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Electric field direction and hovering charged object

  1. Sep 2, 2010 #1
    1. The problem statement, all variables and given/known data

    A small object of mass 3.64 g and charge -18.1 µC is suspended motionless above the ground when immersed in a uniform electric field perpendicular to the ground. What are the magnitude and direction of the electric field?

    2. Relevant equations


    3. The attempt at a solution

    I solved for the magnitude and was fine with that but was wondering why the electric field direction would be downwards?

    Why couldn't it be upwards?

    E = m*g / -q = -#

    My logic was that a - electric field upwards would make the charged object hover since - and - repel.

    I guess if it was going downwards the same effect would still occur, but I don't understand (if this is even right...) how to determine if up or down.
    Am i completely off track on how to think about this?
  2. jcsd
  3. Sep 2, 2010 #2
    The direction of the force depends on the electric field and the sign of the charge. Positive charges experience a force in the direction of the electric field, and negative charges experience a force in the opposite direction.
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