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Electric Field and the weight of an electron

  1. Mar 15, 2007 #1
    What is the magnitude and direction of the electric field to balance the weight of an electron, a proton?

    well electric field = F / q
    so the force is gravity and the charge is the charge on each

    so E = (mass of electron * 9.8) / charge on electron

    similarly for proton

    but how do i kno the direction??
    electron is negative
    proton is positive

    would the field have to be in same direction (from equation) or in the opposite direction to balance the forces??
  2. jcsd
  3. Mar 15, 2007 #2
    The direction depends on the source charge. For an electron and positive field, for example, you'll have to place the electron at a suitable distance between the Earth and source charge so that both forces cancel.
  4. Mar 15, 2007 #3
    but the electric field needs to balance off the electron/proton
    how does the sign on the field work?
    ie. is it same/opposite as sign of charge
  5. Mar 15, 2007 #4
    No information about the field has been provided in your question. Is this part of larger question? Remember, field lines of a positive charge "spread out" while those of negative charge converge. And a postive charge always tends to move along the direction of field.
  6. Mar 15, 2007 #5
    that is why i am confused, the question is the same as posted and the direction can either be up, down, left, right, in, or out

    but i didnt think the direction would be constant for the entire field
  7. Mar 15, 2007 #6
    Well, the only answer one can possibly give, with the information provided, is that it is opposite to the direction of the gravitational field at the point.
  8. Mar 15, 2007 #7

    Doc Al

    User Avatar

    Staff: Mentor

    No need to worry about source charges. For this problem, all you care about is the direction of the field at the location of the particle.

    To balance the weight of the particle, which always points down, the electric force must point up. To get an upward force, which direction does the field have to point?

    Hint: A positive charge in an electric field will experience an electric force in the same direction as the field.
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