====== Photoelectric Effect ======

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A gold leaf electroscope with a flat zinc electrode at the top is given a negative charge and later discharged by illuminating the zinc electrode with ultraviolet light. The fact that only ultraviolet light causes discharge and not visible room light serves to demonstrate a relationship between the light's wavelength and the discharge rate of the electrode. If the electrode is given a positive charge, the ultraviolet light causes no discharge. This suggests that the free charge carriers in metals are negatively charged electron. (These are emitted when they interact with a photon (quantum of light) with energy greater than the metal's work function.)

===Setup===


The electroscope is charged by induction. To give it a negative charge, charge a glass rod by rubbing it with silk; the rod will have a net positive charge. Ground yourself to the zinc electrode by touching it with one finger and bring the positively charged rod near, but do not touch, the zinc electrode. Electrons from your body will flow to the plate since they are attracted to the glass rod. While maintaining the rod near the plate, take your finger off it. The plate should now have a negative charge, indicated by the separation of the gold leaf. To give it a positive charge, repeat procedure rubbing a PVC or amber rod with felt, fur, or brown paper towel. Electrons will now flow from the plate to your body and a net positive charge will be left on the electrode, indicated by the separation of the gold leaf. 

To perform the demonstration, first turn on the UV lamp and aim it to the zinc plate. Make sure the lid is closed for now (it absorbs UV radiation, so no UV photons will be hitting the plate at this point). Charge the plate specified above and then open the lid. If the plate was negatively charged, you should see a rather quick drop of the gold leaf; if it was positively charged, the gold leaf should remain in place. (In humid days, the charge may start leaking before opening the lid, which will make it difficult to observe the photoelectric effect and the difference between negatively and positively charged electrodes. This demo is best performed in cold, winter days.)

Note: Zinc can oxidize rather quickly and develop a patina which can look greyish at first, making it difficult to distinguish it from zinc's color. Before performing the demo, it is recommended to brush the surface of the electrode with a scour pad to maximize surface contact between metal and light.

J5, G0, L0

PIRA DCS 7A10.10