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Experiment - pepper and soap

  1. Jun 21, 2014 #1
    I have a question... I think that you know the experiment with pepper and soap.
    (you put water on plate, than you put a lot of pepper on in, than you put on your finger some soap and than you touch the plate). The pepper goes to edge of the plate. Only some of them fall in the water. It happend because the surface tension was reduced by soap. However, if is the surface tension reduced, than is reduced surface force also. The surface force compensated the gravitational force of pepper at the beginning of the experiment. However, now is the surface force reduced, so it can not compensated the gravitional force. If it is right, the all pepper had to fallen to water, but it is not true, the pepper went to edge of the plate.

    Pls, can you tell me why the pepper went to edge of plate? Don't send me a link to wikipedia about surface tension. I understand. I want a straight answer to this problem.

    Thank you very much!
  2. jcsd
  3. Jun 21, 2014 #2


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    The soap doesn't remove surface tension everywhere instantaneously and totally. It takes time for the area with reduced surface tension to cover the whole surface. I believe what happens is that the pepper moves faster than that area can expand.
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