Electric Current Electric current I is the rate of the flow of charge Q through a cross-section A in a unit of time t. A + - Wire +Q t One ampere A is charge flowing at the rate of one coulomb per second.

Example 1. The electric current in a wire is 6 A Example 1. The electric current in a wire is 6 A. How many electrons flow past a given point in a time of 3 s? I = 6 A q = (6 A)(3 s) = 18 C Recall that: 1 e- = 1.6 x 10-19 C, then convert: In 3 s: 1.12 x 1020 electrons

Factors Affecting Resistance 1. The length L of the material. Longer materials have greater resistance. 1 W L 2 W 2L 2. The cross-sectional area A of the material. Larger areas offer LESS resistance. 1 W 2A 2 W A

Factors Affecting R (Cont.) 3. The temperature T of the material. The higher temperatures usually result in higher resistances. R > Ro Ro 4. The kind of material. Iron has more electrical resistance than a geometrically similar copper conductor. Ri > Rc Copper Iron

Resistivity of a Material The resistivity r is a property of a material that determines its electrical resistance R. Recalling that R is directly proportional to length L and inversely proportional to area A, we may write: The unit of resistivity is the ohm-meter (W·m)

Temperature Coefficient For most materials, the resistance R changes in proportion to the initial resistance Ro and to the change in temperature Dt. Change in resistance: The temperature coefficient of resistance, a is the change in resistance per unit resistance per unit degree change of temperature.