## Lecture (25 minutes)

Describe a simple gas in a piston system and make a large sketch on the whiteboard. Emphasize to students that even though they will spend a lot of time working with the ideal gas, we are going to think about general gas properties, and so we will not be reasoning from the ideal gas law.

On your small white board, write a quantity you can measure for this system. Focus on gathering Volume, Temperature, and Pressure. As you discuss each one, label the graph and ask students how they would measure that quantity (in particular for this system). How would they cause a particular change to each quantity? How would they hold each quantity constant?

Tell the students that there is an analogy between the PDM and the gas in a piston. Ask the students to discuss this with their group and try to identify what is analogous. It is okay if the students cannot come up with direct variable analogies, but in the end we prefer to associate pressure with F_{L} and volume with x_{L} (this matches the convention used on the homework and in the elevator cycles activity). Briefly talk about the fact that what they know so far doesn't really tell them what temperature should be, but does indicate that there should be a fourth variable, which turns out to be entropy. Briefly discuss how entropy can be held constant by insulating the system and emphasizing that this is different from holding the system at constant temperature.