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Homework Help: I think this is coulomb's law

  1. Jan 18, 2006 #1
    ok, im trying to do this "prelab" thing for my lab tomorrow, and there is one question i just can NOT figure out. the question is

    The weight of a 0.6 kg object of charge Q is just balanced by another object of equal but opposite charge fixed to a support 107 cm above it. What is the magnitude of the charge Q (in μC)? [Note that 1 μC=1x10-6C.]

    i understand how coulomb's law works, with F=k(q0q1)/r^2 but what i am having trouble figuring out, is what can i do to get an answer here? i have the mass, and the distance (r). i was thinking to use the force of gravity formula, and set it equal to coulomb's, but i looked at an example in the book where they are given q0 and q1, and they found both gravity and coulombs, and the answers were not equal. so i figured that must mean that is not the right way to do it.....

    any ideas? sorry so long, its been bugging me for a while now... any help is appreciated

    thanks in advance
  2. jcsd
  3. Jan 18, 2006 #2


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    It tells you that the object's weight is just balanced by the electric force on it from the opposite charge. You already know the object's weight, right? So from that and Coulomb's law you can easily calculate the required equal and opposite electric force to balance it.
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