Reference: EM28
Category: Electromagnetism
Purpose: Demonstrates the concept of capacitance.
Description: Two parallel brass plates are mounted so that they can slide towards or away from one another. First show that the 220V mains supply connected between the plate and the case of a gold-leaf electroscope produces only a very small deflection of leaf. Then connect the two brass plates to the terminals of the electroscope. Bring the two plates as close together as possible. (They are varnished to prevent electrical contact – if leakage occurs, place a thin plastic sheet between them.) Connect a 12V car battery across them, and then disconnect it again. The leaves of the electroscope show nothing. Now pull the plates apart and the leaves diverge, showing that a large potential difference arises from ‘stretching the lines of force’ between the very small charges on the plates. Alternatively, start with the plates far apart. Charge one up to a high potential by stroking it with a charged Perspex or Teflon rod. (It is best to also stroke the plate of the electroscope.) Then move the plates together and show how the leaves fall together as the potential difference falls and the capacitance increases.
