Common mistakes on HW9

Dear class,

If you would like your HW9 back, you can retrieve it at the review session tonight or see me in my office tomorrow. Here are a few common mistakes we noticed in grading it:

Problem 49:

Some students did not realize that entropy increases for the nonideal compressor case. The corresponding curve in the Mollier diagram should start at the same place as the ideal case, but then move both up and over to the right, in the direction of increasing entropy. The endpoint will not be the same, although it will intersect the same isobar.

Problem 50:

Some students calculated for the turbine:

W_real = W_isentropic / efficiency
This is the right expression for a pump or compressor, but for a turbine it should instead be:
W_real = W_isentropic * efficiency

Cheers,
MSS

HW9

Hi class,

Please do attach the Mollier diagram that you work in problem 49 to your solutions. If you would like another copy, you can print one out from the online pdf of the homework assignment.

Cheers,
MSS

Final and review session

Dear class,

The final will be Wednesday, March 16, from 12-3pm. It will be open book and one page of notes, similar to the midterm. You should bring a calculator, your book, writing utensils, and your notes page. If you would like to use the first page of the fundamentals handout, and write your additional equations on the back, that would be fine as well -- but in total you should only have one page of notes to bring. The format of the final will be very similar to the midterm, but longer in proportion with the three hour time block.

I will hold an open question-and-answer review session on Monday, March 14 from 6-8pm in Engineering II 1519. Feel free to stop by whenever is convenient during that time to ask questions.

I will also be available to chat during office hours on Tuesday, March 15, from 10am-12pm and from 3:30-5:30pm. My office is Engineering II 3321.

In addition, I will be handing out grade statistics sheets for each student on Friday at the end of class so that you will be able to see how you are doing in the course going into the final.

Cheers,
MSS

Common mistakes on HW6 and HW8

Dear class,

We noticed that there were a number of mistakes on HW6 and 8 involving unit conversions. It is essential that you carry units throughout all of your calculations so that can keep tabs on conversion issues. To avoid a lot of writing, you can keep your equations in analytical form until the final step when you plug in actual values of variables.

Cheers,
MSS

Common mistakes on HW7

Hi class,

Here are some common mistakes that were made on HW7. Please check to see if you made these so as to avoid them in the future.

Problem 32: "well-insulated" implies "adiabatic" - which means Q = 0, not necessarily that delta(T) = 0 (although with an ideal gas in a process that also does no work, delta(T) will equal zero).

Problem 33: need to use absolute temperatures with the efficiency, Qc, and Qh equations.

Problem 34: need to use consistent units with the Q=n*cp*delta(T) equation

Problem 38: constants in table c.1 have units (e.g. B has units of 1/Kelvin and C has units of 1/Kelvin^2); basically another reminder to take care with units and make sure that they match up.

Cheers,
MSS

HW8, problem 42

A slight correction to the problem statement:

In part (b), please calculate the entropy change for the world, i.e., that due to both the system and environment entropy change. The point is to show you that supercooled water freezes, the entropy of the world increases and thus this process does not violate the second law, even though "disorder" may seem to be decreasing.

Cheers,
MSS

Some Notes on Homework #6

Problem 28

When calculating the percent error, compare your calculated enthalpy at 25 °C to the tabulated value at 0 °C (1189.5 J/g). Please ignore the fact that this tabulated value is not at the same temperature as your results. You do not need to rescale the tabulated value to 25 °C.

Problem 31-b

For this problem, assume that no phase changes occur - all components remain gases over all temperatures. Also assume that the heat capacity coefficients given in Appendix C.1 are valid over all temperatures. There is a typo in the problem, so don't worry if your answer seems off, just approach the problem the same way you've learned in class.