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Small clarification: applying Coulomb's Law

  1. Sep 5, 2013 #1
    1. The problem statement, all variables and given/known data

    I'm studying electric charges and fields, and I have a quick question about this example in my book: Untitled-1_zps8eb88ce6.jpg

    Everything is straightforward except the subscripts (13 and 23) on r in the denominators of initial Coulomb equations for F1 and F2. In the next step they just plugged in the distances, so those numbers don't seem to do anything, and there are no subscripts in the original formula. So what are they?

    2. Relevant equations

    F = k [(q1q2)/r2]
  2. jcsd
  3. Sep 5, 2013 #2
    They just indicate that the force one is from charge 1 to charge 3. And the force 2 is from charge 2 to charge 3.
  4. Sep 5, 2013 #3
    Duh, I knew it had to be something obvious. Thank you!
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