A Simple Example from Introductory Physics

The glass of water shown in Figure 1 is exposed to a constant air temperature of 20 degrees Celsius. There are 1,000 cubic centimeters (cc) of water in the glass, and the water temperature is 19 degrees Celsius. Suppose we know that the air temperature remains constant at 20 degrees. What would you expect the water temperature to be if we come back in a couple of hours? You might answer 20 degrees, the same as the air temperature. Or you might think that the water temperature may not yet be at 20 degrees. But if we wait long enough, it will reach 20 degrees. Now consider a longer term question. Suppose we come back in 6 months to measure the volume of water in the glass. Will there still be 1,000 cc in the glass?

Figure 1. Glass of Water.

You probably realize that evaporation will gradually remove water from the glass, so there will be less than 1,000 cc after 6 months. And you are probably aware that heat is used to evaporate water. If the heat comes from the internal energy stored in the water, we might ask yet another question:

Will the water temperature become cooler and cooler over time?

These pages develop and test a system dynamics model to help us address these questions.