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Temperature Coefficient of Resistance
10 questions By Tony R. Kuphaldt
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Question 1 of 10
It is a common phenomenon for the electrical resistance of a substance to change with changes in temperature. Explain how you would experimentally demonstrate this effect.
Reveal answerIt is a simple thing to demonstrate a substance’s change in resistance with temperature. I’m interested in finding out how you might discern a quantitative measurement of this change. That is, how would you design an experiment so as to “attach a number” to the effect of resistance changing with temperature?
Notes:This question is an excellent starting point for an in-class experiment. There are several ways in which this effect could be demonstrated.
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Question 2 of 10
An electronics instructor wants to demonstrate to his students the effect of electrical resistance changing with temperature. To do this, he selects a carbon resistor about 3 centimeters in length and 5 millimeters in diameter, black in color, with a wire at each end, and connects it to an ohmmeter. Whenever he grasps the resistor between his fingers, the ohmmeter instantly responds by showing a greatly reduced resistance.
What is wrong with this experiment?
Reveal answerIf the change in resistance is truly due to a change in resistor temperature, it should not be instant.
Notes:I must confess, the genesis of this question was an experience from my own education. This really happened! I still remember staring at the demonstration, perplexed that the resistance would change so quickly and so greatly when the instructor grasped the resistor. I also recall the mild insult the instructor directed at me as I attempted to communicate my confusion: “What’s the matter? Too complicated for you?” Please, never treat your students like this.
Some students may believe the experiment is flawed because they expect the resistance to rise with increased temperature, rather than fall. This, however, makes a fundamental assumption about the nature of temperature-induced resistance changes, which is a bad thing in science. Let the experimental evidence tell you how the phenomenon works, don’t tell it what it should do!
Discuss with your students what they think the real mechanism of resistance change is in this experiment, and how they might modify the experiment so as to isolate temperature as the only changing variable.
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Question 3 of 10
If we were to plug an electric saw into a very long extension cord, and then plug the other end of the cord into a power receptacle, we would notice a decreased level of performance from the saw, as compared with how it performs when directly plugged into the same receptacle (with no extension cord).
Determine if the saw’s performance becomes better or worse as the ambient temperature increases, and explain your answer.
Reveal answerThe saw’s performance worsens as ambient temperature increases.
Notes:Discuss the nature of the problem, with reference to Ohm’s Law. Ask your students to explain the effect in terms of Ohm’s Law, and the cord’s ability to deliver electrical power to the saw motor.
