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Electrostatic Field concept question

  1. Jun 1, 2009 #1
    1. The problem statement, all variables and given/known data

    A ping pong ball covered with a conducting graphite coating has a mass .005 kg and a charge -4 microC. What electric field will exactly balance the weight of the ball?

    2. Relevant equations

    qE = mg

    3. The attempt at a solution

    I know how to get the magnitude of the electric field using the equation above. I just don't understand the conceptual part of this question: Which way is the electric field going (up or down)? Any help would be appreciated...thanks
     
  2. jcsd
  3. Jun 1, 2009 #2

    rock.freak667

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    since weight acts downwards, I'd assume the electric field move the charge upwards.
     
  4. Jun 1, 2009 #3
    It would have to act parallel and in the opposite direction to the force of gravity.
     
  5. Jun 1, 2009 #4
    well another similar problem had the electric field pointing downward with the particle having a negative charge.
     
  6. Jun 1, 2009 #5

    rock.freak667

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    So which way will the electric field point to move the -4uC charge upwards?
     
  7. Jun 1, 2009 #6
    I would assume upwards but I'm not really sure. This seems like a really simple concept but i can't figure it out.
     
  8. Jun 1, 2009 #7

    rock.freak667

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    Electric fields move from positive to negative. So if you place a negative charge in that field, it will move towards the positive side.
     
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