A special generator which is probably the best demonstration in any Physics or Science class.

It consists of a large, silvery metal globe atop an insulated cylinder connected to a box with a motor and belt that stirs electrostatic charge.

It works as such: The belt has a number of steel wool tips on the base of the belt, which deposits a negative static electricity charge onto the belt. Like rubbing a comb onto a rubber balloon to get a charge on the rubber. When the belt gets to the top, steel wool tips 'scrape' off the electrons and charge the metal globe.

Now, you have a charged metal globe, what to do with it? If you take a steel rod and bring it close, sparks will jump, like in a tesla coil. Nifty.

But what you really need is this. Take a student/bystander, preferably with long or free hair, and have them stand on something insulated, like a desk, rubber mat, milk crate, or sneakers (may not work as well). With the machine off, have the volunteer stand on the insulation and hold one hand up so they touch the globe (discharge it first).

Turn the machine on. Within seconds the globe will fill with electrons and through the arm of the person. Their hair should stand on end within seconds. Take a picture, they look like the statue of liberty. Better yet, repeat it with 2 people. Shut the machine off, first.

Once you turn the machine off, the person is still charged. Oh, they'll be shocked once you touch them, and their hair should promptly fall.(down, not out)

Now a Van de Graaff is a wonderful thing, but even better, you can make your own. It's easy and allows you to play with tools.

DISCLAIMER: Please don't put your VDG next to a computer, and use common sense. Your VDG can make huge voltages, but the current should be very low. When in doubt, ask someone who knows what he's doing (a category to which I do not belong).

The basic concept is that you have a base and a top with a moving belt between them. The base and the top are made of particular materials chosen for their place in the triboelectric series and shininess. There is some sort of non-conducting belt moving between the base and the top, and there is some contact between the belt and the end pieces. This makes a static charge through a process called something I can't describe very well and of which I'm hazy on the details.

Practically, you'll want to have some sort of non-conducting rod seperating the two pieces. Try PVC. Now down to business. Let's have some ingredients.

  • Note: all materials are listed in descending order of preference.
  • One top sphere, ~9-18"d: Try aluminum or steel. If you could use lead glass or lead that would work wonderfully.
  • One lower terminal, ~9-18"d: a sphere, hemisphere, or whatever made from teflon, silicon (how'd you get a sphere of that?), vinyl, or polypropylene.
  • One belt, you figure out the dimensions: something non-conductive. Surgical tubing is good. You may want to try an old vacuum cleaner belt if it has high enough resistance (most don't).
  • One motor: just something to drive the belt. You will probably run off AC, so get whatever additional supplies you need to make the motor run. Faster motors are better.
  • One upper axle: Try a nylon paint roller.
  • One lower axle: PVC is your friend.
  • Two conductive brushes: Just use a whole bunch of very fine copper wire.
  • One shaft, 2.5+' length: Something non-conductive that will enclose the belt. Don't use wood, as it will become conductive at high voltages.

You now have all the materials. You should attach the top and bottom terminals to the shaft. Attach the axles to their respective terminal, and make sure they have a good connection, both mechanically and electrically. Install the motor such that it drives the axles (of course). The motor should be electrically separate from the generator itself. Attach a brush to each terminal. The brushes should almost touch the belt and should, electrically, be part of their respective terminals.

Turn it on and play with electrons.

OK, I know that every kid who ever played with one of these suckers knows they are the coolest science toys ever invented.

I also think there might be one or two people out there who studied poetry and other girly stuff * who have no idea what the kewl kids are going on about when they talk about the VdG generator.

So, to bridge the gap of understanding here, I've tried to give two answers to every question you might ever have had about these little balls of fun.

Q1: What is a VdG generator?

A1a: it is simple machine capable generating very high voltages at low current by employing the physical principles of electrostatics.

A1b: They are the totally coolest way of making your hair stand on end and making really massive voltages. And you can build one yourself. And they are totally legal!

Q2 What sort of voltage are we talking about here?

A2a: In a simple home-built machine, 10 000 volts is easy to achieve. Larger scale machines on display in museums can easily get into millions of volts.

A2b: I reckon on about 30 000 volts per inch (10 000 volts/centimetre), but that's enough to blast holes in pieces of paper and make sparks two inches long and make a really loud crack!

Q3 Wow, that sounds like a lot of volts. Is it dangerous?

A3a: No, provided you follow the safety guidelines given at the bottom of this node.

A3b: I put my hand on the machine and charged it up to a million volts and I was like Emperor Palpatine, zapping everyone with sparks and force fields. Feel the power of the Force, Skywalker!. Zap!

Q4 Isn't that a bit childish? Sparks, blasting holes in bits of paper and all?

A4a: Yes.

A4b:Are you kidding? You just try it. Then see what it does to your hair when you touch the ball.

Q5 I'm going to regret this, but what does it do to your hair?

