Stirling engine ET representing energy density

In E2, 130815, Thursday AM, the teachers were asked to enact a Stirling engine in an energy theater. They had been shown the engine the previous morning and started sketching energy diagrams, but had not carried out an ET.

The engine consists of gas contained in a cylinder,separated into two compartments by a styrofoam wall. If you heat the gas in one of the compartments with a hot cup of coffee, it will expand, which moves the separating wall. The wall, in turn, is conncted to a balanced wheel with a rod, so that the wheel starts to rotate when the wall moves. Finally, the wheel is connected to a piston, which compresses the gas in the compartment that was not heated, so that the styrofoam wall returns to its starting position. This closes the cycle of the engine.

I attach the entire ET (25 minutes, three clips), since I think it gives much insight into how the groups gradually developed the idea of representing "energy density" in the form of participants' distance from each other within a particular object. The closer they are to each other, the higher the energy density. This can be compared to another representation, posted by Abby in 2011 (http://scherrenergyproject.blogspot.com/2011/08/shrinking-people-to-represent.html) of people "shrinking" vs. speading their arms, indicating the same amount of energy, but more or less useful to do work.

They enact the entire process in three steps: First, the cylinder starts go get heated, so that the engine starts to move. Then, there is the steady-state operation, where thermal energy goes to surrounding air, driven by a gradient in temperature and different thermal density at the heat source vs. heat sink. Finally, when the coffee has reached room temperature, the engine will stop and thermal energy will be evenly distributed.

 

The notion of "energy density" is found useful by the entire class, including the instructors, and this ET pair of groups is later asked to enact their ET in full class. Energy density contributes to understanding the difference in usefulness of a certain amount of energy in different circumstances.

Another thing to note: Gail is checked out most of the time.

Group 3's energy diagram:

A synthesised energy diagram, when the engine is driven by a temperature gradient between the atmosphere and a pack of ice, based on those of all groups: