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Homework Help: Confused about coulomb law!

  1. Feb 3, 2014 #1
    1. The problem statement, all variables and given/known data
    You have two charges, q1= -15uC and q2= 3uC, separated by a distance d= 3m. We want to calculate the electric field, E, at a location x relative to charge q2 located on a line connecting the two charges. Note that x could be anywhere on that line. Also need to calculate the Coulomb force experienced by each charge. Give Both magnitude and direction. And furthermore find the location x where the electric field E= 0

    2. Relevant equations



    3. The attempt at a solution

    FOR electric field I got -12,000. Not sure if this is right. and I have no clue on how to find the mag and direction.

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

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 3, 2014 #2
    You have to find the net field in this problem, and you're given two field-generating charges. That means you have to superimpose one field over the other (i.e. calculate the field of one point charge, then the other charge, then add the two together vectorially. Try starting there.
  4. Feb 3, 2014 #3
    For the magnitude, that is simply the result you get when you use your equations. To find the direction, you have to think a bit more.

    For electric fields: what do you know about the direction of electric fields created by positive and negative charges?

    For electric force: look at the charges you have. Under what circumstances will the particles attract? When will they repel?

    In both cases, it is especially helpful to draw out a diagram when solving for the direction.
  5. Feb 3, 2014 #4
    ok So i drew it out but still confused if I got the electric field rite.
  6. Feb 3, 2014 #5
    Usually, electric fields are pretty large numbers. They are in units N/C. One Coulomb is huge. You're working in microcoulombs so you should expect a large number. For the direction, just think about electric fields. Do electric fields point toward or away negative charges? What about positive?
  7. Feb 3, 2014 #6
    electric field go from positive to negative right?
  8. Feb 3, 2014 #7
    for q1 I got 15,000?
  9. Feb 3, 2014 #8
    Field depends on r, so you won't get a constant value. It will be a function of r in some way. Describe the steps in your reasoning so we can see how you're getting your answers.
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