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Capacitor with Dielectrics
# 5C20.10

Description: An electroscope is connected to a parallel plate capacitor to show how capacitance increases when a dielectric is inserted between the capacitor plates.

When the dielectric is inserted, the dielectric polarizes and attracts more charge to the capacitor plates (increasing its capacitance). This effect can be seen by the fact that electroscope shows the presence of less charge, indicating that more charge is now on the capacitor plates.

When the dielectric is removed, its capacitance decreases and some charge leaves the capacitor and goes back to the electroscope.

Instructions: Connect a wire lead from one plate to the top terminal of the scope (this is the plate that you will ultimately charge). Warning: this wire lead should "float" in the air, i.e. it should not touch the table or anything else, or the plate will lose its charge. Connect the other plate to the ground terminal at the bottom of the scope (it's alright if this lead touches the table, etc.).

Charge a PVC rod with fur, and rub the rod on the plate that should be charged. You can now insert the dielectric between the capacitor plates and observe the effect on the electroscope. A wooden board is used as the dielectric.

Parts: parallel plate capacitor, dielectric, charged rods, electroscope

Purchased/Built: built

Setup Time: 5 min

Quant. in Demo Room: 1

Demo References
The Video Encyclopedia of Physics Demonstrations Disc 18-20 Charge a parallel plate capacitor with a rod, insert dielectrics and observe the electroscope. Animation.
Sutton E-70 Various dielectrics are inserted between two charged metal plates to show the difference in deflection on an electroscope.
Freier and Anderson Ed-2 The voltage is measured with an electroscope as dielectrics are inserted between parallel plates of a charged capacitor.

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