## Wednesday, 8 September 2010

### Phys. Ed.

Keeping It Current

Li'l Neph was over last weekend, looking for some more physics tutoring. He's moved on to the "Movement" module, which is some fairly basic stuff about speeds and accelerations, forces, masses, weights, friction, and a lotta balls (golf, tennis, football and cricket feature prominently). It's the third time I've been over this module with him, and he seems to have quite a good grip on the material. [Update, Sep 13: pass!]

So far I've resisted the urge to demonstrate principles of statics by pushing him around, or hanging heavy weights off of him. On the other hand, during a previous "Electronics" module, I did indulge in a little bit of practical demonstration fun. In the middle of our treatment of Ohm's Law, I was suddenly grasped by some phantom teacher's inspiration, ran upstairs, and grabbed my multimeter and a spare, tungsten filament light bulb. We used our knowledge of the mains voltage and the bulb's power rating, to make a prediction of the filament's resistance:
R = V2/P = (240V)2/60W = 960Ω.
Of course, when we then measured the bulb's resistance using my (t)rusty multimeter, it turned out instead to be closer to 100Ω, some order of magnitude too low, and I gave my student a puzzled look. In response, a peculiar expression, a surprising combination of embarrassment and sympathy, flashed across his face, as he began searching for some excuse to exonerate his doddery old uncle...

The ensuing conversation was certainly a rewarding enough outcome from our little empirical investigation. We spoke about the variability of resistance with temperature in a hot filament, and the related issue of catastrophic bulb failure due to asynchronously switched cold surge currents. Also about technological obsolescence: how lucky I'd been to even find an incandescent bulb in the house! That led to a useful diversion, concerning how much of his textbook was, for an educational resource, irredeemably out of date. I mean, fax machines? CRT televisions? FM radio? Telephone dials?

But his look of sympathetic embarrassment, that's the one part of the lesson that I won't be forgetting about in a hurry!