1. 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.

2. 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 C, then convert:
In 3 s: x 1020 electrons

3. 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

4. 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

5. 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)

6. 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.