5B30.25 - Faraday Ice Pail
Demonstrate the principle of the Faraday ice pail.
Pull out an aluminum grounding plate, grounding plug, and piezoelectric squeezer. Use the small, open-air electroscope with red-ish measurement scale. Connect the outer ring of the electroscope to the grounding plate with a banana cable. Remove the flat-plate electrode from the top of the electroscope and connect the large metal coffee can directly to the electroscope. Pull out the copper sphere with handle to use as the charge transfer surface. Set up a camera to view the scale on the electroscope. The backplane of the electroscope can be placed in the groove that puts the backplane on an angle to the electroscope in order to catch the most ambient light to assist in the viewing of the scale on the electroscope. Be sure to zoom in on the scale of the electroscope.
With the large metal coffee can connected to the top of the electroscope, make sure the scale on the electroscope reads zero. Then use the piezoelectric squeezer to deposit charge on the copper sphere attached to the handle. If the charged sphere is now brought up to the open end of the can and carefully held inside, the induced charge at the outside of the can is shown on the electroscope. If the sphere is removed, the charge disappears. If the charged sphere is then brought into contact with the inside of the can, no jump will be seen on the electroscope, but the charge will remain on the can when the sphere is removed.
Then, if you use the piezoelectric squeezer to deposit the opposite charge onto the sphere and carefully hold it inside the coffee can, but not contacting the can directly, then the outside of the can is momentarily grounded. The ice pail can now be shown to have the opposite charge as the sphere since the positive and negative charges would cancel one another out. Lifting the sphere out again will show that the initial charge still remained on the can.
**Be careful not to let the sphere come too close to the can or the spark will jump accidentally. It is often necessary to work at getting the right amount of charges so that the effect is pronounced but a discharge does not take place.**
This is a simple electrostatic experiment performed by Michael Faraday demonstrating the effect of electrostatic induction on a conducting container in 1843. The experiment shows that an electric charge enclosed inside a conducting shell induces an equal charge on the shell, and that in an electrically conducting body, the charge resides entirely on the surface of the conductor. It also can demonstrate the principles behind electromagnetic shielding such as employed in the Faraday cage.