Monday, December 14, 2009

Energy Labs: Mechanical and Heat Energy Lab

The Einstein's Big Idea teaching guide activities included a lab to demonstrate the conversion of mechanical energy to heat energy. It involved stirring cups of glycerin to raise the temperature a "few tenths of a degree." Since I doubted (a) that we had a thermometer which could measure such a small change accurately and (b) that anyone would be impressed by this, I went looking for a different lab we could do to demonstrate this phenomena. We did two labs, quoted below, from Arbor Scientific's CoolStuff newsletter. Both were simple, dramatic, and worked as described. They were part of a larger lesson on thermodynamics. I hope to do more of the lab another time!

Heating up a Hanger

The conversion of mechanical energy into heat may be dramatically demonstrated by simply bending a coat hanger. First cut a 30-cm length of coat hanger with wire cutters. Grab the ends of the wire in each hand and rapidly bend it back and forth several times. Now touch the point on the wire where the bending occurred. (Caution! The coat hanger can sometimes get surprisingly hot, so only touch the hot spot briefly.)

Stretching Exercise

Place a rubber band loosely looped over the index fingers in contact with skin just above your upper lip. Now quickly stretch the rubber band. What do you experience? Now let the rubber band relax quickly. What do you feel now?

When the rubber band is stretched quickly, work is done on it, causing its internal energy to rise. This rise reveals itself as a small increase in temperature. When the rubber band is allowed to quickly contract, it performs work and suffers a reduction in internal energy which produces a cooling sensation.