Van de Graaff (Static Electricity) Demo

[this introduction is more appropriate for older students, ~5^th grade and up] Who can tell me what atoms are made of? (electrons, protons, neutrons) And what are the charges on those particles? (negative, positive, neutral, respectively). What happens if I try to bring two electrons near each other? (they repel because they have the same charge). And what happens if I bring an electron and a proton near each other? (they attract). Now, who can tell me what electricity is made of? (moving electrons). In this demonstration, we’re going to investigate static electricity. Static electricity refers to electrons standing still, instead of electrons moving through wires.

Has anyone seen one of these machines before? It’s called a Van de Graaff generator. It makes static electricity. Have you ever rubbed your shoes on the carpet (show action) to build up a static charge, and then get a shock when you touch something else? That’s exactly what this machine is doing. It has a big rubber belt that’s rubbing against a piece of carpet on a roller at the bottom. So it’s building up a lot of static electricity, and it’s putting it up on this metal ball on top (charge up generator).* So now we have a lot of static electricity to play with.

[for the younger students] The particles that make up electricity are called electrons. They always have a negative charge, and when you bring two negative charges near each other, they want to get away from each other. If I had a negative charge and a positive charge, they would want to move towards each other. This is a very important law of physics to remember: like charges repel, and opposite charges attract.

How do we know that the electrons on the sphere want to repel? I can’t see anything happening now. I’ve told you they repel, but that’s not how science works. We don’t just make stuff up and tell each other that’s how it is. We have to show that it works. I can think of an experiment we can do to find out. I have this fake hair that we can put on the Van de Graaff generator. What do you think will happen? Why? (get answers). Let’s find out what happens. (put the wig on the generator, and turn it on). So the hair stuck up. It was like the hairs were trying to get away from each other. When we turned the generator on, static electricity built up on the sphere, and some of it got stuck on the hairs. Then the hairs were all negatively charged, and because they were negatively charged, they wanted to repel, so they stuck out to get as far away from each other as they could. So we just saw that electrons like to repel from each other.

[the packing peanut and pie plate experiments show the same thing as the wig, so they can be skipped if time is short] I have another experiment to try. I have this plate full of Styrofoam packing peanuts. Now using what we just learned in the last experiment with the hair, we can figure out what is going to happen to the packing peanuts. If the electrons get onto the packing peanuts and repel each other, what is going to happen? (they’ll jump off the plate). Let’s try it to find out. Sometimes it’s a good idea to try more experiments to make sure we know what we’re doing. (put the plate on the generator, and turn it on). Cool! We just used the science we learned in the first experiment to predict what was going to happen in this experiment. (You can do the same experiment with a stack of 3-4 aluminum pie plates. The plates float off of the sphere one by one when you turn the generator on).

By now you might be wondering about this wand I’ve been using. You can see that it’s connected to a wire, which is plugged into the power outlet. In power outlets, electricity comes in one wire and goes out another wire, and usually there is a third wire that is connected to the ground. It is actually connected to a big metal bar buried in the ground. Remember that the electrons want to repel from each other. Well, if they go through this grounding wire and get into the ground, which is a pretty good conductor, they can spread out really far. They can go to the other side of the world if they want. Since the electrons like to stay away from each other, they will go into the ground any chance they get. So this wand I’m using here is just a path for the electrons to get to the ground. So every time I touch the wand to the sphere, I’m letting all the charges go to the ground so it’s safe for me to touch the sphere.

Now I have one last thing to show you with the wand. I’m going to hold it close to, but not touching the sphere on the Van de Graaff generator. (turn off the lights for this experiment if you can; it looks cooler that way. Do the experiment. Move the wand to different sides of the sphere so everyone can see the mini lightning bolts). The electrons on the sphere wanted to get to the ground, but they weren’t connected to the wand. They actually jumped through the air to get to the wand and into the ground. Did that remind you of anything you would see in nature? (lightning) In fact, we did just make lightning. Lightning happens when there are a lot of charges up in the sky. They make the jump all the way to the ground so they can spread out, just like the charges on the sphere jumped to the wand.

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. Be sure that you are wearing rubber-soled shoes (the thicker the better) when using the VdG, or else the shock may hurt more than a little. 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.

*Technical notes: our Van de Graaff generator may actually collect positive charges instead of negative charges, but we haven’t bothered to figure it out. Since it does not matter what the charge is for any of the experiments we do, it is easier to simply ignore this situation. Mentioning it at this level will only confuse students. The same is true with lightning, in which charges often jump from the ground to the sky.

There’s a pinwheel that we can put on top of the sphere (use silly putty to hold it in place). When we turn the generator on, the pinwheel starts spinning away from the points. The pinwheel experiment is problematic because nobody knows why it works. It’s best to skip it until we figure it out.