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Bouncing charged ball experiment

  1. Jun 11, 2013 #1
    Hello, I would like to share a cool experiment... but not yet experimented for me.
    Suppose you have a ball of metal between two plates.
    The upper plate is charged - and the lower plate is charged +
    There is constant potential between the plates.
    When the ball touches the lower plate, it charges positively and it follows the electrical field in the upward direction. Then it bounces against the negative plate.
    Let's suppose we have an elastic collision here. At the collision, the ball loses its charge and becomes negative. At the same time, the negative plate is connected to a constant potential putting positive charges away.
    After bouncing, the ball will go downwards and continue to accelerate because now it has taken a negative charge. Then it bounces on the lower plate, etc. etc.
    At the end the ball would bounce really quickly and frequently between the plates.

    Is this true?
    Does this experiment have a name? I would like to call it mine otherwise...
  2. jcsd
  3. Jun 12, 2013 #2

    Simon Bridge

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    Welcome to PF;
    It's a variation on an electric pendulum.
    The collision does not need to be very elastic - the voltage can be adjusted to compensate.

    The ball touches the -ve plate becoming negative, it is attracted to the +ve plate, touches it, becomes positive, now it is attracted to the negative plate, and back and forth it goes :) it's be at a pretty steady rate.

    I've seen it done on a track as well.

    If you do it vertically, then the ball will hit the bottom plate harder than the top plate - do you see why?
    If you build it, you should find some way to confine the ball - put it in a clear tube or something - so it won't bounce away on you.
  4. Jun 12, 2013 #3
    It's much clear now. I know why the ball hits harder the bottom, it's because of gravity :)
    Thanks for the information.
  5. Jun 12, 2013 #4
    Very cool experiment. I would like to try it sometime.
  6. Jun 12, 2013 #5

    Simon Bridge

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    There are lots of examples on youtube ... doing vertically would be trickier but adds an extra eye-warp to it if the all you see is the ball bouncing between "floor" and "ceiling". It would make it look like this:

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