Static Electricity Demonstration
This demo deals with static electricity, so some of the material may be too advanced for some of the younger kids. Use your judgement to see what material in this description needs to be simplified even more, or just skipped. In general, don't expect kids to know as many of the answers to our questions as with some of the other demos.
We want to start by talking about electric charges. You can ask your audience whether they know what electricty is made of. Don't expect to get many right answers here, so after some guesses tell them it's made of something called electrons and ask if they have heard of them. Explain that electrons and protons are both part of an atom and ask if they know what some of the differences between the two are. Explain: electrons have what we call a negative charge and protons have a positive charge. Does anyone know what happens if I bring two electrons together? Depending on your audience, they might acually know the answer to this one. Otherwise, just explain: when two of the same charges are near, they try to go away from each other, but when two opposite charges are near, they attract each other. You also need to make sure to point out that all electrons have the same charge -negative- so they will try to go away from each other.
Next, we want to explain what the van der Graaf generator is and what it does. You can explain that when you turn it on, the belt will move and will rub against a piece of carpet that we have put at the bottom. Ask the audience: have you ever rubbed your feet on carpet for a while and then touched a door knob? What happened? After their answers you can explain that rubbing your feet on the ground makes you pick up charges from the carpet, and that the van der Graaf will do the same thing. Then, following the belt with your fingers, explain that those charges will travel to the big metal dome we have at the top. Then ask: since all of these charges are the same, will they want to stay toegther or go far from each other? Explain that they will go to every inch of the dome, so they can be as far apart as possible, and that whenever they get a chance to go even farther, they will take it. To show them this, present them the wand, and show that it it connected to the base of the apparatus. Explain: this base is connected to the wire, which is in turn connected to the wall. Have you ever noticed how some plugs have a third, big, fat leg? Feel free to unplug the generator and show the plug, so the kids know what you are talking about. Explain that that leg is connected to the ground, so that if the charges follow the wire, they'll be able to go all the way to the ground, so they can be much farther apart. To show that this is indeed teh case, you will turn on the generator and bring the wand close to the dome. The effect is much more impressive if the lights are off, but this is only possible when we are on stage. If you're doing this during a science night, be sure to ask for some room before turning the van der Graaf on, so that no one gets shocked. If you let it charge up a bit before bringing the wand near, the sparks will be much more impressive and visible. After showing the sparks, explain once again that they are caused by electrons wanting to go to the ground, and then ask if they've seen something similar happen in nature. If you don't get any right guesses just say: this is actually the same thing as lightning! We are making miniature lightning bolts! In a storm, the clouds get charged up, so all those charges want to go to the ground. When they do, we see a big spark, just like this one here. You can then repeat the experiment with the other machine, when we bring it along. The principle is the same.
The rest of the experiments are all about the same principle, which we can see in different ways. The next one will be using the wig. Notice that atmospheric conditions can make this one be a lot less impressive at times. Put on the wog on the dome of the generator, and ask the audience what they think will happen if we allow the charges to go onto the hairs of the wig. Wait for some answers and then say: in science, when we are not sure of something, we test it, so let's see what happens! After you turn on the generators, the hairs of the wig stand up. While it's on, bring the wand to the wig, and point out how every time they touch the wig, they go back down, only to go up again once you take the wand away. Turn it off and explain: since all the charges are of the same type, they try to go away from each other. When they go on the hairs, they pull so much away from each other that the hairs end up standing up. But when I bring the wand to them, all the charges travel through the wand all the way to the ground, so the hairs can go back down. When we are on a wooden floor (like on a stage) or some other type of insulating material, you can try this experiment with a person instead of the wig. Make sure to make it very clear that they need to be touching it before you start it, or they will get shocked. They need to leave their hands on it until after you've turned it off and discharged it. If you're doing a science night, you can point out how people's arm hairs stand up when they get close enough.
Next, you can use the pie tins. Stack 3 or 4 of them on top of the dome, upside down and ask the audience what they think will happen. You can even prod them a little by saying: remember that all these charges, which are all negative, will go onto the pie tins. Regardless of answers, try it out. Sometimes the tins will get stuck because they are stacked in an awkward way. Just stop the generator, move them a bit and try again. If it's still not working, you got a bad day! After showing how they fly off one by one, explain: because they are now all full of like charges, they want to go away from each other. But they are too heavy to all go at the same time, so instead they start separating one by one. You can now repeat the same experiment, but using the styrofoam packing peanuts instead. Make sure to use the tin with some tape on the bottom, so it stays on the dome. As always, ask what they think will happen before you do the experiment. By now, some of them might give you the right answer! After you're done, explain it once again: all these styrofoam bits get charged up with the same charge, so they want to be far away from each other. That makes them fly off the plate!.
Safety notes: Be sure to always discharge the Van de Graaff generator immediately after each part of the demonstration, because it's impossible to tell whether it is charged or not just by looking at it. You wouldn't want yourself or one of your fellow presenters to get accidentally shocked. A shock from the VdG is more of a surprise than anything else, and only sometimes hurts, but who wants that, anyway! If you wear rubber-soled shoes (the thicker the better) when using the VdG, the shock will hurt less. One quirk of the VdG is that the power switch will often give you a shock when you're trying to turn it off. To prevent this, hold the grounding rod to the sphere while you are turning off the generator, or just turn it all the way down using the (plastic) knob, and only turn it off after discharging it.
You can end this demo by mentioning that all these experiments use static electricty. That means that they charges are not moving too much. When they do move, they create a current, and the magnetism demo (which normally comes right after this one) will talk all about that.