Electricity and magnetism
I spent two weekly hours on the subject. First, I wanted to show that
most of the electricity around us comes from 2 different sources: batteries
For the batteries,
I did the standard stuff: any acid plus any two metals make a battery. I
had an orange, a lemon, and some scraps of cloth with vinegar. For
different metals, I had thick copper wire, big galvanized nails (for a
zinc surface), steel nails, aluminum foil, pennies. Then, with some
telephone wire and my old analog voltmeter, we made several batteries: the
standard one where you stick nails and wire of different metals into the
lemon or orange. We noted the different voltages, and hooked them up in
series to get a little higher. Unfortunately, one or two lemons is not
enough to light up a flashlight bulb. We made a 'Volta pile' (reference to
the original pile here) by stacking aluminum foil, vinegar-soaked cloth
For the generator part, I had made a simple generator:
- In a board, I screwed four 3" drywall screws in a 1.25x2"
rectangular pattern. Around this I wound a bunch of telephone wire.
- I had a pretty strong fridge magnet, about 2x1x3/8". I fixed an
axle to it by bending a piece of coathanger wire, and tying it
in place with string. The wire stuck out about 2" on either side.
- Two pieces of cardboard, folded, glued to the wood base, and with
slots in the top to hole the magnet, formed pretty good bearings.
- Slip the magnet into place, wrap masking tape around the axle right
next to the cardboard, so that the axle cannot slip sideways. The
masking tape also make a fatter place for your fingers to grab onto
when you twirl the magnet.
- Of course, the dimensions given here all depended on the particular
magnet you happen to have around. The goal is to have the wire as
close to the twirling magnet as possible, while at the same time
have enough clearance so that the magnet can twirl freely.
Twirling the magnet produces enough voltage to register easily on the
voltmeter (as AC, not DC, but I glossed over that detail), but again not
enough for a flashlight bulb. [Would an LED work?]
The next week I realized that I did have a generator that
would light up a lamp. Being a Dutchman, I had in my box of bike parts an
ordinary 'dynamo', or bike generator. This thing mounts on the front
fork of your bike, and when engaged, presses against the tire.
Out of the same box came a front bike fork, and to complete the
demonstration, I borrowed a wheel from my garden cart, which I had built
using 2 front bicycle wheels. I could mount the whole assembly upside
down with some scraps of wood, and light up a bike light brightly with
even a gentle turn of the wheel.
In the real world, what turns generators?
We talked about dams, and generators powered by oil, gas, coal and nuclear
power. The children were clever enough to add solar and wind power to the
list. They were well aware of the fact that fossil fuels cause plenty of
environmental problems, but not that these fuels were going to run out in
their lifetimes, and that they had a big problem to solve when they grew
up to be engineers.
31 March, 7 April 98, 2 and 9 Feb 2000