Courses: P1, P4, P15, P19, Astr4
Charge a zinc plate using an electrophorus, then discharge using an arc lamp which produces a great deal of ultraviolet light. If a glass pane in placed between the light and the zinc, the zinc will not discharge, as the glass blocks the UV. The gold leaf electroscope, which shows the charge on the zinc plate, is projected onto a screen (or the wall) by placing it in line with the light from the arc lamp.
To use this setup: Rub the fur on the plastic. This leaves a net negative charge on the plastic plate. Place the electrophorus on the plastic -- the charges in the metal plate will polarize, with the electrons being pushed toward the top, but the charges will not flow from plastic to metal. Touch the top of the metal plate to draw electrons off. This will leave a net positive charge on the electrophorus. Move it near the plate of the electroscope, but don't let the two touch! If they do touch it will draw electrons off the zinc plate (which is attached to the electroscope), and the electroscope will show a net charge, but it will be a positive one. The UV light will try to knock more electrons off the plate, making it even more positive, and you won't see any change in the electoscope. Instead, hold the electrophorus above the electroscope, and close to it, which will polarize the charge in the electroscope (negative charge will be drawn toward the electrophorus, leaving the bottom of the electroscope plate positively charged). Then, while holding the electrophorus in this position, touch the bottom of the electroscope's plate, allowing electrons to be pulled from you onto the electroscope. This leaves the electroscope and the zinc plate with a net negative charge, and ready for the lamp to knock electrons off.
Turn on the power supply. Tell students not to look directly at this lamp! It generates a lot of UV light, which is very bad for the optic nerve. Turn the knob to bring the carbon rods together. As soon as they touch, and they begin to spark, back them off a small amount to generate bright and even light. The electroscope will show the charge being drained away.
Do the same thing again, but hold the glass plate between the arc lamp and the electroscope. No charge will dissipate. You can pull the glass plate out to see the foil leaves start to move together, and reinsert it to stop the process.
Setup: The speed with which the zinc discharges is affected by how much oxide has formed on the plate. Use fine steel wool to remove the oxide before setting up the demo -- but don't remove too much or it will discharge faster than ideal for the theatrical experience! Also, check the state of the carbon rods, and adjust or replace them if needed.
Lots of details in this video:
This next video shows the typical classroom demonstration:
