Hot or Cold?

Teachear's Guide

This experiment is about heat and temperature, two concepts easily confused. The experiment can also be performed without measuring temperatures, as described in most textbooks, but the measurements and the graphical representation of the phenomenon add a lot to its educational value. It makes observations less superficial and allows for a deeper analysis that goes beyond perception, though being based on it.

Students should be divided into groups, if possible, so that each of them could individually perform the “classical” experience before carrying out the measurements. A good alternative is to let them perform the first part at home, asking for a brief report on the experience and personal sensations.

An important feature is that the probes, as all temperature measuring instruments, show a detectable time of response. In this case, for example, probes detect a value for temperature near enough to the actual temperature of the liquid where they are immersed after 30 seconds at least. This should encourage to reflect on the meaning of temperature measurements when time intervals between measurements become very short. In the above case it is clear that the probe detects its own temperature instead of the temperature of the liquid. The experimental plot shows indeed that the probe immersed in hot water gives heat out (and cools down) when immersed in lukewarm water, whereas the one that was immersed in cold water takes heat in (and warms up).

Through this activity the students should realize that hot and cold sensations are also linked to the energy in transit (heat) and not only to the temperature of the object. This should help avoiding the confusion, that often occurs, between heat and temperature.

Fig1: Plot from sample data

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