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Homework Help: Rearranging coulombs law equation

  1. Apr 15, 2010 #1
    I have just started the physics part of a science foundation course and im just really struggling how to rearrange equations

    I have been given a quesiton to rearrange the coulombs law equation as below, to make K the subject and i have been sitting here for hours and i am no further forward. Can anyone walk me through it please and you will be saving my sanity.


  2. jcsd
  3. Apr 15, 2010 #2
    Right. You need to get K on one side by itself, with nothing else on that side of the equals sign.

    F=K*Q1*Q2/r2 (I don't know the Law, i'm assuming Q2/R2 should be in brackets)

    Whatever you do to one side, you have to do to the other side as well, so its equal.

    Divide by Q1 on both sides

    F / Q1 = K * Q2/R2

    See how there is less on K's side of the equals sign now?

    Times by R2 on both sides

    (F / Q1)*R2 = K * Q2

    Divide by Q2 on both sides

    (F / Q1)*R2*Q2 = K

    See how for each step we remove one thing that is on the wrong side of the equation?
  4. Apr 15, 2010 #3
    I can't understand what you mean to say but I guess this would be your answer:

    K = F* r^2/( Q1 * Q2)
  5. Apr 15, 2010 #4
    Oh....I thought the R2 referred to something you put into the equation. Not R^2. I suppose I shouldn't really comment when i don't know the Law. :( Sorry.
  6. Apr 15, 2010 #5
    OP didn't ask us for the law. He asked to rearrange and anybody can try to explain him. No fault of Yours.

    Edit: And your way of explanation is really amazing.
  7. Apr 15, 2010 #6
    Thanks. :)
  8. Apr 15, 2010 #7
    Thanks for the really quick replies - im sorry i wasnt clear enugh still getting the hang of trying to type equations on the board.

    I tried to follow what u said bobbajob

    i got something along the lines of K=(sqr root)of F/Q1Q2 but im guessing im miles off
    The equation in its true form is at http://en.wikipedia.org/wiki/Coulomb's_law and is the first one on the page if it makes it clearer.

    Thanks again
  9. Apr 15, 2010 #8
    No worries, this will sound a bit patronising but start with some easier equations (Now that you know your answer.)

    Y'know, REALLY easy ones like the DST triangle and stuff.
  10. Apr 15, 2010 #9
    yeah i guess its just practice but my brain seems to be wired back to front when it comes to physics - ill have nother go at this one and see how i get on. i thought i would have to use a square root when i moved the r^2 across.

    thanks for the advice guys
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