First, an ideal gas in a cylinder is compressed with a piston and held in the compressed state until thermal equilibrium has been reached with the sorrounding. Now, we put a plastic block next to the piston and let go. The gas will expand and thereby transfer kinetic energy to the piston and the plastic block, so that the block shoots out. It's basically an air gun.
Now, imagine the same scenario, except for that we compress the gas further, and once again let it cool off to equilibrium. Since the temperature is the same as in the former scenario, the thermal energies of the gases will be identical. Yet, the second more compressed gas will shoot out the platic block further. Why is that? A stab at an explanation might be that there is a greater pressure gradient in the second scenario, which leads to the energy transfer being more efficient. Abby's explanation is that it has more free energy (as opposed to degraded energy that cannot be put to use of benefit to us). She has designed the task and has made an AAPT poster about student interaction in relation to it:
In the following clip, they have enacted the ET and made energy diagrams group-wise. Rachel is excited since they have come up with two qulitative different models of the energy flow. Two groups argue that elastic, potential energy is stored in the compressed as (by analogy to a compressed spring), while the other groups explain it in terms of transfer from thermal to kinetic energy. She match them up together for comparison and eventually reconciliation of the two models (a theme that continues through Monday PM and is not settled until Tuesday AM).
Melanie holds up the diagram of one group and Madonna goes through their flow. [I apologize for some erratic "panning" in the beginning].
First, thermal energy is added to the system as it gets compressed, next, thermal energy leaves the system as the temperature decreases to reach equilibrium. They also briefly mention the elastic energy involved.
The strange think is the next link in their argument. As the piston is released and the gas expands, Madonna says that the temperature decreases and thermal energy returns to the chamber. Sid, from the other group, tries to follow her: "So you are saying, when the piston is released, there's an increase in temperature in the gas?" Madonna corrects: "Decrease in temperature." Sid wonders: "So why is there thermal energy entering the chamber?" Madonna: "Because if it becomes cold here [points to gas (?)], and it's hot out here [points to surrounding (?)], energy is gonna flow into the chamber." Sid (?): "Oh, I see... [hesitant]. But if there is a change in volume... That does not seem right to me for some reason..."
I side with Sid that the explanation does not make sense. I think that Madonna's group are one step ahead in their causal change of events.
During the event of expansion, the gas performs work on the surrounding, which leads to decreased kintetic/thermal energy of the gas, which goes hand in hand with decreasing temperature of the gas.
In the next step, the created temperature difference between the gas (now cool) and the surrounding will lead to thermal energy returning to the gas (provided thermal contact), by means of heat conduction.
Why were they tempted to include this future return of thermal energy into the expansion event? This puzzles me...
Another reflection is that the exercise of trying to understand the other group's qualitatively different,but not necessarily more wrong or right, and possibly try to negotiate or reconcile the two models led to very fruitful discussions. I talked to Lane and Adam from E1 about this, and they haveused this tactic extensively in the past, but this year, they have found that their four groups and two energy theater "ensembles" often come up with similar solutions, and that there was a greater variety of explanations previously when they worked with more parallel groups.
Mary and Madonna continue comparing the models in the following clip:
This was super important to the teachers in Teaching Seminar this past year as well. I think maybe because they don't want to leave the end of the scenario in an "incomplete" state - where stuff will still happen. It isn't in equilibrium until there is that flow back to the piston. Very cool. I am glad that you find this interesting as well! It is a great scenario! :)
ReplyDeleteA test balloon:
DeleteCould the confusion in part relate to the ambiguous meaning of the word "then"?
On hand, it may relate to causation thoughout one single process: if thermal energy leaves the gas, then it decreases in temperature.
On the other, there is succession: First, the temperature went down (and thermal energy left the gas), then thermal energy entered the gas (to regain equilibrium)?