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
Text problems Ch. 27: 10, 52