A5a: The charge passes from the charged ball up your arm and onto your body. Each individual hair becomes charged to the same voltage and, because like charges repel, each hair tries to move as far away from its neighbours as possible. Your hair starts to stand on end in a very frizzy hairstyle that would make your trichological consultant faint.

A5b: it gives you this totally crazy look like a mad professor or something. You can laugh at all your friends, then get zapped by their mad Force-wielding spark zapping skillz.

Q6 OK, I can see what you like about it, tell me about this sparking business.

A6a: it's nothing to do with the Force. But it is a very effective demonstration of the way an electric field distorts as the surface geometry changes. A spherical surface allows the electric field to be symmetrical and smooth, which in turn means you can build up a large potential difference and a lot of charge without ionising the air. Once you concentrate the field around a pointed geometry (like a finger—or a lightning rod) the electric field strength exceeds the insulating capability of the air, and the electricity will discharge to earth. If the only route to earth is through another person, then the electricity will jump from your fingertips to that person, creating an ionised path between the two bodies. The path is illuminated by a plasma, a bit like lightning, as the charge balances itself out.

A6b:Whoa. I just lay my hand on the dome, charge the thing up and point with my other hand. Zap! If you turn the lights down, it looks just like in the movies. It tingles a bit, but it's not painful. Well, not unless I aim at their earlobes or some other sensitive bit of skin. Heh.

Q7 All these sparks and high voltages, why on earth is it not dangerous?

A7a: Seems a bit odd, doesn't it. But the thing about voltage is it is just a potential difference. It's like being at the top of a hill. Simply being there is not dangerous at all. The danger lies in how quickly you might fall to the bottom. So if your hair is charged to a high voltage, then it just means you climbed a high hill.

The thing about a VdG generator is first that it has a very high internal resistance, and second that it can only store a limited amount of charge. The high resistance means that even at huge voltages, there is very little current (and its usually the current which does the damage). While the lack of charge means that there is not enough electricity to do much damage, anyway. So using the hill analogy, the high internal resistance means there is no way you can fall down the hill. It's like the hill is surrounded by very gentle slopes on all sides. The lack of charge means that the only thing that can get to the top of the hill is a feather, or an ant.

A7b: Aww, c'mon, get a life, just try it!


Oh, err, yeah you guys like history and stuff.

The VdG generator was invented in the Autumn of 1929 by Robert Jemison Van de Graaff, who was working at the time in the Palmer Physics Laboratory at Princeton University. He later became a professor at MIT. The original idea was to use the machine to generate very large voltages used in early research into atomic structures.

The largest such generator—at least the largest one to be insulated by normal atmospheric air—is at MIT, housed in the Thomson Theatre of Electricity, where it is demonstrated a couple of times a day to students and tour groups. The machine originally used two electrodes, each an aluminium sphere, 5 m in diameter. Each of these could be charged to 2.5 million volts, giving a total potential difference of 5 million volts. Later, however, the mechanism was removed from one of them, and the two spheres welded together to form a single electrode. Because of the two inherent safety factors in the design, even this monster, will not kill if you get hit by a spark. This one, however, will hurt, quite a lot. Apparently most of the demonstrators have been hit by a spark at one time or another, and the biggest challenge after receiving such a belt is to avoid saying rude words in front of the audience of schoolchildren.


And here, from http://hypertextbook.com/eworld/vdg.shtml are some safety tips for everyone who wants to use a VdG generator

Operation and Safety

  • Do not allow students to use the Van de Graaff generator unsupervised.
  • People with cardiac pacemakers should never operate the generator or come in contact with it.
  • Obviously, we are dealing with high voltage here. Stay about three feet away from the collector while it is charged. Full intensity, white-hot sparks can jump as far as 15 inches, less intense, red-purple sparks can jump 20-30 inches. While the current is too low to injure you, a surprise spark is no fun. Keep the generator at a safe distance from the outlet where you plan to plug it in. If you're too close, you won't be able to turn it off safely.
  • Always discharge the collector dome between experiments and when you are finished. Use the discharge wand for this. Connect the alligator clip to a gas main or similar grounded object. Hold the discharge wand by the handle. Do not touch the grounding strap when discharging the generator. The voltage is so high that the current can pass through the insulation into your hand.
  • The motor produces a lot of heat that could damage the belt or the motor itself. Do not run the generator continuously for long periods of time. Turn it off when not in use.
  • Leave the upper and lower combs alone. They are not supposed to touch the belt. Do not bend them or mash them.
  • Keep the entire device clean and dry. Dust and moisture degrade the generator's performance.
  • Handle the aluminum parts with care. The collector, housing, and discharge wand are easily dented.

Sources:

  • http://hypertextbook.com/eworld/vdg.shtml
  • http://www.mos.org/sln/toe/toe.html
  • http://tvdg10.phy.bnl.gov/vandegraaff.html

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