Thermal Equilibrium Between Two Bodies


The time necessary for this measurement can be very long according to the system considered. It is advisable to start the experiment and to have the class working on another activity meanwhile.

The class can be divided into groups and a different system can be given to each of them. It is useful to ask the students to prepare the adiabatic system by themselves using materials from everyday life. Various containers can be used (thermos, boxes of Styrofoam or other materials and of different shapes) that are likely to be more or less adiabatic; liquids can be changed (water, oil, alcohol, etc.); the smaller container can be changed (a plastic balloon, insulating or not insulating glasses); the masses of the liquids can also be changed. Each group should take care of measuring and recording the masses of liquids they use that must be chosen also according to the type of system.

Also a very long measurement (12- 16 hours) of the temperature of a glass of hot water can be set up. It is surprising how long it takes for the water to reach thermal equilibrium with the surroundings.

By setting different activities the students are encouraged to look for analogies and differences between one situation and the other.
When data collection and analysis are completed, each group illustrates and share with the others the results obtained; the teacher can stimulate the discussion and invite all students to compare the different cases analysed. Students get accustomed to discussion and collaboration, which are basic elements of scientific research.

The following concepts can be made clear and reinforced:

This experiment provides a nice example of a phenomenon that continuously decreases gradually getting slower and slower. It is an opportunity for introducing exponential behaviours that are frequently found in phenomena from different fields of Physics (discharging a capacitor, emptying a tube full of water, …)
Data analysis can be performed differently according to the age of the students. For the younger ones the description of cooling and heating curves and the comparison of different cases will be enough; students more competent in Maths can develop the linear description of the phenomenon and calculate the parameters of the model.

The experiences are also described in:

A more detailed analysis of the cooling curve of a body is described in the module “Newton’s Law of Cooling”.

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