Saturday, September 15, 2012

Physical phascination

I'm teaching physics labs this fall, and, true to my druthers, am getting as many thought-provoking, memorable, and hands-on experiments in the mix as I possibly can. Bangs are good. Explosions, even better (as long as no-one gets hurt). And anything that whizzes is fair game.

I well-remembered the many memorable physics labs we did in high school (most British students take the material that Americans take in college physics during high school, either at O- or A-level or both).

My high school had the full endowment of traditional science lab equipment -- the Van der Graf generators, the cathode ray tubes, the oscilloscopes and so on. It was all lots of fun, and very hands-on. A sense of danger and derring-do added to the fun and helped us learn, although I'm sure that the instructors were really very careful with safety precautions behind the scenes.

I even remember the chemistry professor rather gleefully waving a sample of uranium over the Geiger counter probe as it clicked away, all the time explaining the harmful effects of gamma wave radiation!

I doubt we could easily get away with radioactive elements in today's American physics lab without someone complaining, but we might certainly try to have some fun, safe experiments.

Last week's was a demonstration of vectors in real life. The first quarter of the text is essentially laying the groundwork for Newton's Laws, and so there's a lot of material on vectors and motion. Wind turbines are a demonstrator of summed vector forces. There's the lift force created by the actual wind hitting the front of the turbine, and passing over each airfoil, but also the lift force created by the blade itself as it speeds through the air.

The efficient pitch, or "angle of attack" of the airfoil is thus many degrees closer to the perpendicular than it would be if the airfoil were not turning.  This can be experimentally derived with a model adjustable pitch turbine.

Unfortunately our model turbines were not quite as well-designed as their creator hoped, and so the blades did develop an unfortunate habit of flying off! Most of the groups coped, however, and got data from which they could solve the problem.

Here's Ryan trying to glue his blades in place. The red marking is to facilitate counting of blades to determine revolutions per minute.

Janet, our lecture instructor, and Emily with a coarse pitch setting. The blades should turn only slowly.

Lily carefully measures the angle of a fine pitch setting. The blades should spin around pretty swiftly at this setting. However, the blade profile is reversed. You can see the convex side of the blade is facing the fan. It should be facing away from the fan.

Here's Oliver using a hand-held anemometer to determine the actual wind speed at the blade hub.

Part of the problem was for students to identify how the same principles of lift and "apparent wind" work in a sailboat and an airplane propeller, as well as in a wind turbine.

Most were able to "vector" on to the answer smoothly enough.

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