Lecture T3: Finished Heat & Work, Started First Law

"We spent half the lecture on a point that was adequately explained on the mud rather than the new topic of the lecture" (1 student)

I think I spent too much time reviewing some of the items from the last lecture and talking about some of the muddy points. It was about 1/2-way through the lecture before I got to new material. I need to strike a good balance between spending extra time explaining things for some people and boring others. I am not sure I have the balance right (please comment if you have thoughts on this). For future lectures I will try to limit the review and mud discussion to 5-10 minutes. If there are things that people are still very unclear about I encourage them to contact me or one of the TA's directly. Note that you will also get practice with these ideas on the homework and we can talk about them more in recitation.

Responses to 'Muddiest Part of the Lecture Cards'

(58 respondents)

2) First PRS question, not (1) because of psys; so what???--and related questions on psys vs. pext (4 students). Please see T2 mud, response 2) and the link within. Why would there be enough information if the process was quasi-static? If the process were quasi-static then psys=pext. So I would have everything I need to calculate work (the behavior of pext and v). What is an example of a situation where psys would give you the wrong answer for the work done on the system? Any time the process is not quasi-static you will get the wrong answer. The example we discussed in class where pext=0 and the pin holding the piston in place is removed. If I integrate (psys)dv I get a finite value, but the work is zero (wrong answer). Also note that this topic will come up repeatedly in lectures and homework so you will get a chance to get more comfortable with it as we proceed.

3) Sometimes it is hard to tell if you are using uppercase or lower case letters on the board. (1 studen1) I will try to be more careful, but you will also become more comfortable with switching between the two through working the homework problems. Just remember: check the units in the equations you use to make sure that you are using consistent variables (intrinsic or extrinsic).

4) The black box problem was unclear. (18 students) First, is this situation possible? Can you think of a scenario in which you could produce this? Sure, we could put an electric motor and a battery inside a rigid cooler (thermally insulated) and allow it to pull the weight up. Now what about the energy in the box? Why does it have to be decreasing? The First Law ( DE=Q-W) tells us it must. Whatever is in the box is doing work to raise the weight (so if we consider the black box our system, work is crossing the system boundary via the fine thread). And by the statement of the problem, the box is in every other way isolated from its surroundings, so heat transfer to the box is zero. So DE=-W where W equals the work done by the system, which in this case is positive (it lifts a weight). So DE is negative, the energy in the box must be decreasing (at the same rate that the potential energy of the weight increases since the energy of the entire box + weight system is constant). Finally, the second answer (heat transfer down the thread) was just a red herring. Your intuition should tell you that it is very difficult to transfer heat down a a very fine thread. The waste heat produced by whatever is doing the work in the box would go towards heating up the contents of the box.

5) The signs of Q and W in the E equation were unclear (1 students). The signs are a convention. They also make intuitive sense. Positive Q is heat added to system (increases E). Positive W is work done by system (decreases E).

6) DPE is what happens when you lift a box of molecules to a new height? (1 student). Yes, this is the same thing as the potential energy you are used to working with.

7) How do you quantify DU? Units? (1 student) Internal energy has units of Joules, or when written as an intrinsic variable (lowercase "u") the units are Joules/kg.

8) What exactly does the First Law mean by "a function of the state of the system"? (1 student) Good question. This means that energy is not a function of path, but only a function of the thermodynamic state (just like p, T, & v). It also implies that internal energy can serve as one of the two independent properties needed to define the state of a system.

9) Internal energy is directly related to T or Q? Why not just use Q? (1 student). Internal energy is related to T not Q. Internal energy is a thermodynamic property and a function of the state of the system. Q is heat added to the system, a path dependent quantity that can change internal energy (work is another path dependent quantity that can change internal energy).

10) If you add heat does it increase or decrease the pressure? (1 student) If I add heat to a fixed volume, the pressure and temperature go up (per the First Law and the ideal gas law).

11) I guess I must be confused about how everything relates in terms of energy and work. I have trouble seeing the cause and effect in the thermo systems. (1 student) The relationship between heat, work and energy is expressed by the First Law of Thermodynamics. As we introduce new parameters and specialize these equations for particular situations it is easy to get confused. If you do, always go back to the simplest statement of the First Law and work forward from there.

12) At the beginning of the lecture you mentioned something about radiative versus convective heating. What is the difference? (1 student) I encourage you to look in the library for a more detailed answer to this question (or wait until 16.050 where this material is taught). In short, conduction is heat transfer through matter by molecular motion, convection is heat transfer by conduction but with a moving fluid, and radiation is transfer by E-M waves (like heat from the sun--no intervening matter is required).

13) In the ideal gas law pv=RT does the constant R account for n, the # of moles of gas? (1 student). Please read this section of the notes again.

14) Why does "choric" stand for volume? What is the origin of the word? (Don't feel obligated to answer this.) (1 student) The best reference for word origins is the Oxford English Dictionary, and they didn't have isochoric, so you are out of luck.

15) No mud (22 students). Good!