Northwestern Physics Demos                                 Go back to Electrostatics 
DCS# 5A10.20NWU# ES-5Location:    10 D 2
title: Electrophorus
aka:  charge by induction

A conducting disk is attached to an insulating handle.  A sheet of insulating material with a ground plane of aluminum foil is charged by rubbing.  (Dry paper will do.)  Polystyrene (pink or blue foam insulation) will be charged negatively when rubbed with paper, Acrylic plastic will be charged positively when rubbed with paper.
Then the disk is placed on the plastic, insignificant charge is transfered by conduction, but the electric field induces charge separation.   When the disk is touched to ground, (the foil beneith the plastic) charges similar to those on the plastic are repelled.  When the disk is lifted by its insulating handle, the decreased capacitance causes a very large increase in potential and the disk is charged oppositely to the plastic.
   A small flouescent tube will blink to show the charge transfer if one end is held in contact with the ground while the other is brought near the disk.

   Balloons take on a negative charge when rubbed with dry skin, hair, or fur.   What type of plastic would be needed to have the electrophorus disk repell a charged balloon?
electrophorusacrylic model