Concepts

Capacitors, capacitance

Overview

The Leyden jar is a dissectible capacitor made of aluminum and glass that poses an interesting question of where the stored charge resides. A series of experiments can be done showing that the charge stays on the glass insulator, though this is not generally true for capacitors.

As with all electrostatic experiments, unless conditions are ideal (dry, cold), the demonstration may perform worse or even fail.

Details

Equipment

• [1] Leyden jar
• [1] Discharge wand
• [1] Van de Graaff generator
• [1] Slightly damp sponge
• [1] Tongs
  
• [1] Extension cord

Classroom Assembly

1. Put the Van de Graaff somewhere away from computers and plug it in.

Important Notes

• People with pacemakers, insulin pumps, cochlear implants, or other critical devices should not come near the Van de Graaff.
• Ensure the Van de Graaff is not near any computers or other electronics.
• Users should not be have their phones or other electronics.
• Test the demonstration just before show time to avoid untimely failures.

Script

Presentation technique can really elevate this demo. It helps to play up the tension of you or a volunteer possibly getting zapped. There are three basic experiments:

Charge and discharge the Leyden jar.

1. Dim the lights.
2. Touch the interior electrode of the Leyden jar to the Van de Graaff.
3. Turn on the Van de Graaff.
4. After a few seconds of charging, turn off the Van de Graaff.
5. Discharge the capacitor using the wand.

1. Charge the Leyden jar again, as in the previous section.
2. Remove the interior electrode using the plastic end of the wand and lift the glass insulator out of the exterior electrode.
3. Use the wand to show no discharge occurs between the electrodes.
4. Reassemble the Leyden jar, performing step 2 in reverse.
5. Discharge the capacitor using the wand.

1. Charge the Leyden jar again, as before.
   
2. Remove the interior electrode using the plastic end of the wand and lift the glass insulator out of the exterior electrode.
3. Wipe the charge off the glass using your hands, the sponge, and tongs.
   
4. Reassemble the Leyden jar, performing step 2 in reverse.
5. Use the wand to show little or no discharge occurs between the electrodes.

Additional Resources