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Homework Help: Microcoulombs Question

  1. Apr 14, 2009 #1
    Ok, the question is:

    A, B and C are three small insulated conductors all carrying positive charges of equal size. A exerts a force of 2.5micronewtons on B. The distance between A and B is 2[tex]\sqrt{3}cm[/tex] and the distance between B and C is 2cm. What is the magnitude of the force exerted by B on C?

    I know how to do all of it, since all the charges are the same, I've worked out the charge of A, B and C to be 5.77x10^-9C (I've checked and re-checked again)

    So then I worked out the force between B and C and I continually get 7.5x10^-4N. When converting this to micronewtons, it is 750micronewtons.

    According to the textbook however, I am wrong and by two decimal points actually. Apparently, the answer is 7.5micronewtons.

    I've checked time and time again, made sure that I converted the micronewtons to Newtons (SI units) when calculating and converting the cm to m in regard to SI units. I don't know what I've done wrong. Could someone please check for me, because the textbook is prone to errors sometimes.
    Last edited: Apr 14, 2009
  2. jcsd
  3. Apr 15, 2009 #2


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    Homework Helper

    I got 7.5 micro newtons.
    So please show your calculations.
    I can rewrite the above question in this way.
    Two identical charges repel each other with a force of 2.5 micro newton when they separate by a distance 2*sqrt3 cm. If their distance is 2 cm, what is the new force of repulsion?
  4. Apr 15, 2009 #3
    Don't worry, I know what I did wrong. I squared 2*sqrt3 and THEN divided by 100 which I wasn't meant to do. I had to do it the other way round.

    Such small mistakes are quite costly.
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