General comments
I was pleased that the class (self-reported) that they had all read the material. I tried too many PRS questions (6) and didn't leave enough time to explain the answers. I will try reducing it to 3 or 4 questions next time and allow more time for discussion and explaining concepts on the blackboard.
Please go back and think about some of the answers to the PRS questions (they are inserted into the web page thermo notes).
" Please show the graphs after every PRS evaluation and tell the class when the answers are being graded." Unless otherwise noted, all the PRS responses in my lectures will be graded on participation only. Regarding showing the graphs, I will try to do this in all circumstances where appropriate.
Responses to 'Muddiest Part of the Lecture Cards'
(70 respondents)
1)What is the justification for two properties defining a state? (5 students) It is an empirical fact for pure substances. It does require that the system be steady or quasi-steady (slowly varying) so that one unique state can be defined (versus it being different in different parts of the system). All of you have worked with the model for ideal gases p=rRT: if you know p and T, you can find r, etc. Thus knowing two properties, fully defines the thermodynamic state of the system.
2) How does 30% of the fuel energy actually move the airplane? It ends up as friction losses but what is the intermediary? -- and related questions (7 students) The fuel energy is used to (eventually) accelerate the gases leaving the engine. If the gases leave at a higher velocity than they came in, a reaction force results (thrust!). This thrust acts in opposition to all the drag forces on the aircraft. The drag forces are related to viscous dissipation around the aircraft (you will learn much more about this in Fluids) which goes to heating the air. The sound energy emitted is very, very small compared to the thermal and kinetic energy.
3) For the airplane problem, how do we know that the atmosphere heats relatively more than the airplane? (3 students) Certainly the frictional forces around the aircraft heat the structure (to the extreme for a re-entry vehicle). However, much of the viscous dissipation happens in the air that is just off the surface of the vehicle and thus it is less effective in heating the body. Good question.
4) What is specific volume? (13 students) I had incorrectly assumed that this was a familiar parameter for everyone. Specific volume is the volume per unit mass (e.g. how much space one kilogram takes up). Its units are m3/kg. Note that it is the reciprocal of density (kg/m3) which is mass per unit volume. It is often more convenient to work with specific volume than with volume itself.
5) The experiment with the metal and the plastic containers was unclear: (6 students) The experiment was designed to reinforce the requirements for thermodynamic equilibrium: thermal equilibrium and mechanical equilibrium. The plastic bottle continued to bounce (thus taking longer to reach mechanical equilibrium). It is also likely that it would take longer to reach thermal equilibrium because metal has a higher thermal conductivity than plastic.
6) The engine p-v diagram question was the muddiest part. (17 students) This problem was designed to illustrate the importance of understanding the thermodynamic path that a system (in this case a chunk of air) follows in various energy conversion devices. We will spend much more time with this concept as we proceed with the thermodynamics lectures, so if it was a little unclear at this stage that is okay. In sum, the air enters the engine and is compressed (note how the area that the gas flows through gets smaller and smaller). During this process the pressure and temperature of the gas increase and the specific volume decreases. In the combustor, energy is added to the gas at constant pressure (this isn't something you are expected to know off hand--but the problem was set up so that there were several other clues to what the correct answer was). This causes the gas to expand (increasing the specific volume). Downstream from the combustor, the gas is expanded through a turbine (note the increase in the area of the flow path) further increasing the specific volume. Energy is extracted to drive the compressor. During this process the pressure and temperature decrease. At the exit of the engine the pressure is the same as the inlet, but the temperature is higher and consequently the specific volume is higher (pv=RT).
7) I am a bit confused about the nature of energy conversion, I guess we'll get to that more later. (1 student) Yes we will. Hang on for a few lectures.
8) Whenever the energy is transferred into kinetic energy, does that mean the energy was converted to heat? (1 student) No, not necessarily. Think of dropping a rock. The potential energy is converted into kinetic energy as the rock falls. Conversion to thermal energy doesn't happen until the rock hits something. Or for a second example, think of an ideal pendulum--kinetic and potential energy are traded back and forth on every swing.
9) What was the equation next to the airplane on the board? (1 student). It was the equation for overall propulsion system efficiency as described in the Breguet Range Equation notes.
10) What is the difference between questions 3 and 4? (1 student). One asked what you think will happen (without seeing the experiment). The other asked what you think happened (after seeing the experiment).
11) Better define the difference between useful work and waste heat (1 student). We will talk more about this later, but in short, anytime I have a temperature difference (say I have a hot body and a cold body) there exists the possiblity of extracting work from it. Alternatively, I could arrange it so I get no work out of it (e.g. put the hot body, next to the cold body and let the thermal energy flow from one to another without extracting useful work). The high temperature gases that exit a gas turbine engine are not used for anything, thus they can be considered to carry waste heat.
12) Unsure of differences between power and energy (1 student). Power is a rate of energy flow (J/s versus J). Take a look at a physics text to see lots of examples, and discussions.
13) No clear thermodynamic equilibrium definition (1 student). As noted above, I was too rushed. I should have written this (and a few others things) on the board. Please see the notes and the text. If these don't clear it up for you, let me know.
14) The explanation of how to do the homework (1 student).
15) Altoids are curiously strong mints (1 student).
16) No mud (9 students). GOOD!