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In Millikan's oil drop experiment why oil drop only pick negative charge from gas between both charged plates? Why not oil drop pick positive charge?

Abhishek
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  • No, both types exist and it will be very easy to see that they are moving in the opposite directions on the microscope. To get a very good data, Millikan's team kept swapping the polarity on the plates so as to keep following the same droplet for hours. That is what led the experiment to give such precision. – naturallyInconsistent Jun 25 '23 at 09:56
  • @naturallyInconsistent your statement goes against the wiki article on the first experiment, where the negativeness is stressed https://en.wikipedia.org/wiki/Oil_drop_experiment .do you have a reference for your statements of both charges being present? – anna v Jun 25 '23 at 12:00
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    @annav I don't know. I was referring to the modern replicants thereof, which may have changed the way the charges are induced onto the oil drops. I have done the experiment in an undergrad setting and it was very easy to see that it would always have drops of both charges in my case. – naturallyInconsistent Jun 25 '23 at 12:45
  • @naturallyInconsistent if you have X rays in your system then you eventually have lot a electrons and if free electrons are there that means they came from any atom present in atmosphere and if electron detached from atom the atom become positive (cation) ... I'm sorry if I'm wrong I'm new – Abhishek Jun 27 '23 at 11:46
  • @Abhishek one would have expected that the X rays kicked an electron from the oil droplet itself and thus made a positive charged droplet rather than negative charged droplet. – naturallyInconsistent Jun 27 '23 at 11:48
  • @naturallyInconsistent no never x ray only ionize air present in chamber go through the experiment again I guess the ray only ionize the air ...but never the oil drop buddy – Abhishek Jun 27 '23 at 11:50
  • @Abhishek If you learn about how X rays work, you can never block the X rays from ionising the oil droplets. – naturallyInconsistent Jun 27 '23 at 13:24
  • As I know X rays can ionize things easily but in this context X rays were used to ionize air because if oil drop ionized that means electron knocked out from oil drop then it carry positive charge (oil drop) and in original experiment positive plate is on upside while negative is downside so the positive drop just go down... I guess Millikan only interested in those drops which are near positive plates which shows presence of negative charge on it ! – Abhishek Jun 28 '23 at 05:04

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Millikan measured the time for a droplet to fall a particular distance under the influence of gravity and aerodynamic drag, then switched on his electric field and measured its travel time over a similar distance under the influence of gravity, drag, and the field. One source of the precision that allowed him to observe quantization of charge was that he could watch the same droplet for hours, raising and lowering it by turning the electric field on and off, and irradiating the chamber to change the charge on the droplet without changing its size or its drag coefficient. To repeat the experiment with positively charged droplets, he would have needed to reverse the polarity of his battery — a big change to a delicate setup. Otherwise the positive droplets would fall under gravity and fall faster with the field, ending up on the floor in short order.

Nowadays we have inexpensive bipolar DC power supplies, and we can reverse the field on a Millikan-type capacitor pretty easily. But that hadn't been invented when Millikan did his experiment.

rob
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