Typically I'm very much against the inductive-learning-good, deductive-learning-bad philosophy that has seemed to invade modern scientific theory. It takes too much time if done every lesson, it does little to correct pervasive alternative conceptions and it ignores half of how science works.
But a mixture of inductive and deductive learning works for Van Der Graaf machines. At the bottom of this post are some investigations that can be done. With good timing and a dollop (5 minutes) of deductive-style learning, you can complete most of them in a single one hour lesson.
Parts of a Van Der Graaf Machine
1) hollow metal sphere
2) upper electrode
3) upper roller (for example an acrylic glass)
4) side of the belt with positive charges
5) opposite side of the belt with negative charges
6) lower roller (metal)
7) lower electrode (ground)
8) spherical device with negative charges, used to discharge the main sphere
9) spark produced by the difference of potentials
Preparation
Firstly, waiting for perfect conditions makes a Van Der Graaf presentation a hell of a lot easier and a lot more impressive.
You want cloudless weather that is very dry. Ideally straight after a cool night, and warming up during the day. Hot, dry, windy weather works as well. Air humidity above 30% will pretty much shut down really large charge build-up. If you get shocked three or four times walking across a carpet, cancel your other lesson plans and break out the Van Der Graaf machine. It is actively better to leave the Van Der Graaf out of your teaching plan during wet seasons, and show it months later during ideal conditions.
Secondly, a Van Der Graaf must be well maintained in order to work well.
At my last school (Cairns State High School), the laboratory technician used to store the rubber belt for the Van Der Graaf in the refrigerator to prevent them stretching, melting and perishing. A fresh belt makes a big difference. A belt that has been sitting on the machine since last year probably won't work very well.
The dome should be washed with warm soapy water to remove mold, dust, greasy fingerprints and grime. Either leave the dome to air dry, or if you're in a hurry, hand dry with low-lint paper towels. Ideally you should take it over to the manual arts department and give it a once-over with a belt polisher. The presence of fine particles on the surface will cause the electrostatic charge to escape very quickly. Which is why it's ironic that most school Van Der Graaf machines are stored uncovered, in hot, humid and dusty back rooms and only taken out once a year and expected to work immediately. Giving the discharge sphere a once over is also a good idea.
Finally, the upper electrode on the top of the Van Der Graaf must be adjusted. This is best done once the machine has been on for a few minutes. The electrode should either just touch the belt, or stand off the belt by 1/2 a millimeter. Too strong a contact causes the belt to slow down, and doesn't increase charge transfer at all. Once you've made this adjustment, make sure that the main sphere is properly reinstalled. There is usually a pin on the inside of the sphere that slots into the top of the upper electrode.
Safety
The Van Der Graaf machines used in school environments are highly unlikely to be lethal. They generate very high voltages but the current is minuscule. You'd need to hook up a Leyden Jar to get lethal currents (Hint - don't, at least not without proper one-on-one training).Having said that, when you and your students step up to the Van Der Graaf machine, make sure that everyone involved removes electronic devices and cards with magnetic strips. I have had a debit card and a wrist watch wiped out by having it near a Van Der Graaf machine. (That year we had excellent Van Der Graaf weather).
Don't forget to check with students about the presence of expensive and sensitive devices such as hearing aids. These cost about $1000+ to replace, and students with hearing difficulties usually have families that are under financial pressure to begin with. Don't exclude such students from participating in the demonstration, just make sure you go through all the instructions with them at the start, and that they hand the aid to someone not participating in the demonstration. Thankfully I have yet to wipe out a hearing aid in the eight years I've been doing this demonstration, but I have had a student forget they were wearing one until I prompted them.
Anyone with a pace-maker or heart disorder should probably give this a miss.
Beyond this, there are some simple rules to follow, to avoid zapping the students and yourself ... well, at least until you want to. (Cue evil laughter):
- Turn off the machine between demonstrations, or at the least follow rule 3. (It can save you time if you leave the machine going during demonstration change over, but it's best to switch off the first time you handle one).
- Pick up the discharge sphere by the middle of the insulation. I've had several students who ignore this instruction, and get a jolt when their hand strays too close to the metal contacts at the far end of the discharge sphere. I've done it myself a couple of times as well.
- Make sure to touch the discharge sphere to the main sphere and hold it there before (and while) touching or handling the main sphere between demonstrations and at the end of the lesson. Even if the machine is off! In good Van Der Graaf weather, the sphere can hold a zapping charge for 20-30 minutes. Note that you should give the little rivet at the top of the machine a once over as well - if the rivet has come lose, the electrical connection can be poor. (Yup, the voice of personal experience again.)
- Avoid touching any metal (desk handles, metal bench, water taps) while near a charged machine. Even if you're not touching the main sphere, you can still pick up a bit of an air charge. This dissipates quickly once you move away from the Van Der Graaf, but can zap you if you're close enough to the machine.
- Anything touching the sphere has the same charge as the sphere. Treat it with the same respect as you do the main sphere. Having said that:
- Discharging the sphere will discharge any objects attached to it.
- Even when someone is charged up, they won't get zapped if there isn't a way for the charge to move quickly to ground. Hence when charging a student (Einstein hair demonstration), make sure to discharge the sphere before the student touches the sphere, make sure the student doesn't touch any metal or other people (see rule 4) while charged, and make sure the student doesn't remove their hands until discharged through the sphere again. Reinforce this constantly with the student, because guaranteed, one will forget.
Demonstrations
Make sure that students have been exposed to (or done investigations on) the following. It is possible to teach these at the very start of the demonstration series, but this takes up valuable time and can switch students off. Best do it a lesson (or lessons) beforehand.
- Electrostatic charges can be generated by friction of dissimilar materials.
- A review of atomic and subatomic theory, in particular that electrons can be removed or added to atoms easily, that electrons are negatively charged, and the atom nuclei are positively charged.
- Like charges repel, unlike charges attract.
The following links point to the different types of demonstrations you can do with a Van Der Graaf machine, and their explanations.
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