey + A p1c5: 14
negy nd owe dw dw dt dq dt + - + - esistnce del emf owe delived y ( out of +) o to ( into of +) ttey p1c5: 15
owe nd el Souces Dischging Bttey owe deliveed extenlly: Chging Bttey + owe deliveed y extenl souce : + p1c5: 16
p1c5: 17 eff + + + + + + + 1 1 1 el Bttey with Lod A mx "mpednce Mtching" d d 0
Complete Cicuit xmple 1 Ω 4Ω A p1c5: 18
Theoy of Metllic Conduction Constnt Acceletion etween ndomizing collisions (momentum, velocity ndomized) ρ J J nqvd F q m m v v + τ τ men time etween o q collisions vvg τ τ vd m men fee pth nq τ J nqv m ( v o ) vg τ d m ρ nq τ p1c5: 19
xmple: Wht is the men time etween collisions nd the men fee pth fo conduction electons in coppe? 8 ρ 17. 10 Ω m n 85. 10 m 91. 10 19 q e 16. 10 C v o 1x10 6 m 8 3 31 m s kg p1c5: 0
hysiologicl ffects of Cuent Neve ction involves electicl pulses cuents cn intefee with nevous system ~.1A cn intefee with essentil functions (hetet, e.g.) cuents cn cuse involunty convulsive muscle ction ~.01 A Joule Heting ( ) With skin esistnce dy skin: ~ 500kΩ wet skin: ~ 1000Ω p1c5: 1