CURRENT AND RESISTANCE

I. Current - A flow of charge

  A. Charged particles under the influence of an electric field travel through a wire, undergo collisions with atoms and other   charges (this produces a resistance to the flow), and, as a result, have a "drift velocity" along the wire.

  B. The current is the total net charge flowing through a wire cross section per unit time - I = dQ/dt

  Units:1 [coul/sec] = 1 ampere

II. Current density, J, is the current per unit area - J = I/A. If v is the drift velocity, n is the number of carriers per unit volume, and q is the charge per carrier, then

   J = nqv
 
 
 
 
 
 
 
 
 

III. Since v will depend on the strength of the effective E-field within the conductor and the type of conductor. we can define the resistivity as

   resistivity = E/J

  A. Since E and J have the same direction - E = (resistivity)J. This is a form of Ohm's Law.
 
 
 
 
 
 

  B. How is conductivity related to resistivity?

IV. How is resistance (R) related to resistivity?
 
 
 
 
 
 
 
 
 
 
 
 

  A. The more familiar form of Ohm's Law is

   V = IR

V. Power in electrical circuits

  A. A battery "pumps" charge from a low potential to a high potential and supplies current to a circuit. The power, or rate of electrical energy transfer, is

   P = iV

  B. If the current flows through a resistor, then the power dissipated in the resistor is

   Pr = i2R = V2/R
 
 
 
 
 
 
 
 

EXAMPLES

Text problems Ch. 27: 10, 52