5C10.20 • Capacitors : Parallel Plate Capacitor

Photo Shows the Demonstration Apparatus Used by the University of Texas at Austin

Set Up Instructions / Notes:

• Demo includes a Wimshurst machine, an adjustable parallel plate capacitor and an open electroscope.
• (Step 1) A red wire is attached connecting the electroscope to the non-moving plate of the capacitor.
• (Step 2) A second red wire is then attached to the first red wire and one of the electrodes of the Wimshurst electrostatic generator.
• This second red wire will be pulled of the fixed plate of the capacitor after charging the demo with the Wimshurst.
• Therefore, the second red wire must fit loosely into the first red wire, if it is to be easily removed from the circuit.
• (Step 3) A double set of black wires are connected to the adjustable plate of the capacitor, electroscope and the second Wimshurst electrode.
• Note: This demo uses static surface charges - which require very dry air conditions in order to work effectively.
  • If the air in the classroom is humid - then the surface charges will bleed away rather quickly.

Photo Shows the Demonstration Apparatus after charging and subsequent removal of the second red wire from the circuit.

Operating Instructions:

• Prior to charging the capacitor - adjust the spacing between the plates so that they are separated as far as possible from each other.
• To charge the apparatus - turn the crank on the Wimshurst machine clockwise until the Electroscope needle deflects upwards.
• Quickly remove the red wire lead from the Wimshurst to the parallel plate capacitor, taking care not to disturb the red wire between the electroscope and the capacitor plate.
• To vary the strength of the E-field - decrease the spacing between the plates and observe the transfer of electric charge from the electroscope to the large parallel plate capacitor.
• Increase the spacing between the parallel plates and observe the needle on the electroscope as it returns to its prevous position.
• A set of dielectric plates are provided with the demonstration to show the effect of inserting a dielectric material.

The 5 minute video below is intended for staff use.

Last updated on April 23, 2018