Chapter 26 CURRENT ELECTRICITY Electric Current (I) –rate of positive charge passing through a surface I=dQ/dt –units Coulomb/Second = Ampere (C/s = A)

Current Flow of Charge

Atomic Model for current Number of charges moving with a velocity through the areaNumber of charges moving with a velocity through the area Metallic conductorsMetallic conductors –Negative charges move –drift velocity is slow –electrons collide with atomic cores frequently –collisions transfer energy (cause heating)

Atomic Model for current n = number of charges/vol e = magnitude of charge take limit as  t goes to 0

Current Density Current per area vector quantity relation to current via dot product

Problem 12 Find current in section bounded by “dr”

Ohm’s Law Vector field equation Conductivity and resistivity Ohm’s Law vector form Resistance and resistivity Scalar Equation

Resistance Units: Ohm = Volt / Ampere (  = V / A)

Ohmic devices vs. no Ohmic

Sample problem 26-4 Find resistance of rectangular block of iron……

Temperature Dependence of Resistance Define temp coefficient of resistivity If  is small and constant Units of  is ( o C) -1

Microscopic view of Ohm’s Law Resistivity is a constant Free electron Theory

Low temperature behavior of resistance Superconductivity Semiconductor

Electric Power Units: watt = amp  volt Substituting Ohm’s Law yields: