The Van de Graaff generator
Topic
Electricity - demonstration of the effects of static charge on a human body
Concept Note
A Van de Graaff generator is an electro-static generator which uses a moving belt to
accumulate very high voltages on a hollow metal globe on the top of the stand. It was
invented by American physicist Robert J. Van de Graaff in 1929.
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, so it can produce a visible
electrical discharge to a nearby grounding surface which can potentially cause a "spark"
depending on the voltage.
How to make it
-what is needed
For the construction of Van de Graaf generator you require the following things:
Aluminium/Steel Bowls (2 Nos.) for making domes.
Belt- Nylon or surgical tabing
Acrylic or Glass tube
Upper Pulley –preferably made of Nylon
Lower Pulley – made of Nylon covered with Silicon tape
Electric Motor
Insulated Copper wire (2 pieces) for upper and lower brush attached
Regulator Switch
Regulated power supply for motor
-how it is put together
1.Take two Aluminium/Steel bowls, cut a circular hole (the size of glass /acrylic tube) in
the bottom bowl for the pulley and belt to pass through.
2. Fix the lower bowl to the top end of the glass/acrylic tube.
3.Mount the upper nylon pulley inside the bottom bowl exactly in the centre. The pulley
should face the lower pulley mounted in the casing at the lower end of the tube. Both the
lower and the upper pulley should be aligned parallel to each other.
4. Make a metal housing for the motor, pwer supply and lower pulley. In the centre of the
housing cut a hole exactly the size of acrylic tube. Fix the bottom end of the tube to the
housing. Mount the motor driven lower pulley parallel to the upper pulley.
5. Wrap the belt around the upper and the lower pulley.
6. Cut the insulated copper wire in two parts with one end of insulation peeled to make
brush with wire strands. Other end of the wire to be peeled for aaching it with the upper
bowl and meal casing.
7. Fasten one end of the wire to the top bowl and rest the other end (Brush side) with the
nlon belt on top roller. Similary rest the brush end of the second wire with the lower roller
and fasten the other end to metal casing.
8. Connect the motor through speed regulator with regulated power supply.
9. Connect the power supply terminals to the mains. Switch the power on . The motor will
rotate the belt resting on both the rollers
Collateral for Volunteers
-what does the experiment do
1.The VdG generator is a charge separator that seperates electric charges mechanically.
2.When turned on, the belt begins to move, picking up the negative charge deposited by
the metal comb at the base of the experiment.
3.The belt then transfers the charges up to the metal comb at the bottom of the generator.
4.The metal comb is attached to the metal sphere and thus transfers the negative charges
to the sphere.
- why
1. Lower roller rubs against belt and becomes negative. Belt becomes posiitvely charged.
2. Negative roller pushes electrons on the brush away to the housing. Tips of brush
becomes posititvely charged.
3. Positively chaged brush tips strip electron from the air.
4. positively charged air ions then get attracted to the negatively charged roller but belt is
in the way.
5. Belt intercepts positive air ions and becomes more positive.
6. Belt carries this positive charges to the metal sphere.
-safety advice
1.Do not allow students to use the Van de Graaff generator unsupervised.
2. Children with cardiac pacemakers should never operate the generator or come in
contact with it.
3. Obviously, we are dealing with high voltage here. Stay about three feet away from the
collector while it is charged.
4. Always discharge the collector dome between experiments and when you are finished.
5. 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.
6. Leave the upper and lower combs alone.
7. Keep the entire device clean and dry.
- concept
The Van de Graaff produces a high voltage with a low current. Voltage is commonly used as
a short name for electrical potential difference. Which means it is a measurement of the
energy contained within an electric field, or an electric circuit, at a given point. This Van de
Graaff generator can produce anywhere from 10-200 thousand volts (V). It is safe because
the current is quite low.
- help
If Van de Graaff generator does not seem to be charging properly, make sure that it is
clean. Avoid oils or debris. You can also use a hair dryer on it to remove any moisture.One
should go through this ritual every time they want to use Van de Graaff generator. You will
be amazed at the difference it can make. You may want to turn off all of the lights and run
the Van de Graaff generator in the dark. You will see bluish-purple sparks shooting out
where ever you have leakage. Try to eliminate the leakage with tape, epoxy or silicon. It
may even take combinations of the three, but it will be worth your while to do so.
What should it look like?