The Van de Graaff generator is a common sight in the science lab. Van de Graaff generators operate using the following principles:
When different materials rub together, one loses electrons to the other. The material that gained electrons will have a negative charge and the material that lost electrons will have a positive charge. This is called the triboelectric effect.
Negative charges repel each other, but are attracted to positive charges. Similarly, positive charges repel each other, but are attracted to negative charges. When charges are repelled, it is called electrostatic repulsion.
When positive and negative charges are separated from one another, a potential difference is created. With a potential difference, electrostatic discharge can occur as electrons jump from the negatively charged object to the positively charged object.
Positive generators (like the Van de Graaff) create a positive charge by pulling electrons from the top of the machine toward the bottom. The positive charge surrounds the dome. Check out the fun demonstrations you can do with a Van de Graaff generator, all with this kit, perfect for every science lab!
Materials: Van de Graaff generator and discharge wand, pith balls, tape
Steps:
Plug the discharge wand into the base of the generator.
Tape the ends of the pith ball strings onto the dome of the Van de Graaff generator. You can put all the strings into a bundle or tape them around the dome at intervals.
Start the generator and observe how the pith balls behave. Bring the discharge wand close to the dome while the generator is still running. Turn off the generator and observe the behavior of the balls.
Discharge the generator and observe the behavior of the balls. Observe how the pith balls react to discharge.
Materials: Van de Graaff generator and discharge wand, spiked arm wheel
Steps:
Place the pointed plug into the top of the dome and place the spiked arm wheel on top of the point.
Move the discharge wand as far away from the charging dome as possible. Turn the Van de Graaff on for about 10 seconds and observe what happens.
Does the wheel only rotate one way? Move the wheel in a circle clockwise and move the discharge arm so it is touching the collecting dome.
Turn the Van de Graaff on and then observe the wheel.
Now move the discharge arm away from the collecting dome so that charge begins to collect on the dome.
Observe the motion of the wheel. What happens?
This demonstrates how charge will collect on the outside of a charged surface and accumulate at a point. The points of the wheel will have a greater charge density and therefore more charge will emanate into the air from the point. Since charge has mass and that mass is leaving from the end of the wheel’s points, the wheel is propelled forward.
This fits with Newton's third law: for every action, there is an equal and opposite reaction. The charge leaves from the tips of the stars moving tangent and clockwise and the star therefore is pushed counterclockwise around in a circle.
Materials: Van de Graaff Generator; dancing ball accessories in a 6" long and 2" diameter glass tube; pith balls, vermiculite, or aluminum foil
Steps:
Place the pith balls, vermiculite, or aluminum foil into the cylinder. Attach the cylinder to the top of the generator.
Switch on the generator and observe the contents in the cylinder for 30 seconds.
Avoiding the dome, bring a finger near to the metal top cap. Observe the change in behavior of the cylinder’s contents. What do you feel?
Move the finger further away then bring it back. How does the behavior of the contents of the cylinder change?
Because you are in contact with the ground (earth), you can supply electrons through your body, to the top cap of the tube. Why do the balls go down the tube, when your finger is near the top cap?
Materials: Van de Graaff generator and discharge wand, neon bulb pointer
This demonstration is best done in a darkened room.
Steps:
Start the Van de Graaff generator.
Bring the neon bulb toward the dome of the generator.
Adjust the distance between the wand and dome, and observe how it affects both the interval between discharges and the brightness of their glow.
Turn off and discharge the generator.
How does it work? As you bring the wand near the generator, the electric field is strong enough to flow through the tube, exciting the neon atoms. As the atoms relax, they give off light, causing the wand to glow with neon’s characteristic red-orange color.
Materials: Van de Graaff generator, a wooden chair, a wooden stick (e.g., meter stick)
Steps:
Set the Van de Graaff generator on a table and place the wooden chair a few feet from it.
Have someone sit or stand on the chair with their feet off the ground and the generator off (people with medium to long, fine hair make the best volunteers!).
Have the volunteer place their hand on the bulb of the Van de Graaff.
Turn on the generator. What happens?
After the demonstration has concluded, switch the generator off, then touch the volunteer with the wooden stick to draw off the remaining electric charge.
