What I particularly like about the Van de Graaff (or VDG) is that it’s a combination of a few discrete scientific principles and some mechanically produced current, making it an interesting study. For example, did you know that its voltage is limited mostly by the diameter and curvature of the dome? That’s why a handheld one is harmless but you want to avoid getting zapped by one with a 15″ diameter dome. What follows is a journey through the workings of this interesting high voltage generator.
The Big Picture
The big picture in terms of how a Van de Graaff generator works is that the whole thing acts like an electromechanical pump for electric charge. The outer surface of the belt is charged at the bottom, one side of the belt moves upward, carrying the charge with it, where it’s removed at the top and deposited on the outer surface of the dome.
Before looking at specific areas, here’s a quick look at it as a whole. There’s a roller at the bottom and another roller at the top, both held in place by a cylinder made of a non-electrically conductive material. A belt is wrapped around those rollers with a motor to rotate the bottom roller, moving the belt and in turn rotating the top roller too. The dome encloses the entire mechanism at the top, in this case by sitting on a ring. In the image above, the dome has been removed and is shown upside down so you can see the opening in its bottom.
Very close to the belt at the bottom roller, but not touching it, is a metal brush with sharp points facing the belt. That brush is usually connected to ground, though it could instead go to a second dome as you can see in the handheld Van de Graaff in the image above. Similarly, near the top roller is another brush with sharp points which are close to the belt but not touching. That brush is connected to the dome through contact with a metal plate that sticks out a bit and runs down to the ring that the dome sits on.
Now let’s go through the different areas and electric effects, starting at the bottom.
The Bottom Roller And The Belt
One key thing about the bottom is that as the roller rotates, the outer surface of the roller continuously makes and breaks contact with the inner surface of the belt. Also important are the materials those surfaces are made of. Together this causes the triboelectric effect to take place.
The triboelectric effect is the same effect that gives you a charge when you rub your socks against a carpet, provided the socks and carpet are the right mix of materials. At a molecular level, that rubbing means that contact between molecules is repeatedly being made and broken. When contact is made, electrons are moved from one to the other and when contact is broken, the electrons remain on that side. That leaves one material positively charged and the other negatively charged.
The triboelectric series is a list of materials with the things that become negatively charged on one end of the list and the those that become positively charged on the other end of the list. The further from the middle of the list, the more charged the material will become. In this Van de Graaff generator, the bottom roller’s outer surface is teflon, which is near the negative end. The belt is rubber, which is in the direction of the positive end relative to teflon.
And so with the constant making and breaking of contact at the bottom of the Van de Graaff, the surface of the roller becomes negatively charged while the inner surface of the belt becomes positively charged, but less so since it has a much larger surface area than the roller.
The Belt And The Bottom Brush
But that’s not good enough. We need to charge the outer surface of the belt. That’s where the bottom brush comes in. The negative charge of the roller repels electrons from the brush’s sharp points, making them positive. Whenever you have a charged object with sharp points, the charge is more tightly packed at the points. The end result is a strong electric field near the points.
That strong electric field ionizes the air by ripping electrons from atoms in the air resulting in a bluish corona between the belt and the points. That corona is an area of electrically conductive air, filled with ions and free electrons. Electrons are repelled from the outer surface of the belt and attracted to the brush where they are further repelled to ground. That leaves the outer surface of the belt positively charged.
And since the belt is moving, that positive charge is carried up to the top roller.
The Top Roller And Belt
The top roller is either made of metal or it’s made of a material that’s on the opposite end of the triboelectric series from that of the bottom roller. In this particular Van de Graaff, the top roller is metal. In either case, the end result is the same.
Since both the outer and inner surfaces of the belt are positively charged, electrons come from the metal roller to try to neutralize them, leaving the roller with a built up positive charge and the inner surface of the belt on the way back down with a neutral charge.
The same behavior between the belt and brush at the bottom happens at the top, just with opposite charges. A corona forms in the gap between the surface of the belt and the sharp points of the brush and electrons are pulled from the brush to the surface of the belt, neutralizing the outer surface of the belt. Where do those electrons come from?
Charging The Dome
Those electrons come from the outer surface of the dome. Notice that the top brush is connected to a metal plate that extends down the side of the cylinder where the dome makes electrical contact with it.
The dome acts like a Faraday cage and what happens is the same as happens in Faraday’s ice pail experiment. That experiment demonstrated that any charge deposited on the inside of the dome, or ice pail in Faraday’s case, will end up on the outside of the dome. The inside of the dome will remain neutral. Key here is that since the inside is always neutral, you can keep depositing more and more charge to the inside and there will be no build up of like charge to repel it. Any deposited charge immediately moves to the outside.
And that’s why the voltage limit for a Van de Graaff is determined by the diameter and curvature of the dome. You can keep pumping charge to the dome and it will keep taking it. And just as with the brushes, if the dome had sharp points then it would easily form strong electric fields with the surroundings and form a leaky corona. So the bigger and rounder the dome, the weaker the surrounding electric field will be. It’ll be harder for corona to form and for the surrounding air to become conductive.
There are still limits of course. For example, the electric field between the dome and the grounded parts at the bottom of the Van de Graaff could become strong enough for corona to form, followed by a spark, briefly neutralizing the dome.
But not all Van de Graaff generators come in this shape. We’ve seen one that uses a soda can for the dome and is held in the hand like a wand and another that uses a Christmas tree ornament for the dome. Shocking!