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Millikan
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Look at the diagram included in animation window 2, which is taken from Millikan’s book The Electron, published in 1935. The model screen above examines some aspects of Millikan’s original experiment. There are parallel plates between which a varying potential difference may be established. A uniform E field exists in the space between the plates. The charged oil drops can be suspended, the electrical force balancing their weight. If you read Millikan’s account he introduces viscous and upthrust (Archimedes) forces in addition to weight and the electrical force. These are included in this model.
- Obtain balance voltages for several charged drops. You can change the charge on the drop by altering "n", the number of discrete charges on the oil drop. Obviously Millikan, who was using x-rays to achieve this effect, had no direct control on how much charge the drops acquired. Explain why the field direction has to be in the direction shown for a given sign of charge.
- Work out what you would expect to find for the ratio of balance voltages as the number of charges on a drop changes and test this prediction using the model.
- For a given charge play with the voltage on the plates and use free fall to position the drops conveniently at the mid point between the plates. This techniques was used by Millikan but in a real experiment this was difficult and required great skill!
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Download this Resource
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Package:
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Millikan.exe (348Kb)
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Contents:
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millikan.mdl, millikan.doc and millikan.gif
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Instructions:
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Download the self extracting ZIP archive. Locate the file using Windows Explorer and double click. The self extractor will then start. The default installation path is C:\AP Revision. Having set the path press the Unzip button. After the files have been extracted change directory to C:\AP Revision. To start the model double click on millikan.mdl.
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