Sunday, January 23, 2011

Fleece Sheets

I had the idea for this post as I went to bed last night.  My wife and I sleep in the basement where it's really cold (especially these days in MN).  We've tried multiple comforters etc to try to stay warm while we sleep but the best investment we've made is fleece sheets.  They're awesome!  With normal sheets I get into bed and freeze because the bed is so cold.  With fleece sheets, they feel warm right away!  What got me thinking is how do they do that?

What I'd really like to talk about is how we present thermodynamics concepts to students.  If you look at a typical intro physics text the thermo section will start with a definition of temperature and go from there.  You'll likely find phrases like "what do cold and hot mean" and "what does it mean to be warmer than something else."  My sheets helped me crystallize something about why this has bothered me when teaching it.  Our sense of hot and cold is much more a measure of heat flow than "average energy per mode".  In other words, my fleece sheets are much worse heat conductors than normal sheets and so I don't feel that heat flowing out of my body, a sensation we call "cold".

Here's another common example that texts often get around to but not until later.  In the morning your bathroom tiles are cold but the carpet isn't.  That's certainly the way my kids would say it, at least.  My snarky response is to say "no, they're both the same temperature."  That doesn't seem to help my kids very much.  What's really happening is that tiles conduct heat much better and you get that "cold" sensation as heat leaves your body much faster than on carpet.

There are a couple of interesting points here.  If you focus on conductivity as being central to the notion of human temperature understanding you can then start to ask questions about why heat flows.  You can also address issues of equilibrium (why I'm so confident that the carpet and the tiles are the same temp, for example).  Heat flows **not** to conserve energy but rather to maximize entropy.  Entropy is a measure of statistics, nature moves to distributions that are more common and cold things gaining energy increases their entropy way more than the loss of entropy for a hot thing losing energy.  This then leads to heat engines and on and on.

So how do you teach temperature?  When does conductivity come it?  What would happen if you did it first? Does anyone do that?

Friday, January 14, 2011

Fake data labs

One thing I've noticed over the years of teaching upper-level physics labs is that the students see the experiment "working" as the finish line.  They seem to plan their semesters with that as a goal, determining when to order equipment, when to test it, when to set it up etc.  And often it works out, the experiment "works" right at the end of the semester and they're happy.  Of course, I'm usually not because there's not enough time for them to do a decent amount of data analysis and write-up.  Don't get me wrong, I think the experience they get by having to plan and execute their own experiments is good, I just needed to find a way for them to experience the other stuff too.

My solution, which I've used for the last several years, is to have them do a fake experiment.  This is in our Modern Physics lab so they're sophomores typically.  I have them choose from a list of seminal modern physics experiments, things like the photoelectric effect, Compton scattering, Michelson/Morley etc.  I then ask them to plan a 21st century version of the experiment and to assume that money is no object, since they're not going to do the experiment in real life anyway.  I have them plan a fully computer-controlled experiment (we teach LabVIEW in this course) and ultimately they have to turn in a functioning LabVIEW program, a formal lab report, and a Mathematica document showing their data analysis.

Now this last part is where I come in.  About half-way through the semester they have to turn in to me what their raw data would look like.  Typically this is a description of a data file or files explaining what all the columns and rows would be, as saved by their LabVIEW program.  They have to figure out things like the motor step size they'll use, what wavelengths of light are necessary etc. I then spend spring break creating fake data for them.  I do this in Mathematica with a random number generator providing appropriate levels of noise.  I also choose my own values of things like Planck's constant and the speed of light so that I can know that they've done their data analysis correctly.  I especially like choosing a direction for and relative speed of the luminiferous ether.

What's great about this project is that the students always have time to do the data analysis and write up because they don't spend any time on the actual experiment.  Of course if this is all we did it would be a problem but they get a lot of experience with real experiments both in and out of this class.  Being somewhat early in the program students get a sense of the type of planning they'll have to do for future long-term projects and they get a real sense of the difference between raw data and a result.

I always tell the students that their last sentence in the report should be something like "Planck's constant is ___ +/- ____".  I tell them that's their goal, not getting the experiment to work.

Saturday, January 1, 2011

Flipped classroom evaluations

I just read my student evaluations for the fall 2010 classes.  This semester I taught General Physics II in the "flipped classroom" or "naked teaching" philosophy for the first time.  I felt the class went pretty well and the student ratings and comments mostly bare that out.  I thought I'd make them available (unedited) to anyone who's interested in flipping their classroom.  If you want a pdf copy, email me (, twitter me (@arundquist), or let me know in the comments below.

I made a few changes in the course as the semester went along.  By the end, we'd gotten into our groove pretty good.  We had daily quizzes on the material from the last class (a 4-sided die randomly deciding which problem to choose), 10-15 minutes of me answering questions they'd submitted online about the new material (which included ~20 minutes of screencasts along with reading the book), and then 30 minutes of the students working in small groups with a smartpen recording what we called roadmaps for how to do the problems. Here's a typical daily outline.

There are a lots of comments praising the flipped classroom but there's definitely a few people who strongly favored traditional lectures.  Check out the spread of scores on some of the questions if you get the pdf.