Page 36 - Basic Electrical Engineering

1.7 THE EFFECT OF TEMPERATURE ON RESISTANCE

Resistance of pure metals like copper, aluminum, etc. increases with increase
in temperature. The variation of resistance with change in temperature has

been shown as a linear relationship in Fig. 1.2.
The change in resistance due to change in temperature is found to be

directly proportional to the initial resistance, i.e., R − R ∝ R . Resistance
0
t
0
(R − R ) also varies directly as the temperature rise and this change also
t
0
depends upon the nature of the material. Thus we can express the change in
resistance as,

R − R ∝ R t
0
0
t

or, R − R = α R t, where α is called the temperature coefficient of
0
0
t
0
0
resistance at 0°C.

or, R = R (1 + α t) (1.5)
0
0
t

Figure 1.2 (a) Shows the variation of resistance with temperature; (b) resistances at two different
temperatures

This expression can be applied for both increase and decrease in

temperature. From the graph of Fig. 1.2 (a) it is seen that resistance of the
material continues to decrease with decrease in temperature below 0°C. If we

go on decreasing the temperature to a very low value, the material attains a