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You probably have seen Van de Graaf generators in high school,
where long sparks are shown to jump between the two globes,
one of which is grounded. Spark jumping is best seen in the
dark where you can observe the fascinatingly intricate patterns
the spark makes as it jumps the gap of almost six inches.
The main concept is high voltage and low current. In spite
of voltages as high as 300,000 volts, the current is only
about 0.1 microamp (0.0000001 Ampere) , not much more than
what you receive on doorknobs on winter days. That's why it
may hurt a bit if a spark strikes your hand, but you won't
die. Contrast that to your car battery which is low voltage
but capable of delivering high current of well over 250 Amperes.
The principle behind most Van de Graaf generators is generation
of charge on the belt by triboelectricity --- dissimilar materials
can strip electrons from one material by coming into contact
and "peeling" apart. Rubbing just creates more surface
area. This is very similar to rubbing a balloon on your hair
and sticking it to a wall. This is NOT due to friction, since
if it were similar materials would generate charge. A second
principle is that the charge for these big sparks is coming
from ionized air --- a Van de Graaf will not operate in vacuum!
Also, remember that the electricity does NOT come from the
motor or from the wall outlet. Some Van de Graaf generators
can be hand cranked.
In a typical generator, the rubbing element is a plastic
or Teflon pulley in the base of the generator. As the belt
passes over the pulley, it causes the pulley to become strongly
positively charged, much like rubbing a balloon on your hair.
A needle-like device held near the belt becomes negatively
charged in response, the strong electric field ionizing the
air and causing negative ions to stick to the rubber belt
as they try to make it to the positive pulley. The belt transfers
the negative charge to the top, where inside the globe the
belt passes over a neutral (usually metal) pulley. Another
needle-like device sits close to the top of the belt and due
to the negative ions on the belt, becomes positively charged.
This sets up another intense electric field which ionizes
the air between the belt and the needles, causing this time
negative ions to flow into the needles. The needles are connected
to the globe, transferring the negative charge to the outside
surface of the globe.
In some generators, the bottom rubbing pulley is felt, becoming
positively charged, with the whole process being opposite
in sign --- the globe becoming positively charged. Whatever
the materials for rubbing, if they are dissimilar and far
enough apart in the "triboelectric series", charge
will be separated, creating the fields, ionizing the air and
transferring charge to the belt.
Click here for more Van
de Graaf demonstrations.
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