Final grades

Dear class,

We have finished grading the final and your course grades have been computed.

The statistics for the final are:

• average - 59% (63% with bonus)
• standard deviation - 16% (17% with bonus)
• maximum score - 92% (98% with bonus)

The statistics for the course grade are (with bonus):

• average - 68%
• standard deviation - 13%
• maximum score - 92%

The letter grade distributions are as follows:

• A - 23.6%
• B - 29.1%
• C - 27.3%
• D - 9.1%
• F - 10.9%

A detailed plot of these distributions can be found on the course webpage.

I enjoyed teaching you all this quarter and wish you the best of success in your future endeavors!

Cheers,
MSS

Final review problem solutions

Dear class,

The numerical answers to the review problems suggested for the final are posted online.

MSS

Problem #48

On problem #48, since we are given that the gas is ideal (not steam, or methane, etc.) with a constant heat capacity and that the expansion is adiabatic, can we use deltaH = Cp*delta T to find the shaft work? But since this is also the expression to find the isentropic work, would the efficiency be 1.00?

The efficiency measures the work relative to the isentropic work. In the isentropic case, the exit temperature will be different from the actual temperature given. So, to compute the efficiency you will need to compute the new output temperature.

MSS

Review session

Class,

I will hold a question-and-answer review session for the final on Monday from 5:30-6:30pm in our usual classroom.

Cheers,
MSS

Problem #47

When solving for V in terms of V-vap and V-liq, is it ok to assume that V-liq will be negligible?

This is probably a pretty good assumption that will simplify your calculations, provided the temperature is sufficiently sub-critical and V-vap and V-liq differ quite a bit in magnitude (e.g., V-vap is ~1000 times as large).

MSS

HW9 question

For problem number 47 once 25% of the steam has condensed do we use the evap. column in the steam table to find the volume and pressure of the new state?

Think about what kind of process this is. It states that the condensation takes place in a rigid vessel. That means that the *overall* molar volume of the system must remain constant. That places a constraint on the overall molar volume of the final state, which is a combination of liquid and vapor molar volumes (via the quality). The other constraint is that the system is on the saturation curve. Those two constraints should help you find the final state, using the steam tables.

You will also need to apply the first law to figure out how the heat removed relates the properties of the system. Note that Delta H = Q / n does not apply here because it is not a constant pressure process.

-MSS

HW 8 Problem 40

Dear Class,

You should assume that the device in problem 40 operates adiabatically.

-Nathan