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. One ampere A is charge flowing at the rate of one coulomb per second. A + - Wire +Q t
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: 1.12 x electrons
Factors Affecting Resistance 1. The length L of the material. Longer materials have greater resistance. 1 1 L 2 2 2L 2. The cross-sectional area A of the material. Larger areas offer LESS resistance. 2 2 A 1 1 2A
Factors Affecting R (Cont.) 3. The temperature T of the material. The higher temperatures usually result in higher resistances. 4. The kind of material. Iron has more electrical resistance than a geometrically similar copper conductor. RoRoRoRo R > R o R i > R c CopperIron
Resistivity of a Material The resistivity 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 ( The unit of resistivity is the ohm-meter ( m)
Example 3. What length L of copper wire is required to produce a 4 m resistor? Assume the diameter of the wire is 1 mm and that the resistivity of copper is 1.72 x . m. A = 7.85 x m 2 L = m Required length is:
Temperature Coefficient For most materials, the resistance R changes in proportion to the initial resistance and to the change in temperature t. For most materials, the resistance R changes in proportion to the initial resistance R o and to the change in temperature t. Change in resistance: The temperature coefficient of resistance, is the change in resistance per unit resistance per unit degree change of temperature.
Example 4. The resistance of a copper wire is 4.00 m at 20 0 C. What will be its resistance if heated to 80 0 C? Assume that = /C o. R o = 4.00 m t = 80 o C – 20 o C = 60 C o R = 1.03 m R = R o + R R = 4.00 mm R = 4.00 m m R = 5.03 m