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Question about common math operations.

  1. Apr 2, 2010 #1
    Okay say we know from magnetism.

    E = F / q
    F = KQq / R^2

    Then we can say that.

    E = KQq / (R^2 / q)

    Then wouldn't we say
    E = KQq * q / R^2
    E = KQq^2 / R^2

    Its just like saying.
    5 / (1 / 2) = 5 * 2 / 1 = 10.

    Why is it we cancel the q's instead of squaring them, because my textbook does that and ends up with this equation.
    E = KQ / R^2

    Because the answers are different. Thanks.
    Last edited: Apr 2, 2010
  2. jcsd
  3. Apr 2, 2010 #2


    Staff: Mentor

    No, we can't say this.
    It's really E = (KQq/R^2) / q, which is the same as (KQq/R^2) * (1/q). The order in which you do the division is significant.

    It might be easier to see formatted as it usually is in print.
    [tex]E = \frac{F}{q} = \frac{1}{q} F[/tex]
    [tex]= \frac{1}{q} \frac{KQq}{R^2}[/tex]
    [tex]= \frac{KQ}{R^2}[/tex]

    The q factors cancel because you have one of them in a numerator and the other in a denominator.
    This is not right, either. 5/(1/2) = 5*2 = 10
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