What it shows:
The basic principles of the parallel plate capacitor made large.
How it works:
The capacitance C of a simple parallel plate capacitor is given by
the ratio of the magnitude of the charge Q on either conductor to the potential difference between the conductors V or equivalently the ratio of the area to plate separation multiplied by the permitivity ε.
This capacitor has C = 88 pF for l = 10cm.
The capacitor consists of two circular 0.08" (2mm) aluminum plates 1m in diameter (figure 1). Each is mounted on a rectangular tube pillar by way of a machined back plate (figure 2). It is electrically insulated from the pillar by using Teflon attachment screws and an insulating Teflon sheet. One plate is attached directly to the base platform, with the other attached to a 60×30×1cm wooden base that is mounted on brass drawer slides so that the plate separation can be varied. Each plate also has a 3cm long aluminum tube attached to its back with bore size to receive the banana clip leads from the electroscope.
figure 1. The capacitor
The most common use of the demo is to apply a charge to the plates and see the effect of varying plate separation on the voltage. Apply the charge using a simple chemical cell 1 (whose terminals can be touched to the plates using alligator leads) and measure the voltage with a high impedance electrometer. 2 A sheet of 1/4" masonite is on hand to show the effect (sometimes temperamentally) of a dielectric. The masonite has a dielectric constant of about 11.
figure 2. Plate attachment detail
Setting it up:
The capacitor assembly is mounted on a dolly for easy maneuvering. The electrometer needs to be bench mounted with a camera on the display; this means quite long (2m +) banana plug leads are needed between the cap and electrometer.
Usually works very well but can be affected by stray fields and charges, especially static from the user who has to lean over the plates when using it. Think about what you're wearing for this particular demo!
1 a good one is a 67.5V Ever Ready #467 cell.
2 Keithley 617 Programmable Electrometer