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How does a Van de Graaff generator work?

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2010-03-27 13:13:59

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 current

travels 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.


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