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What is the magnitude and direction of the electric field?

  1. Nov 27, 2009 #1
    1. The problem statement, all variables and given/known data
    A small object of mass 3.80 g and charge -18uC (-18 x 10^-6 or microcoulombs) "floats" in a uniform electric field. What is the magnitude and direction of the electric field?

    2. Relevant equations
    F = ma
    F = Eq

    3. The attempt at a solution
    I already have the magnitude down. It's 2070 N/C

    Why does the electric field point down though? I don't fully understand the direction of electric fields with positive or negative charges in them. can someone explain this to me?
     
  2. jcsd
  3. Nov 27, 2009 #2

    kuruman

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    Positive charges move with the electric field lines and negative charges move against the electric filed lines. So if the electric field is down, a negative charge will experience an upward force against gravity and therefore float.
     
  4. Nov 27, 2009 #3
    so if the electric field is down, that means it's of a negative charge?
     
  5. Nov 27, 2009 #4

    kuruman

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    Say at a point P in space you are told that the electric field is straight down and that it is produced by a single charge. This can mean one of two things:

    (a) There is a positive charge directly above point A or
    (b) there is a negative charge directly below point A.

    Now if I place a negative charge at point A it will experience a force up regardless of whether (a) or (b) is the case.
     
  6. Nov 27, 2009 #5
    it wasn't very clearly explained in my book. Much thanks!
     
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