4-TEMP COEF OF RESISTANCE

4-TEMP COEF OF RESISTANCE - TEMPERATURE COEFFICIENT OF...

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10 TEMPERATURE COEFFICIENT OF RESISTANCE OBJECT To measure the temperature coefficient of both thermistors and copper wire. APPARATUS Thermistor assembly, copper wire assembly, electrically heated water bath, temperature sensor, slide-wire Wheatstone bridge, galvanometer, Power supply, resistance box, eight wires THEORY Most metals used as electrical conductors exhibit a linear relation between resistance and temperature. The temperature coefficient of resistance of such metals is defined by the expression = o o t tR R R (1) where t R : value of resistance of the metallic conductor at temperature t C o R : value of resistance of the metallic conductor at temperature 0 C Of course, equation (1) may be rewritten as R t = R o t + R o (2) when we recognize immediately that equation (2) is simply the slope-intercept form of a straight line, namely, y = mx+b. Thus, if we were to plot a series of values of resistance versus temperature for a given specimen, the resulting straight line would intercept the resistance axis at the value R o while the attendant slope divided by R o is the value of Most metals have a small positive temperature coefficient of resistance. In contrast with metals, thermistors have large negative temperature coefficients. Moreover, with thermistors the change of resistance with a change in temperature is non-linear. The resistance of a thermistor is given by the equation R = R a ) / 1 / 1 ( a t t where R: resistance in ohms at temperature t (K)
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4-TEMP COEF OF RESISTANCE - TEMPERATURE COEFFICIENT OF...

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