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?


  1. Thank you for this post. I just finished creating lessons on thermal energy without addressing it in this manner. I think your description makes it more real to students and gives reason to why thermodynamics is taught at a high school level. You have inspired me to go rework the lessons to make the connections in the order you do to help make thermal energy more real to students.

  2. Thanks for the comment, Deana. I think if you ask your students what they think of as cold to the touch, everything would be a decent heat conductor. Even an ice cube is only cold once you start to liquify a little layer. At first it's not that bad.

  3. I love it! I've spent a lot of the day thinking about how cold it is (-21 with windchill this morning), especially since my apartment is so poorly insulated that I can feel a draft from the window halfway across the room. If I were a high school student, today would be a great day for a lesson on thermodynamics.

  4. I've been thinking about wind chill lately. In my applied math lab I'm going to have them predict the lake ice thickness in the region (Mathematica can get the weather database with some simple commands) and wind chill is an issue for that as well. We're back in positive territory here in MN for actual temps, thank goodness.