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Homework Help: Coulomb's Law/Newton's Law of Universal Gravitation (Precalculus?)

  1. Jul 9, 2013 #1
    1. The problem statement, all variables and given/known data
    The electric field, E, a distance D away from a charged particle is directly proportional to the size of the charge Q, and inversely proportional to the square of the distance D. If the charge is increased by 40% and the distance is increased by 30%, by what percentage does the electric field change?

    2. Relevant equations


    3. The attempt at a solution

    I'm actually pretty upset with how rusty I am at this but for some reason when I read these problems (and precalculus gravitational problems), I can't seem to get Classical Mechanics and Electromagnetism equations out of my mind. Although, I know that the answer will pretty much apply Coulomb's Law. Maybe I'm overthinking this..

    ED = Q = 1/D^2


    Is all I'm interpreting from the question.
  2. jcsd
  3. Jul 9, 2013 #2


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    What do +.35(Q) and +.2(D) mean?
  4. Jul 9, 2013 #3
    Ah, I'm sorry.

    I meant to write +.4Q and +.3D.

    I'm increasing the charge by 40% and the distance by 30%
  5. Jul 9, 2013 #4


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    If you increase Q by 40% then Q becomes 1.4Q. The original electric field is E=kQ/D^2. Compute the ratio of the new field to the original field.
  6. Jul 9, 2013 #5
    I know that, I'm just separating the added charge and added distance from the setup because I'm not even sure if I have that done correctly.
  7. Jul 9, 2013 #6


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    That you understand it would be pretty hard to tell when you write things like +.35(Q) and +.2(D). If you know the ratio of initial and final charges and distance then use Coulomb's formula to compute the ratio of electric fields. This is pretty straightforward.
  8. Jul 9, 2013 #7
    I think I figured it out, with your help. Thanks
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