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 two PRS questions. Q1 was designed as a warm up and to get you thinking about energy. Q2 was intended to emphasize the requirement for thermal and mechanical equilibrium for defining the state of a system (where "mechanical" is an improvement over the word "dynamic" I used in lecture). Most students correctly answered these questions.
Responses to 'Muddiest Part of the Lecture Cards'
(69 respondents out of 81 students in attendence)
1) What is the justification for two proper ties defining a state? -- and related questions (3 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. Could have gone into p=rRT a little more. (4 students) Read over this. If it doesn't clear up the confusion let me know.
2) Explain the conversion process for the engine again -- and related questions (9 students) Some fraction of the fuel energy is used to do work to propel the vehicle. Let's call this part A. The remainder (part B, say) ends up as internal energy and kinetic energy in the exhaust. Both parts eventually end up as increased internal energy in the atmosphere through which the airplane flies. Let's consider part A first. The fuel energy is converted to internal energy and kinetic energy in the gas flowing through the engine (in the combustor). The various pieces and parts of the engine then cause this flow to accelerate producing pressure forces on the surfaces of the engine which push the engine forward (doing work on the engine/airplane). This thrust force 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 more about this in Fluids) which goes to heating the air. The heating due to viscous dissipation 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. So the portion of the fuel energy that goes directly to providing propulsive power is eventually converted into increased internal energy in the air around the airplane. The remaining fuel energy that does not go into producing propulsive power (part B) ends up as increased internal and kinetic energy in the exhaust -- which also eventually ends up heating the atmosphere through which the airplane moves (since the kinetic energy in the exhaust is eventually converted to internal energy through vviscous dissipation). The sound energy emitted is very, very small compared to the internal and kinetic energy (the ratio is on the order of 1/1000 to 1/1000000). We will discuss more details of what happens within the engine in future lectures.
4) What exactly is internal energy, the first law, and how do we get work from a system, how do we change the state of a system (and related questions). (14 students) We haven't gotten to these topics yet, but we will (see Chapter 3 and Chapter 4).
5) Difference between metal and plastic in terms of thermal equilibrium (1 student) 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) For the "dynamic" part of thermodynamic equilibrium, the lecture mentioned that P.E. and K.E. must be unchanging, but does that mean that they can not be transferring at equal rates? -- and related questions (4 students) First, I was a little loose with my wording in an effort to emphasize the two requirements. It would have been more appropriate for me to say "mechanical" equilibrium versus "dynamic" equilibrium which ends up as a bit of an oxymoron. For mechanical equilibrium, the system must be steady (every time you observe it the properties are the same), but it can be moving.
7) What are examples of different forms of energy? (2 students) chemical (e.g. as might be carried in fuel + oxidizer and measured by the heating value of the fuel in J/kg), kinetic (measured by mass and velocity), potential (measured by changes in distance within a potential field of a given strength), internal (typically measured by temperature and pressure).
8) What is a thermodynamic diagram? --and related questions (5 students) It is a plot of the behavior of two independent thermodynamic variables for a system. We will get more practice with these in the next lecture.
9) Will this course only examine gas systems? (1 student) No.
10) Why did you mention mdot-fuel * h again? (1 student) In reference to the fuel power that is at the start of the energy conversion process for an airplane.
11) Where is the reading on the internet? (1 student) I will assume you have found this now or you probably wouldn't be reading these mud responses.
12) Name of book (1 student) Borgnakke, Sonntag and van Wylen.
13) Why is it always so warm in this room? (1 student) Explaining this is one of our learning objectives.
14) No Mud. (22 students) GOOD!