Electricity and Magnetism > Capacitance > Capacitors

DCS# 5C10.20

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PARALLEL PLATE CAPACITOR

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APPARATUS

variable parallel plate capacitor	113-02-B
electrometer	113-02-F2
low current, high voltage source or other DC power supply	113-02-F1
dielectric (bakelite or acrylic sheet)	214-02-F
camera	202-09-D

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DESCRIPTION

Set the electrometer to its 100 V scale.  Ground the electrometer and connect the leads to the plates of the capacitor.  With the plate separation set to a few millimeters, momentarily connect 30 V across the capacitor plates.  Increase the plate separation to show that voltage increases.

Insert the sheet of bakelite to show that the voltage decreases.  It may be necessary to note the voltage, separate the plates, insert the dielectric, re-establish the original plate separation, then read the new voltage.

NOTES

The electrometer is very sensitive and can be damaged by potentials >100 V.

Place the dielectric on a grounded metal sheet and use it to discharge the dielectric before placing it in the capacitor.

This is not a good quantitative demo since the internal capacitance of the electrometer + leads is comparable to that of the capacitor.