W9

Heat Engines and Refrigerators (Ch. 21)

How the heck do refrigerators not violate the second law of thermodynamics?

Announcements

• Grade concern form: if you want Prof. Tanedo to double check something when finalizing grades at the end of the term, please fill out this short form to remind him of any past excused late assignments. (Examples: students who transferred in during week 2 are exempt from week 1 assignments.) You may fill this form out multiple times, once for each concern. Due: Fri, March 12

• Grade appeal form: if you know that there is a problem with a grade, then please record a 5 minute explainer video making your case for why one of your grades should be improved. There are two types of appeals: reviewer mistakes, and your mistakes. You may fill this form out multiple times, once for each concern. Due: Fri, March 12

Pedagogy Survey

• Post-Class Pedagogy Survey
  Our flipped classroom/explainer video course is experimental. To examine whether this type of course format is beneficial to students, we have a brief survey and with the option for a follow up discussion with an education researcher. Prof. Tanedo would appreciate it if you take some time to fill in this survey. Due: Fri, March 12 (optional, but appreciated)

Lectures

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

• Tue (pre-pdf, post-pdf)

• Thu: part A, part B (pre-pdf, post-pdf)

  • Lecture recording was interrupted due to a power surge. (Was Prof. Tanedo's refrigerator taking in too much work!?)

Example Videos

From TA Ian Chaffey.

• Ex. 21.38

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

• Understand how a refrigerator works conceptually.

• Be able to describe heat engines and refrigerators as cycles in a pV diagram.

• Efficiency is "what you get out divided by what you put in"

• The Carnot engine is a hypothetical maximum efficiency cycle because it only uses reversible processes.

• Calculate work in/out and heat transfer of cycles using our favorite thermodynamic processes.

Things to think about

• Why do refrigerators seem thermodynamically "impossible"? What "trick" do we use to actually make a working refrigerator?

• Why do heat engines and refrigerators have to be cycles on a pV diagram?

• What does reversibility have to do with maximum efficiency?

• What are the conditions for a thermodynamic process to be reversible?

Reading Guide

The book is obsessed about calculating efficiencies. That is not the interesting part of the chapter. In fact, it's just using the tools from previous chapters: area under the pV curve, using the first law to determine Q. Instead, what is more important is that you can describe how a refrigerator is able to make the cold part inside the fridge even colder while dumping heat into the warmer environment outside. You should also appreciate what makes a process reversible and why the Carnot engine (reversible) is the "best" engine.

Sections of this chapter:

• 21.1: background information (skim)

• 21.2: introduction to heat engines and refrigerators and the book's funny diagrams. The book makes a big deal about these heat flow diagrams—you should understand how to read them, but I don't think they're actually all that helpful.

  • Be sure to understand why an heat engine/refrigerator is a cycle.

  • Get used to the definition of efficiency. The efficiency is always "what you get divided by what you put in," but for a heat engine you get work and put in heat; whereas for a refrigerator you get heat (out of the fridge) and put in work.

• 21.3: this is the main part of the chapter where the book goes through complete thermodynamic cycles. This section has really useful examples, but it can be easy to get lost in the details. Make sure you appreciate the big picture (Tuesday lecture) of what the cycles are trying to do.

• 21.4: connects reversibility to maximum efficiency. This section is conceptual and important, but it's written a bit funny. First it argues how reversibility implies maximum efficiency, but it doesn't explain what makes a process "reversible" until the second half of the section. (You may want to peek at the second half first.)

• 21.5: Carnot cycle. This is the main "payoff" of the chapter: the "best" thermodynamic cycle. If you have a solid understanding of section 21.3, then this section is just one specific application where you only use adiabatic and isothermal processes.