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## Always, Sometimes, or Never True: Instructor's Guide

### Main Ideas

• The first and second thermodynamic laws
• The thermodynamic identity
• Intensive and extensive variables
• State functions

Estimated Time: 60 minutes

Small groups of students are given an “Always, Sometimes, or Never True” handout and asked to decide if statements about four different scenarios are always, sometimes, or never true.

### Prerequisite Knowledge

• Solid understanding of the first and second thermodynamic laws.
• Knowledge of the difference between intensive and extensive properties.
• Understanding of the consequences of a process being reversible or irreversible.

### Activity: Introduction

Before performing this activity, a brief review of the first and second thermodynamic laws is helpful to put students in the right mindset. Students should be placed into small groups and given the activity handout. Before the groups begin analyzing the scenarios, be sure to emphasize that the first and second thermodynamic laws, the thermodynamic identity, and the internal energy expressions will lead to the answer for each statement.

If students are struggling to progress in the worksheet, bring the class back together and answer the statements in the first scenario using the thermodynamic laws so the groups have a solid example to refer to for future reasoning in the worksheet.

### Activity: Student Conversations

• One troubling aspect of the Sometimes/Always True activity was that the T.A. and the professor answers, sometimes, didn't align when helping the individual groups. It would be useful if the T.A. and professor met beforehand to ensure they gave the students the same answers and reasoning. - Teal, Jeff and Andy (Students)
• All three of us felt that we did not receive definitive answers for most of the questions on the activity even after completing the activity. It would be beneficial to complete Sometimes/Always True activity as an entire class, or at least present the answers to all the questions to the entire class. - Teal, Jeff and Andy (Students)
• It may also be beneficial to break the activity into more pieces. Perhaps, the students could complete a section, then present, and then multiple more sections could be completed in this manner. Simply letting groups loose on the activity caused groups to jump back and forth between activities which resulted in confusion for many of the students. - Teal, Jeff and Andy (Students)
• The balloon scenario, in particular, sticks in our mind as a vivid example of when there were contradictory interpretations of the same problem. In the end, the problem was the students never resolved the differences between the two interpretations of the scenario, in particular the aspect of the problem involving whether or not the balloon stretched when sat upon.
• I've only done this activity once. I think it is a good idea, but getting the questions right is tricky, and it's absolutely necessary to go over them with the TA in advance. So I'd be happy to hear any experiences you have with this, but can't recommend anyone try it without very careful consideration (as I intend to do this coming year). – David Roundy

### Activity: Wrap-up

Bring the small groups back together as a class and ask which scenarios or statements were the most challenging and why. Because this worksheet is based on reasoning rather than computations that give a definitive answer, students often find this worksheet very difficult. Answering some of the more difficult statements at the end of the activity will help students follow the same reasoning to finish the worksheet later for additional practice in physical reasoning. Re-emphasize the the first and second laws of thermodynamics are just as important for making physical reasoning as they are for mathematical computations.

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