Magnitude of electric field at a point

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

    A force of 5.7 N. is exerted on a 8.8 microCoulomb charge in a downward direction. What is the magnitude of the electric field (in N./C.) at this point?

    2. Relevant equations

    E = (kel * abs(q)) / r^2\
    E = Fel / q

    3. The attempt at a solution

    8.8 microcoulombs --> 8.8e-6 Coulombs

    At first I tried using:
    E = (8.99x9)(8.8e-6) / r^2

    But I didn't understand how I was supposed to find the radius. If it were a square, rectangle, triangle etc. and given some more information I could use trigonometry to determine the radius. The equation is for a point charge, though.

    I then used E = Fel / q and simply plugged and chugged to get the right answer.

    My question, though, is when do I use the E = Fel / q and when do I use the E = kq/r^2 equation?

    Is the point charge for when two points are being compared to each other and the E = Fel/q is for one item?
     
  2. jcsd
  3. Doc Al

    Staff: Mentor

    The first equation (Fel = Eq) is true in general--it relates the field to the force produced by that field on a particular charge. The second equation is for finding the field due to a point charge. (Which wasn't relevant here, as you were not given any information about the source of the field, only the force it produced on a given charge.)
     
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