W7

Work, Heat, and the 1st Law of Thermo (Ch. 19)

Lectures

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

• Tue (pre-pdf , post-pdf)

• Thu + addendum (pre-pdf, post-pdf)

Example Videos

From TA Ian Chaffey.

• Exercise 19-39

• Exercise 19-52

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

• Thermal energy is really just the microscopic kinetic and potential energies of the ~Avogadro's number of particles that make up a gas.

• The first law of thermodynamics is that energy is conserved. That's it.

• Heat is a change in energy. It is not a temperature. In fact, it's a kind of work.

• The work in a thermodynamic process (e.g. pressing down on a piston to compress a gas) is the area under the curve of a pV diagram. The overall sign depends on whether you're asking about the work done on the gas (negative sign) or the work done by the gas (positive sign); the book asks about the work done on the gas.

• Adiabatic means no heat (Q=0). Add this to the list of "important kinds of processes" along with isochoric, isobaric, and isothermal. You should know the shape of an adiabatic process on a pV diagram; the derivation is technical (sec 19.7), but the result will be important.

Things to think about

• The trickiest thing in the chapter is the sign of work. This is due to whether we are asking about the work done on the gas (which is usually what the book asks) versus the work done by the gas. These differ by an overall minus sign.

• The next two weeks we'll be working up to figure out how refrigerators work. This means we want to figure out how to lower the temperature inside a box, even though the box is already at a temperature lower than the outside. This week, we learn about heat as a way that energy is transferred.

Reading Guide

• 19.1 - 19.3 are general background information. In my opinion, these three sections are too verbose and end up making the science unclear. You should skim these sections. I suggest ignoring anything with a bar chart.

• 19.4 defines the first law of thermo. You can compare it to the discussion in 19.1. Pay attention to this section: be sure that you can calculate the work in our favorite thermodynamic processes (isobaric, isochoric, isothermal). It also introduces a new "favorite" thermodynamic process: adiabatic. This is going to show up a lot, please spend some time getting used to it.

• 19.5 is an introduction to specific heat. It answer: how is heat related to a change in temperature?

• 19.6 is simply an application of the ideas from 19.4 and 19.5. Armed with the ideal gas law and the 1st law of thermo, we can keep track of energy transfer from heat. This is called calorimetry. Like many things in this book, it's an unnecessarily fancy word.

• 19.7 follows up on 19.5 and derives some important results related to the specific heats of gases. This is a more technical section, but the results are important. You should be sure to understand a few things: (i) why the change.

• 19.8 gives some results for different ways in which heat transfers energy: conduction, convection, radiation. We won't be doing much with this. Please read this section, but treat it like a collection of facts. You don't have to memorize anything or derive any of it. You may need to use some of the equations on some homework problems

The following article on common misconceptions about the first law of thermodynamics is useful:

• "Investigation of students’ reasoning regarding heat, work, and the first law of thermodynamics in an introductory calculus-based general physics course," David Meltzer, American Journal of Physics 72, 1432