Conceptual Physics

R = resistance of component

ȡ = resistivity of material

L = length of component

A = cross-sectional area

Units of resistivity: Ÿ·m (ohm-meters)

The lamp cord is 0.75 meters

long and contains copper wire.

What is the wire’s resistance?

R = 0.0018 Ÿ

25.6 - Resistivity and temperature

The resistance of many conductors increases with temperature. As the temperature of

the metal coils of the hot plate shown on the right increases, so do the resistivity of the

metal and resistance of the coils. Other materials, such as semiconductors, have

decreasing resistivity with temperature.

Equation 1 shows two equations that reflect the relationship of resistivity and resistance

to temperature. Both include the temperature coefficient of resistivity, represented by Į

(the Greek letter alpha). A table of these coefficients is shown in Concept 2 for some

materials. These coefficients are empirically determined and apply over a specific range

of temperatures. To apply the resistivity equation, the material’s resistivity must be

known at one temperature, T 1. Its temperature coefficient of resistivity must also be

known for the temperature T 1. These values are used to calculate the resistivity at

another temperature.

The second equation is used to determine the change in resistance of a resistor. If the

resistance of a resistor is known at one temperature, the equation can be used to

calculate its resistance at another temperature. This equation can be derived from the

first.

In Example 1, we use this equation to calculate the change in resistance of a nichrome

coil in a hot plate that is heated from 25°C to 375°C.

In many materials:

Resistivity and resistance vary linearly

with temperature

Temperature coefficient of

resistivity,Į

(^462) Copyright 2000-2007 Kinetic Books Co. Chapter 25