Further electrostatic experiments
Practical Activity for 14-16
These experiments could be used at the same time as elementary experiments with charged rods and polystyrene balls. Or, they could be used as revision experiments before a more advanced look at electrostatics.
Apparatus and Materials
- EHT power supply (5000 volts)
- Megohm resistor
- Demonstration meter
- DC dial, 2.5–0–2.5 mA.
- Melinex, metal-coated, 2 strips
- Melinex, plain, un-coated, 2 strips
- Perspex rods, 2
- Retort stands and bosses, 2
- Leads (shrouded), 3
Health & Safety and Technical Notes
Melinex is an aluminium-coated polyester film. It can be obtained from Papersafe, 2 Green Bank, Adderley, Market Drayton, Shropshire, TF9 3TH. Telephone: 01424 871556, Fax: 0870 054 8747.
A fairly large roll, 50 micron thick, costs £8.00 (January 2007).
The strips should be about 12 cm x 2 cm. They must be flexible. 25-guage Melinex is suitable. For steps b and c it must be metal coated on both sides.
For step 4 the strips must be of plain flexible non-conductor (Melinex, or strips of polythene film cut from a sandwich bag).
- Current, driven by power supply: connect the EHT supply to 2 megohms and the demonstration milliameter. Simply point to the current.
- Charges at rest, provided by power supply: hang two strips of metal-coated Melinex from horizontal Perspex rods on stands. Place stands just far enough apart to prevent the strips touching when they attract each other. Carry leads from the EHT's + and - terminals (neither of them are earthed) to the strips, and touch them momentarily.
- Charges of the same kind, provided by power supply: earth one terminal of the EHT supply. Connect the other terminal momentarily to each strip in turn. Move the stands nearer, so that the repulsion is more marked.
- Charges made by 'friction': tape two strips of uncoated flexible plastic to a Perspex rod in a stand. Run dry fingers down the pair of strips, with one finger between them.
- Note: If the strips are already charged, they are easily discharged by waving a lighted match nearby.
- Step 1 above: A power supply can continue to drive a current through an external or an internal resistance. Students might be worried when you say that you are going to connect a delicate milliameter across the terminals of a 5,000-volt supply. This is part of the showmanship of the teacher. The internal resistance of the supply will ensure that the current is small.
- Step 2 above: An EHT power supply pumps charges onto the Melinex. The strips should be far enough apart so that they do not touch when they attract, otherwise a spark will travel up to them and their aluminium surface will evaporate.
- Step 3 above: This will show the strip repelling.
- Step 4 above: When you rub your finger between the two strips electrical forces will pull negative particles off your skin. Both strips will be charged with the same charge and so repel.
This experiment was safety-tested in January 2007