3.8 TEMPERATURE DEPENDENCE OF RESISTIVITY
The resistivity of a material is found to be dependent on the temperature. Different materials do not exhibit the same dependence on temperatures. Over a limited range of temperatures, that is not too large, the resistivity of a metallic conductor is approximately given by,
where
For metals,
Resistivity
The relation of Eq. (3.26) implies that a graph of
Equation (3.26) thus, can be used approximately over a limited range of
Resistivity
Some materials like Nichrome (which is an alloy of nickel, iron and chromium) exhibit a very weak dependence of resistivity with temperature (Fig. 3.10). Manganin and constantan have similar properties. These materials are thus widely used in wire bound standard resistors since their resistance values would change very little with temperatures.
Temperature dependence of resistivity for a typical semiconductor.
Unlike metals, the resistivities of semiconductors decrease with increasing temperatures. A typical dependence is shown in Fig. 3.11.
We can qualitatively understand the temperature dependence of resistivity, in the light of our derivation of Eq. (3.23). From this equation, resistivity of a material is given by
In a metal,
For insulators and semiconductors, however,
Example 3.3
An electric toaster uses nichrome for its heating element. When a negligibly small current passes through it, its resistance at room temperature (
When the current through the element is very small, heating effects can be ignored and the temperature
Using the relation
Thus, the steady temperature of the heating element (when heating effect due to the current equals heat loss to the surroundings) is
Example 3.4
The resistance of the platinum wire of a platinum resistance thermometer at the ice point is
Now,