PURPOSE: To demonstrate the photoelectric effect.
DESCRIPTION: A zinc plate is placed onto an electroscope. Zinc is used because it has a low work function and does not require a more intense UV source. A static negative charge is applied to the metal plate using the triboelectric effect by rubbing a Teflon rod to separate charge by friction and touching the rod to the plate. The needle on the electroscope indicates the initial excess charge. A beam of very intense white light from a carbon arc lamp is aimed directly onto the metal surface. This light contains a high amount of UV and causes a discharge of electrons from the surface of the metal plate. The ejection of the electrons from the surface of the plate is observed by the discharge of the electrometer. This is the photoelectric effect.
A common glass plate inserted between the zinc plate and the arc lamp filters the UV and the discharge will stop. Remove the glass plate and the discharge is allowed to continue. The application of positive charge will prevent the photoelectric discharge. This is repeated using a plate of quartz glass which passes UV. The quartz glass will not filter the UV and will not stop the discharge.
The method used in this demonstration is not in strict agreement with many textbook descriptions of the photoelectric effect . The photoelectric effect will cannot easily cause the metal plate to acquire a charge as electrons are ejected from the surface. This does not work experimentally without placing the metal in a vacuum and using a collector to get the ejected electrons. There are two problems. The cloud of air that surrounds the metal plate at atmospheric pressure will become ionized and cause a barrier to build that will prevent electrons from easily leaving the zinc surface. Also, without a collector near the metal surface to pull the electrons completely away after they're ejected the electrons will tend to get pulled back to the surface. Therefore because of these two problems if the experiment is performed without excess initial charge then the photoelectric is so small that a more expensive and sensitive electrometer would be needed to make the measurement. Both of these problems are overcome by using the experimental method of initially placing an excess of negative charge onto the zinc plate.
With this method the photoelectric effect will cause the plate to lose all of its initial excess negative charge. The metal surface is not initially neutral, and it will not obtain a positive charge as described in the textbook.
EQUIPMENT: Zinc plate 4" x 4", Pasco project-o-scope electroscope, Carbon arc lamp, PVC electrostatic rod and paper towel.
SETUP NOTES:
The oxide layer on the zinc plate must be polished before use. Clean the zinc plate with a scouring pad and alcohol.
Updated by JZ in 4/8/2004