Photoelectric effect

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The Photo-electric effect is a fundamental experiment in "modern physics".
A Gold Leaf Electroscope has the normal plate removed and a zinc plate is used. the electroscope needs to be in good repair. To work, the zinc needs to be freshly scratched to remove the surface oxides. Use a moderately fine abrasive paper (i.e a 180 grit "sanding paper" or similar). NB. Too fine scratching impairs function! The UV light source needs to be of the right type too. A spectral tube will not, normally, work as the UV is normally filtered by the glass.
There is some thought that a well known supplier has advertised in their catalogue a lamp designed for this experiment that does not work. Ensure that UV the output in the range <310nm! The easiest method of charging is to use an EHT unit (at between 2 and 5KV) and momentarily to touch the plate with the negative probe. This method removes any confusion of charge polarity. The EHT can then be turned off. More safety conscious of you can argue whether the probe should be earthed or the case! In my view the case on a table should be earthed.

Charging the zinc rod Using an EHT unit

Charging the zinc rod using an EHT unit The leaf rises and should remain reasonably high (subject to humidity). If the UV lamp is allowed to fall on the plate the leaf falls. Changing the polarity the light will not fall (other than through leakage) as electrons are not caused to be lost from the photon action. The Energy of the light is given by E=hf often written as E=hν in older books. here h is plank's constant.
The Work Function (φ) of the metal is the energy that needs to be imparted to an electron by a photon to just be released by the electrostatic attraction of the metal lattice.
Thus the released electron will have an energy: E=hf-φ. There is no electron release if hf<(φ) so there is a cut-off frequency f=φ/h -which can be worked out.

The work function of metals can be found on the Kaye and Laby online web pages (In section 4.3) You will need to look up the photoelectric column!
As an extension, try looking for safe metals with similar work functions to see if they work. Tin may work but copper is not likely to (aluminium may work but in a vacuum as it forms an oxide layer too quickly!). Bend a cleaned sample and attach to the normal cap of the electroscope with a crocodile clip.
Note the electron Volt, eV, is a measure of energy =1.602 x10-19J


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--D.B.Ferguson 23:24, 2 November 2006 (GMT)--D.B.Ferguson 22:39, 31 October 2006 (GMT)


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