The generator uses a Teflon pulley at the lower end of the
machine, attached to an electric motor. A rubber belt passes over
the pulley. As the pulley turns, the pulley acquires a negative
charge while the inside surface of the belt near the pulley acquires
an equal amount of positive charge. The outside surface of the
belt acquires an equal amount of negative charge by induction.
An electrode in the form of a wire screen (called the lower brush)
drains away these negative charges to the ground. A similar screen
electrode (called the upper brush) at the top of the belt removes
the positive charges and deposits them on the collector dome.
The Teflon pulley retains the negative charges that it acquires.
Positive charges stay on the inside surface of the belt and
travel upwards as the belt moves. At the top, it runs over an
aluminum pulley that conducts the positive charges and retains
them. Free electrons from the metallic pulley flow on to the belt
and are carried down to the the lower plastic pulley. As the belt
keeps running, more charges are deposited on both pulleys. The
positive charges are transferred to the collector dome and the
negative charges are drained into the ground. The belt plays an
important role in transporting negative charges from the upper
comb to the lower comb and positive charges from the lower comb
to the upper comb. There is no transfer of charge from the electrical
lines. This device would work exactly the same if it were powered
by a hand crank.
On the metallic collector dome, the positive charges spread
out due to electrostatic repulsion and become uniformly distributed
due to the dome's spherical shape. The buildup of positive charges
on the dome continues until ionization intensity is reached. At
equilibrium, the potential difference between the collector dome
and the generator housing can reach one-half million volts. The
air between the collector experiences dielectric breakdown and
the generator discharges the accumulated static charge in the
form of a spark. This discharge causes the potential difference
to drop below the ionization threshold but is brought up again
in a matter of seconds. This process continues as long as the
generator is running.
Source: Zafar A Ismail, Van de Graaf Generator: Instructions & Applications
Operation & 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. This is an expensive device and we should try to
keep it operating perfectly for a long time.
Leave the upper and lower combs alone. They are not supposed to touch the
belt. Do not bend them or mash them. They were working fine when last tested
and do not require any adjustment.
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. The collector already has a sizable ding in it from
and accidental drop of only three inches!
Demonstrations
Sparks
When the grounded discharge wand is brought near the collector dome, lightning
discharges will occur, accompanied by a crackling sound. Try varying the
distance between the wand and the collector to see the different types
of sparks the generator can produce. Some are white-hot and quite intense,
others are a red-purple and less intense. Try to see how long you can get
the white-hot sparks to jump. Under the best circumstances, sparks can
leap up to fifteen inches. I believe the rule is three inches for every
100,000 volts.
Current Kills, not Voltage
If you feel adventurous you can try discharging the generator without the
wand. Bring the side of your forearm near the collector. Do not allow the
generator to run long before doing this. If you come up on it pretty fast
it won't have time to accumulate a lot of charge and you can zap yourself
with relatively little pain. You could also try holding a fluorescent tube.
The length of the tube offers some protection from really violent shocks
and the tube will light up. The voltage is still high enough to hurt slightly
but the current is so low it really can't harm you
Hair Raising
Pick a student volunteer with long blonde hair for this demo. Darker hair
is too heavy and short hair isn't very dramatic. Your volunteer should
also have clean, unprocessed, un-moussed hair. Thoroughly discharge the
generator before beginning. Have your volunteer stand on the insulating
footstool in the accessories box and place one hand on the generator. Your
volunteer should not be touching the table or anything else. Your volunteer
should also not be wearing a jacket, hat, or layers of loose clothing.
These can serve a discharge points. Turn the generator on and wait. Your
volunteer's hair will begin to levitate by electrostatic repulsion. Show
you volunteer the results in a mirror, but stay a safe distance away. When
your volunteer looks thoroughly charged have her hold her other hand out
horizontally with spread fingers. Under the right conditions, the charge
will safely leak off her fingers making a crackling noise. When you are
done turn off the generator. Tell your volunteer to remove her hand. Discharge
the generator with the discharge wand. Tell your volunteer to "shake
off" the excess charges before stepping of the footstool. This will
reduce or eliminate the shock.
Lightning Rod
In the accessories kit you will find a coil of heavy gauge wire bent so that
the inside of the coil stands up like an antenna. Place it on top of the
uncharged generator and turn the generator on. Place your hand nearer to
the generator than normal. You will find you can get quite close before
there is a spark and that the spark is mild in comparison to those generated
earlier. This happens as the charge concentrates at the tip of the wire
and discharges continuously, reducing the charge of the collector sphere.
You can also touch the wire with similar results. This illustrates the
operating principle behind the lightning rod. Contrary to popular belief,
lightning rods reduce the likelihood of a lightning strike by discharging
the area around them.
More Repulsion Demos (and One Repulsion-Attraction Demo)
Lay a piece of animal fur on the generator and turn it on. The hairs
will stand up. Bring your hand near the fur. Corona discharge between
your fingers and the fur will discharge parts of the fur. If you do it
right you can make the fur look as if it were being animated by a mad
scientist.
Lay the stack of pie plates on the generator and turn it on. The plates
will rise off one at a time by electrostatic repulsion as if they were
an armada of UFOs.
Drop confetti on the generator and turn it on. The confetti will fly
off.
Tear long narrow strips of newspaper and tape them on to the collector
with scotch tape. Turn on the generator. The strips will try to align
themselves with the electrostatic field. Do the same with the smaller
Van de Graaff generator and put the two generators near each other. You
will get a crude approximation of the field around two positive charges.
Thoroughly remove the tape when you are done to keep the performance
of the generator high.
Bring a lit candle near the collector after the generator is turned
on. The flame will deflect away from the collector. This shows the flow
of ions away from the collector and mimics the solar wind to a certain
extent. If you bring the flame very close, a portion of the flame will
be attracted toward the collector. The ions in the flame are separating
by charge.
Dissectible Leyden Jar
Use extreme caution when performing this demonstration. Do not touch the
Leyden jar when it is charged and discharge it completely when you are
finished. You cannot tell the difference between a charged and uncharged
Leyden jar until it's too late.
Assemble the Leyden jar. Holding the jar by the outer aluminum cup,
touch the terminal of the jar to the generator's collector dome. Turn
the generator on. You may prefer to use the smaller generator. The jar
can also be charged from the static on a TV screen or a charged balloon
Turn the generator off. Place the jar on the table. Stay clear of the
terminal. Use the wooden handled chemical scoop to discharge the jar
by placing it in contact with both the inner and outer aluminum cups.
You will get a vicious sounding spark.
Recharge the Leyden jar as before. Using the wooden handled chemical
scoop, lift the jar by the terminal and remove the middle plastic cup
and the inner aluminum cup together. Let the plastic cup slide off the
inner cup and place all three parts on the table in a row. Show the students
how you can now safely handle all of the parts. Using the scoop reassemble
the jar by sliding the inner cup into the middle cup until they stick.
Quickly lift them and then slide them into the outer cup. Discharge the
fully assembled jar. You will get another vicious sounding spark (the
intensity may be a bit less). It looks like a magic trick. The charge
resides on the plastic cup. Being an insulator, the large quantity of
charge cannot leave easily. When the unit is reassembled, the charges
on the cup can exit violently though the metal surfaces.
Untested Demos
The Van de Graaff generator comes with a newspaper-like set of instructions
and applications. There are a few demonstrations listed there that we were
not able to make work. Maybe you would like to try them. We also have a
Faraday Cage. We do not know how to use it effectively. If you come up
with any useful demonstrations please share them.
