Day 96: Energy RepresentationsPosted: March 11, 2014
Today I gave my version of Kelly’s Energy is Pain introduction to energy. For me this comes after 5 days of empirically showing how work produces changes in energy, though they don’t know those terms yet. I was careful to do two things in this process; first, I wanted to emphasize (and will keep doing so) that work is always done, though often it is within the system and we simply track the energy storage, not the changes due to work. The above picture illustrates this, as I had first (in blue) considered the air outside of the system, thus energy is transferred out of the system via work done by the air and then stored as thermal energy outside of the system, and then in green went back to what it looks like when we consider the air in the system, so that the work simply transfers the energy to thermal energy stored in the system. Same transfer, same storage, but the question is, are we tracking the storage or the transfer?
I slightly modified the typical money example for this purpose. I picked a student and defined our system as including both myself and the student. I said I had two dollar bills initially. Then I gave the student some money without stating how much, and then we both stated that we had a dollar each. Nothing left the system, but money was still transferred. Then I started over defining myself as the only part of the system. I stated openly that I am transferring a dollar to the student, and had him put it away. In that case we tracked my initial storage, how much left, and the final storage. In both cases the transfer and the storage were the same, but how we tracked them were different.