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Homework Help: Proportionality calculations (Coulombs Law)

  1. Apr 25, 2008 #1
    Two unlike charges attract each other with a force of 5 x 10^-5 when placed 12 cm apart.

    1) What is the force between them when they are 4.0cm apart

    2)At what distance apart will the force between them be 2.0 x 10^-4

    now i have attempted these questions, but i cannot find a suitable method of solving these questions that is consistent with all types of these questions...
    Can anyone be willing to show me a method of solving these types of questions.
  2. jcsd
  3. Apr 25, 2008 #2
    you know Coulombs Law?

    [tex] F = \frac { k q_1 q_2 }{r^2} [/tex]

    In this problem you have F = 5 x 10^-5 N if r = 5 cm. What would F be if r = 4.0 cm
  4. Apr 26, 2008 #3
    yes i know coulombs law, its just the question is worked out using proportionality from coulombs law
  5. Apr 26, 2008 #4


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    Homework Helper

    Hi whitestrat,

    Coulomb's law is an inverse square law, so, for example, if you double the distance between the charges the force decreases by a factor of four; if you halve the distance, the force increases by a factor of four.

    Here the distance goes from 12cm to 4cm. So what happens to the force?
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