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Forces in a circle from similar charges?

  1. Jan 2, 2012 #1
    Hey guys,

    New here. It's been a while since I've done any physics. I've been playing around with some mental work in my head and I am trying to figure out why something doesn't work. I know it shouldn't work, but I can't figure out the why.

    What I am talking about is utilizing similar charges to create movement. I know that a negative charge would push away from a negative charge, just like a positive charge would push away from a positive charge, and go together if they are opposite. Much like how magnets work.

    The issue I get into is if I picture doing this in a circle. Say I had 10 items (lets say balls) arranged so that they were in a circle. They can rotate around an axis. I'll throw out random numbers just for getting a general picture, but are not quantitatively the same as it would be in a real situation (that's because I have no clue as to what charges would be required off-hand). If I had all ten balls with a charge of -200, I would assume nothing would happen. Now if ball1 were charged with -1. and ball2 were charged with -20, with increasing charges up to ball10 charged with -200, my mind is telling me that -1 would rather go towards -20 because it is the path of least resistance. And the same with -200 would rather go to -1 than say -180. One part of me says that it would rotate, another part of me says that it wouldn't, because it's supposed to be impossible to have something power itself from itself. I assume there is some principle similar to a fan not being able to blow the sail on a sail boat, but I can't think of it.

    Anyways, can you guys help me out with the logic here?

  2. jcsd
  3. Jan 2, 2012 #2
    If you have two negative charges next to eachother and release them, they'll start to move apart, right?
    Why doesn't that violate the same reasoning ('impossible to power itself from itself' is basically 'conservation of energy').
  4. Jan 2, 2012 #3
    Yes they would move apart.

    But if they are attached and share the same pivot point they could only move in either a counter clockwise or clockwise manner. Maybe my description was bad.

    Here's a picture. Assuming that these were all attached at that middle point and could rotate freely clockwise or counter clockwise. horizontal to the ground to have equal force of gravity on all balls.

    so... if they were attached, would they move?

    Attached Files:

  5. Jan 2, 2012 #4
    I understood. And yes, they would move (Just like two like-charges, initially close)---they would eventually oscillate in some complex pattern.

    Why doesn't that violate conservation of energy?
  6. Jan 2, 2012 #5
    Why is the energy of conservation not violated?

    That's what I'm trying to figure out! haha.

    I know with the fan/sail concept the fan pushes air, > into the sail. The sail wants to go the direction the fan is blowing, but cannot because the act of taking in the air pulls the boat back with the same force, or greater if there wasn't 100% containment of the sail.

    But with the balls, assuming the energy is already there, the force wouldn't change. There would be a given repulsion from ball A for ball B that would be based on the given strengths of the fields. If ball A is in given X,Y coordinates of space, Ball B would have a desire to move out of the field based on the strength of the charge.

    I would see an issue at the highest ball, and the 2nd lowest. The highest ball would clearly want to go clockwise, because the charge is much lower that the one before it. but for the other ones, they would want to go the other direction. if the first is a charge of -1 then the second is a charge of -50. The -50 would rather go towards the -1 than towards the -100. Going from that, and looking at my example, it would look like ten balls would want to go counter clockwise, while two balls would want to go clockwise. If I knew how to do the vector calculations, my basic assumption would be that somehow they would even out.

    The goal would be to have a clear movement in a given direction, either clockwise, or counter clockwise, using charges.
  7. Jan 2, 2012 #6
    Another point,

    two balls would move in opposite directions with the same force.

    I wish I knew how to draw this out. haha.
  8. Jan 3, 2012 #7
    So, can anyone clue me in how that could be set up to actually rotate?

    The only way I can think of, for sure, would be to have an outside force that had a controlled switch that would turn on and off as the ball was past the zenith (probably the wrong word) so that it would controllably push the ball in a circle.

    But the way I have it set up, I can't think of a way to actually have it turn itself.
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