Electrostatics Experiments

These experiments require high voltages. Take great care if you attempt to reproduce them and use a power source which has an output limited to less than 100μA.

When polythene is rubbed with wool, some of the electrons in the wool get transferred to the polythene. The polythene therefore becomes negatively charged. It also follows that the wool must become positively charged. If two pieces of polythene are rubbed with wool, they will each become negatively charged and will repel each other when brought together. Like charges repel and opposite charges attract. Conductors can also carry a charge, but because the charge carriers in a conductor are mobile, the charge is free to redistribute.

If a charged conductor is irregularly shaped, the charge density (charge per unit area) is not uniform. Charge density is highest at convex regions of greatest curvature. Field strength is proportional to charge density. It therefore follows that if you want a very high electric field strength then you need a very sharp point and a high voltage. If you want to avoid breakdown, then you need smooth surfaces.

Spinner

This experiment works in a similar way to a radiometer. Two sharp points are mounted on a pivot so that they are free to move. A high voltage is applied, which causes a very high electric field strength at the tips. Air molecules which strike the tips are repelled away with increased kinetic energy. There has to be an equal and opposite force into the tip and that causes it to move. Without the air surrounding it, nothing would happen. I attempted to prove this by operating a spinner inside a bell jar and then pumping the air out. As the pressure dropped to around 0.1mbar the low pressure air broke down and forced the EHT to drop to about 400V. I therefore had to turn the EHT off and use the diffusion pump to get the pressure lower. When the pressure reached about 10^-4mbar the EHT was switched back on and raised to 12kV. The spinner didn't move.

The spinner was made by bending two pins and soldering them to a small washer. The washer was then wrapped in aluminium foil. A third pin, stood upright with the point uppermost was used for the support. The support pin is pushed through one layer of foil and slightly into the second layer. A small amount of Blu-Tack was used to balance the spinner.

12kV was required to run the spinner. This was produced using a high voltage power supply producing about 12kVrms at 15kHz, followed by a rectifier module. The ac power supply is a home made unit which uses a b/w television flyback transformer with a re-wound primary. This unit is very useful because voltage multiplier networks can be added to produce even higher voltages. Instructions on how to build this power supply can be found in the book 'Build Your Own Infrared and Laser Space-Age Projects' by Robert E. Iannini.

Here it can be seen spinning. It can easily be made to go at several thousand rpm.

High Voltage Ping-Pong

This experiment uses the principal of electrostatic force to drive conductive balls up and down a tube.

To make the balls I used black conductive foam. This material is commonly used for storing integrated circuits. The foam was cut into four cubes, each about a centimeter square. The corners were then cut off of the cubes to make them approximately spherical. Solid metal balls will not work because their mass is too high.

The plastic tube used is 115mm long and 45mm in diameter. The longer the tube, the higher the voltage that is required. The conductive end caps on the tube were made from copper mesh. Just about any conductive sheet can be used for the end caps, but it must be possible to attach wires to it. The end caps were attached to the tube with insulating tape.

35kV was required to operate the experiment. This was obtained by using the ac EHT supply followed by a Cockcroft-Walton voltage multiplier network.

Here the balls can be seen shooting up and down. A slight incline on the tube seems to help to prevent the balls getting stuck in the middle.