Chapter 25: Current, Resistance and Electromotive Force. Charge carrier motion in a conductor in two parts

Transcription

1 Chpte 5: Cuent, esistnce nd Electomotive Foce Chge cie motion in conducto in two pts Constnt Acceletion F m qe ndomizing Collisions (momentum, enegy) =>esulting Motion Avege motion = Dift elocity = v d ~10-4 m/s Typicl speeds ~ 10 6 m/s p1c5: 1

2 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:

3 Connection with micoscopic pictue: q v d E 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 d 1 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

4 Exmple: 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

5 Cuent s esponse to n pplied electic field J J( E) E = conductivity E 1 esistivity J m units = = A m A m m p1c5: 5

6 J slope = 1/ J E J E line esponse Ohm s Lw E nonline esponse diode nonline esponse diection dependence! depends upon mteil E Tempetue f does not depend on E, the mteil is sid to oey Ohm s Lw p1c5: 6

7 Fo cylindicl conducto E J E J A E J A esistnce see lso exmple 8-3 e: ltente geometies Exmple: 50 mete length of 18 guge coppe wie (~1.0 mm in dimete) constnt cuent of A = 1.7x10-8. m find E,, p1c5: 7

8 Tempetue Dependence of T Metllic Conducto T Semiconducto T Supeconducto Fo smll chnges in tempetue: [ 1 ( T T )] T o o tempetue coefficient of esistivity [ 1 ( T T )] T o o p1c5: 8

9 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

10 Electomotive Foce nd Cicuits Stedy cuent equies complete cicuit pth cnnot e only esistnce cnnot e only potentil dops in diection of cuent flow Electomotive Foce (EMF) povides incese in potentil convets some extenl fom of enegy into electicl enegy Single emf nd single esisto: = + - = = p1c5: 10

11 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

12 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

13 Open Cicuit =0 =0 = A Shot Cicuit = = = 0 A p1c5: 13

14 Complete Cicuit A Chging Bttey A p1c5: 14

15 Enegy nd owe dw dw dt dq dt = esistnce = del emf owe delived y ( out of +) o to ( into of +) ttey p1c5: 15

16 owe nd el Souces Dischging Bttey owe deliveed extenlly: Chging Bttey owe deliveed y extenl souce : p1c5: 16

17 p1c5: 17 eff E el Bttey with Lod A mx "mpednce Mtching" d d 0

18 Complete Cicuit Exmple E =1 = A = 4 p1c5: 18

19 Theoy of Metllic Conduction Constnt Acceletion etween ndomizing collisions (momentum, velocity ndomized) E J J nqvd F qe E m m v vo men time etween qe collisions vvg vd men fee pth m nq J nqv m E d m nq = ( v o ) vg p1c5: 19

20 Exmple: Wht is the men time etween collisions nd the men fee pth fo conduction electons in coppe? m n m q e C v o 1x10 6 m m s kg p1c5: 0

21 hysiologicl Effects 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: ~ 500k wet skin: ~ 1000 p1c5: 1