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Electric Force Question

  1. Jan 1, 2010 #1
    1. The problem statement, all variables and given/known data

    002 (part 1 of 2) 2.0 points

    Two identical conducting spheres are placed with their centers 0.3 m apart. One is given
    a charge of +1.3 × 10^−8 C and the other a charge of 1 × 10^−9 C. Find the electric force exerted on one sphere by the other. The value of the Coulomb
    constant is 8.98755 × 109 N · m^2/C^2.

    Answer in units of N.

    003 (part 2 of 2) 2.0 points

    The spheres are connected by a conducting wire. After equilibrium has occurred, find the
    electric force between the two spheres.

    Answer in units of N.

    2. Relevant equations


    3. The attempt at a solution

    So, I input all of the numbers into the formula as shown, but the homework system won't accept the answer. Any clue as to why?

    As to the second part, I assume I total the charges and divide it by two, then use that in the equation as both charges...Right?
  2. jcsd
  3. Jan 1, 2010 #2


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    Homework Helper

    Show your calculations.
    Your approach to the second part is correct.
  4. Jan 1, 2010 #3
    For the first part, are you supposed to enter the magnitude of the electric force or the vector notation of the electric force? If you're entering that correctly, try not rounding your numbers- online homework systems can be very fickle (and annoying). Failing that, your calculations could just be wrong so showing them would be helpful.
  5. Jan 2, 2010 #4
    I did 8.98755x10^9*1.3x10^-8*1x10^-9 / .3^2. I get 1.29820166666e-6. I don't know what's wrong there.
  6. Jan 2, 2010 #5
    And part two is also incorrect. I'm at a loss.
  7. Jan 2, 2010 #6


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    Staff Emeritus
    Science Advisor
    Gold Member

    This answer is correct (EDIT: actually what you've written is meaningless because you didn't include units along with your numbers, but I'm going to let it slide this time :tongue:). I'm afraid it's between you and your software. By the way, are you supposed to be following the rules for significant digits when presenting your answers? Because if you interpret the numbers given as measured quantities having some finite precision (rather than being exact quantities), then you know one of the charges to only one significant figure.
  8. Jan 2, 2010 #7
    Ah, well, units aren't needed for input. But thanks for the slack haha.

    And no, sig figs are irrelevant. So I really have no idea. I might wait until we return Monday and ask the instructor. I'm done with everything besides those two questions so rather then guess and lose points I can get a definite answer. Unless anyone has some suggestions.
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