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Variation statement into graphs- Right?

  1. Jul 29, 2011 #1
    My first post, yay i already like the atmosphere here :P anyway....

    Using the formula F = kQq/R2 sketch graphs between
    a. F and Q (k,q, and R are constant)

    b. F and R (Q,q and k are constant)

    c. Q and R (k,q and F are constant)


    I think i did it correctly but I'm not quite sure.

    a) I wrote that F is directly proportional to Q so i guess its Direct Variation graph.

    b) Is 1 / R^2 inversely proportional? And is the graph the same as y= 1/x or should it be y=SQUAREROOT of 1/x but then it wouldn't make sense?

    c) i got it so that it's R = SQUAREROOT of Q so it's direct root graph?

    I hope that's correct! Can anybody double check?

    ----------------
     
  2. jcsd
  3. Jul 29, 2011 #2

    PeterO

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    Your (a) sounds good

    In (b) you said it was 1 / R^2, then immediately inversely proportional. What happened to the square?

    (c) is a true statement, but why express R in terms of Q, when the question said Q in terms of R ?
     
  4. Jul 29, 2011 #3
    Thanks for your reply,
    I suspected it could be Square.root 1/R.

    and does it really make a difference if you express it in the other way?
     
  5. Jul 29, 2011 #4

    PeterO

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    I have not seen many people claim the radius of a circle varies as the square root of the Area when asked to draw a graph showing the relationship between the Area and radius of a circle.

    The test of whether it makes any difference would be when you change the variables you graph to get a straight line so that you can determine the complete relationship.

    And I don't think the second one is Square.root 1/R. The way you are writing them, it could be Square.root 1/F
     
  6. Jul 30, 2011 #5
    I'm really confused :( I'm trying to understand it but i'm having trouble grasping the concept - F is proportional to R^2 since the R^2 came from F=kQq/R^2. So why doesn't that make it an inversely proportional graph or what is it even supposed to be- this is really frustrating :(
     
  7. Jul 30, 2011 #6

    PeterO

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    F is proportional to 1 / R^2 - but I think you just mis-wrote it.

    if you were to graph y = 1/x you would get a graph showing inverse proportion.

    But there is also y = 1/x^2 - the inverse square, y = 1/x^3 , the inverse cube etc.
     
  8. Jul 30, 2011 #7
    Alright that makes sense, thanks for all your help!
     
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