Why is the van de graff generator used?

Answer 1

A Van de Graaff generator is an electrostatic machine which uses a moving belt to accumulate very high electrostatically stable voltages on a hollow metal globe. The potential differences achieved in modern Van de Graaff generators can reach 5 megavolts. Applications for these high voltage generators include driving x-ray tube, accelerating electrons to sterilize food and process materials, and acelerating protons for nuclear physics experiments. The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a capacitor and a very large electrical resistance.

A simple Van de Graaff generator consists of a belt of silk, or a similar flexible dielectric material, running over two pulleys, one of which is surrounded by a hollow metal sphere. Two electrodes in the form of comb-shaped rows of sharp metal points, are positioned respectively near to the bottom of the pulley and inside the sphere. One electrode is connected to the sphere, and a high DC potential (with respect to earth) is applied to the other electrode; a positive potential in this example.

The high voltage ionizes the air at the tip of second electrode, repelling (spraying) positive charges onto the belt, which then carries them up and inside the sphere. This positive charge induces a negative charge to the electrode and a positive charge to the sphere (to which electrode is connected). The high potential difference ionizes the air inside the sphere, and negative charges are repelled from brush and onto the belt, discharging it. As a result of the Faraday caqe effect, positive charge on electrode migrates to the sphere regardless of the sphere's existing voltage. As the belt continues to move, a constant charging currenttravels via the belt, and the sphere continues to accumulate positive charge until the rate that charge is being lost (through leakage and corona discharges) equals the charging current. The larger the sphere and the farther it is from ground, the higher will be its final potential.

The Basis of Charge Generation

When we "rub" stuff we can generate small voltages that promote the movement of charges, specifically electrons. By rubbing the right kind of materials, we can maximize the generation of this static electricity, which is called triboelectric effect. It's a form of what is called "contact electrification" where moving a material against another creates static electricity that causes some charges to move. Have you ever done this? Sure you have. If you've ever pulled some plastic wrap off the roll, you've participated in a contact electrification experiment. The plastic wrap wants to go all kinds of ways and stick to itself. Static electricity in action. And we've all had a "zap" grabbing a doorknob after crossing a rug on a dry day. In the van de Graaff, the little motor drives the belt up and over the top roller and back down to the bottom one in a continuous loop. At the top, a little metal "comb" up there attached to the sphere and held just above the belt deposits electrons on that belt. They're being stripped off the sphere (by triboelectric effect) and being carried down to the base of the apparatus. A positive charge develops on the sphere. As we run the machine longer, more charges shift and the potential on the sphere (the voltage on the sphere with reference to the base, where the electrons are being deposited) increases.

By connecting a small metal ball on a wand that is tied by a wire to the base of the generator, we can collect those electrons that we've been stripping off the big hollow sphere. Now the voltage is building up between the hollow sphere and the ball. Soon the charge differential between the two surfaces rises to the point where the voltage breaks down the air between the two surfaces. (The air can no longer insulate the two spheres.) The atoms and molecules of air between the two surfaces are ionized and they will now conduct electric current. The current follows the ionized path, and the ionized air will emit light. We've actually created lightning between the two surfaces. And thousands or tens of thousands or volts can be created to cause the air to ionize and allow the arc to occur. This shifts the charge balance toward a more neutral state, and the generator will have to run more to "rebuild" the charge.

The more the distance between the surfaces, the higher the voltage necessary to break down the air in the gap between them and cause the arc. Naturally. Some of the big machines create hundreds of thousands of volts with ease, and can actually generate a potential difference of several million volts under the right conditions.

Wikipedia has a nice article on the Van de Graaff generator, complete with pictures. Need a link? Hey, this is WikiAnswers. Of course we got one for ya.

Answer 2

Ever see someone get electrocuted? You can recreate that on a smaller scale using a bundle of nerves located on the inside of you elbow. Hold the inside of your elbow over the Van Der Graaf generator, and as the static electric charge arcs to your elbow, your hand will involuntarily twitch.

WARNING: (1) This hurts... a lot. (2) This will leave welts on the inside of your elbow that will take several days to heal. (3) If you do this too much, you will have strange random twitches for the next several years, but they will eventually go away.

Cheers! -Mike

Answer 3

the generation of heavy ion beams of several tens of megaelectronvolts, sufficient to study light ion direct nuclear reactions

Answer 4

It's used for an example in science classes for static electricity.

Answer 5

Primarily in science classes to demonstrate static electricity.