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,

ρT = ρ0 [1 + α(T − T0)] .. .. (3.26)

where ρT is the resistivity at a temperature T and ρ0 is the same at a reference temperature T0. α is called the temperature co-efficient of resistivity, and from Equation (3.26), the dimension of α is (Temperature)^−1.

For metals, α is positive and values of α for some metals at T0 = 0°C are listed in Table 3.1.

The relation of Equation (3.26) implies that a graph of ρT plotted against T would be a straight line. At temperatures much lower than 0°C, the graph, however, deviates considerably from a straight line (Fig. 3.9).

FIGURE 3.9 Resistivity ρT of copper as a function of temperature T.

Equation (3.26) thus, can be used approximately over a limited range of temperature T around any reference temperature T0, where the graph can be approximated as a straight line.

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).

FIGURE 3.10 Resistivity ρT of nichrome as a function of absolute temperature T.

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.

Unlike metals, the resistivities of semiconductors decrease with increasing temperatures. A typical dependence is shown in Fig. 3.11.

FIGURE 3.11 Temperature dependence of resistivity for a typical semiconductor.

We can qualitatively understand the temperature dependence of resistivity, in the light of our derivation of Equation (3.23). From this equation, resistivity of a material is given by


ρ = (1/σ) = m/ (n e^2 τ)

ρ thus depends inversely both on the number n of free electrons per unit volume and on the average time τ between collisions. As we increase temperature, average speed of the electrons, which act as the carriers of current, increases resulting in more frequent collisions. The average time of collisions τ, thus decreases with temperature.

In a metal, n is not dependent on temperature to any appreciable extent and thus the decrease in the value of τ with rise in temperature causes ρ to increase as we have observed.

For insulators and semiconductors, however, n increases with temperature. This increase more than compensates any decrease in τ in Equation (3.23) so that for such materials, ρ decreases with increase in temperature.

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 (27.0 °C) is found to be 75.3Ω. When the toaster is connected to a 230 V supply, the current settles, after a few seconds, to a steady value of 2.68 A. What is the steady temperature of the nichrome element? The temperature coefficient of resistance of nichrome averaged over the temperature range involved, is 1.70 × 10^−4 °C^−1.

Solution:
When the current through the element is very small, heating effects can be ignored and the temperature T1 of the element is the same as room temperature. When the toaster is connected to the supply, its initial current will be slightly higher than its steady value of 2.68 A. But due to heating effect of the current, the temperature will rise. This will cause an increase in resistance and a slight decrease in current. In a few seconds, a steady state will be reached when temperature will rise no further, and both the resistance of the element and the current drawn will achieve steady values. The resistance R2 at the steady temperature T2 is

R2 = 230V/2.68A = 85.8 Ω

Using the relation


R2 = R1[1 + α(T2 − T1)]

with α = 1.70 × 10^−4 °C^−1, we get

T2 − T1 = (85.8 − 75.3)/{(75.3) × 1.70 × 10^−4} = 820 °C

that is, T2 = (820 + 27.0) °C = 847 °C

Thus, the steady temperature of the heating element (when heating effect due to the current equals heat loss to the surroundings) is 847 °C.

Example 3.4

The resistance of the platinum wire of a platinum resistance thermometer at the ice point is 5 Ω and at steam point is 5.39 Ω. When the thermometer is inserted in a hot bath, the resistance of the platinum wire is 5.795 Ω. Calculate the temperature of the bath.

Solution:


R0 = 5 Ω, R100 = 5.23 Ω and Rt = 5.795 Ω

Now, t = {(Rt − R0)/(R100 − R0)} × 100, Rt = R0(1 + αt)


= {(5.795 − 5)/(5.23 − 5)} × 100


= (0.795/0.23) × 100 = 345.65 °C