General comments
The lecture covered most of Chapter 2 of the notes including an introduction to energy conversion processes, discussions of thermodynamic systems, defining the state thereof, and thermodynamic equilibrium. We did three PRS questions. Q1 was designed as a warm up and to get you thinking about energy. Q2 was more difficult and focused on energy conversion processes. It was designed to emphasize that energy is always conserved as it is transformed from one form to another. Q3 was intended to emphasize the requirement for thermal and mechanical equilibrium for defining the state of a system. Most students correctly answered these questions.
Most of us really didn't know there was reading for the lecture today, maybe next year let us know where to look online. (5 students) Yes, that was my error. I should have sent out an email since it was the first class. Please read over Chapter 2 (and 3 for the next lecture). For the future, please look on the Unified web page -- almost all the lectures have reading assignments from the notes associated with them.
I liked it when we got to talk to our peers about the concept questions. Those are good. (1 student) We will do plenty more!
Responses to 'Muddiest Part of the Lecture Cards'
(53 respondents out of 68 students in attendence)
1) What is the justification for two proper ties defining a state? -- and related questions (6 students) First note that it is two INDEPENDENT properties (so density and specific volume only count as one since they are the inverse of one another). It is an empirical fact for pure substances. It does require that the system be equilibrium or quasi-equilibirum (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. PV = nRT ... do you really need two variables? Example: if you know pressure and volume, the temperature of the system depends on the moles of gas in the system. (2 students) Read over this. If it doesn't clear up the confusion let me know. How does the definition of the state of a system apply to systems that aren't ideal gases? (1 student) It still applies, i.e. two independent properties are required to define it, but the relationship between the independent properties will not be the same as the ideal gas law.
2) How the useful work from the airplane engine gets turned into waste heat iu the end was a little unclear to me. -- 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 heat due to viscous dissapation is similar to what happens when you move a block along a rough surface (it heats up due to friction). There is friction between the air and the airplane. The sound energy emitted is very, very small compared to the thermal and kinetic energy (the ratio is on the order of 1/1000 to 1/1000000).
3) Some examples of closed and open systems would help. (1 student) We will get to many examples of both. Is open/closed system predetermined by the system itself or chosen by the problem solver? (2 students) Good question. It is chosen by the problem solver, but in certain situations the problem is much more tractable one way or the other. The answer to the Advil/Altoid problem might be different if the system was drawn differently. (2 students) Perhaps, which is why it is always important to start with a clear definition of the system. How does bouncing affect the mechanical equilibrium? (1 student). Small deflections in the walls of the containers. Although mass crosses the boundary of the engine, if it did so at a constant rate, could it be calculated as a closed system since inner mass is constant? (1 student) No. The distinction is whether mass crosses a boundary or not, because mass can bring energy in and out with it. In a closed system, the only way to change the energy is through work or heat transfer at a boundary. I am confused about changes in mass and volume in closed vs. open systems. (1 student) We will have more discussion of this in subsequent lectures.
4) The relationship between energy and work remains a little muddy. (1 student) We haven't gotten there yet, but we will (see Chapter 3).
5) In the experiment the thermal part was a little muddy. (and related questions) (7 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. Metal has a higher thermal conductivity than plastic, therefore when the energy is transferred to the container as heat (and to the floor and the air) the system (the metal container and everything in it) more rapidly reaches a uniform temperature.
6) Can the equilibrium necessary to define a system be dynamic equilibrium or must the thermal and mechanical components of the system be at rest? -- and related questions (3 students) The system must be steady (every time you observe it the properties are the same), but it can be moving. Why is mechanical equilibrium necessary to analyze a system thermodynamically provided that there is thermal equilibrium? (1 student) Because I can arrange a situation where the thermodynamic properties (say pressure and volume) are changing but the temperature is constant. After all, in a bottle of gas, the molecules are accelerating all the time, but you can look at the overall properties. (same student) Correct. We look at the macroscopic properties (like pressure, temperature, etc.) versus tracking each individual atom.
7) No Mud. (9 students) GOOD!