W8

Entropy & The Second Law (Ch. 20)

This is by far the chapter of the book that I find most annoying. It doesn't explain what entropy is or where the second law of thermodynamics comes from. We're going to do something unusual and ignore most of what the book says.

Update: Week 8 grade listings (using your P40B anonymous ID)

Lectures

Additional lectures (from Spring 2020) are linked at the bottom of the page.

• Tue: (pre-pdf, post-pdf)

• Thu: (pre-pdf, post-pdf)

Example Videos

From TA Ian Chaffey.

• to be uploaded

Reminder: TA office hours are on Friday at 1pm, please find the link pinned to the #general channel of our Slack.

Learning Objectives This Week

• Articulate what entropy is as a technical idea, explain what's wrong with the common idea that "entropy is disorder."

• Explain the second law of thermodynamics from the perspective of microstates.

• Connect the second law of thermodynamics to the principle that heat only flows from hot things to cold things.

• Understand the probabilistic nature of entropy, and what we mean by entropy "always" increases.

Things to think about

• It's a common joke that physics parents tell their children to "decrease the entropy" in their room when their bedrooms are messy. Does this make sense? What are the microstates and macrostates in this analogy?

• In what sense is the second law a "deep statement" about nature? Or is it just an "obvious" statement about statistics and probability? What assumptions go into the idea that heat always goes from high temperature subsystems to low temperature subsystems?

• In what way does entropy measure irreversibility?

• In what way does entropy measure ignorance or incomplete knowledge?

Reading Guide

I recommend reading/viewing the following:

• Jeff Phillips' What Is Entropy on TedEd (video).

• "The Misunderstood Nature of Entropy," PBS Spacetime video

• "Entropy and the second law: A pedagogical alternative," Baierlein. Read the first 2 sections; the remaining 3 sections covers next week's chapter very well (though at a slightly more technical level than the book.)
  American Journal of Physics 62, 15 (1994); https://doi.org/10.1119/1.17732

• "No seriously what is Entropy," massgap.wordpress.com (10/28/2018)

This week we're mostly ignoring the book. Here's my breakdown of what is in the book:

• Sections 20.1 - 20.5: motivating the microscopic picture of internal energy and temperature. These are the 'basics,' but they're not particularly useful for us here and you may have already seen these ideas in a chemistry class. Skim this quickly if you want to better understand how the microphysics of a gas contributes to the macroscopic properties in thermodynamics.

• Section 20.6: this is a goddamn train wreck. It gives the fancy names for critical ideas (entropy, second law, irreversibility) but doesn't actually say anything meaningful. As part of your university education, we're going to ignore this chapter and spend the week learning about these critical ideas properly.