70. Faraday Ice Pail

This demonstration uses a Van der Graaff generator, a 10 cm test sphere on an insulating handle, a can mounted on an electroscope (the "ice pail"), and a second electroscope to test for charge. The "medium" difficulty of the demonstration is in remembering to do all the steps in the correct order.

Version 1: The induced charge equals the inducing charge

Charge the small sphere with the Van der Graaff. Ground the Van der Graaff sphere and the pail to make sure there is no extra charge around. Show that the small sphere is still charged by bringing it near the second electroscope.

Lower the small sphere into the pail without touching it. The electroscope deflects. Remove the sphere, and the deflection disappears.

Lower the sphere into the pail again without touching it. The electroscope deflects. Ground the pail by touching it. The deflection disappears. Remove the sphere. The electroscope deflects again. You have induced a charge, opposite in sign to that on the sphere, onto the pail from the ground. We will now prove that the magnitude of this induced charge on the pail is equal to the magnitude of the charge on the sphere.

Lower the sphere slowly into the pail. The deflection goes to zero. Lower the sphere down until it touches the bottom of the pail. If you listen carefully, you can hear the sound of the spark jumping. The deflection remains zero. Now take the sphere out. The deflection remains zero, and you can test the there is no charge on the sphere by bring it to the second electroscope. All charge has been neutralized.

Suggested by Bill Layton

Version 2: Gauss's Law

Start as above. Charge the small sphere and lower it into the pail without touching it. The electroscope deflects. Remove sphere. Deflection disappears. Put sphere back in. Deflection returns. Lower the sphere so it touches the bottom of the pail, and you can hear a spark jump. Deflection remains unchanged. Remove the sphere. Deflection remains unchanged. Finally, bring the sphere to the second electroscope. There is no deflection. The sphere is completely discharged. All of the original charge on the sphere went over to the pail.