Lecture 30

• applications of Gauss’s Law: cylindrical symmetry and conductors

• chapter 28 (Current and Conductivity): basic idea of electron current

Outline

Conductors in Electrostatic Equilibrium: at surface• if not, charges (free to move ) would...

• net charge outside

• If tangent to surface, charges move...

! E "= 0

!e = AEsurface for outside face+0 for inside face (Ein = 0)+0 for wall (E ! surface)!e = Qin

!0; Qin = !A "

Esurface =!

"!0

, ! to surface"

E

Ein = 0

Within conductor...

• excess charge on exterior surface

• inside hole ( inside conductor and no charge in hole): screening

• charge inside hole of neutral conductor polarizes...

E = 0 E = 0

Outline of Chapter 28 (Current and Conductivity)• motion of charges thru’ wire (current): due to electric field inside

wire (relation to charging/electrostatics)

• law of conservation of current

• relate current to conductivity

indicators of current

excess charge moves (current)

Electron Current• charge carriers (that move): electrons in metals

• no current : electrons moving rapidly, no net motion

• push on sea of electrons using : drift speed superposed on random thermal

vd

Ne = i!t

Ne = nV = nA!x = nAvd!t

E

Conservation of current• electron current same at A and B: can’t be created/destroyed/stored: lightbulb uses energy (work done to push electrons)

• wire full of electrons, don’t have to move from plate to plate (just shift/rearrange

Discharge of capacitor is instantaneous