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