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Coulombs Law problem

  1. Sep 10, 2013 #1
    I know almost this exact thing has been asked before but even after looking at the other posts I still can't figure out what I'm doing wrong.

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

    Two very small 8.05-g spheres, 11.0 cm apart from centre to centre, are charged by adding equal numbers of electrons to each of them. Disregarding all other forces, how many electrons would you have to add to each sphere so that the two spheres will accelerate at 25.0g when released?

    2. Relevant equations
    F=ma
    F=k(q1q2/r^2)
    q=ne

    3. The attempt at a solution

    F=ma=F=k(q1q2/r^2)

    F= (8.05g)(25)(9.8m/s)=1972.25

    1972.25(.11m)^2=(8.987e9)Q^2

    (23.86)/(8.987e9)=Q^2

    Sqrt(2.66e-9)=Q

    5.15e-5=Q

    Q=ne

    n=5.15e-5/1.6e-19

    =3.22e14 electrons
     
  2. jcsd
  3. Sep 10, 2013 #2
    What exactly are you asking us to do? Are you getting the wrong final answer?

    I would recommend double-checking to make sure you've used the correct units for your calculations - in particular you need to convert the mass from grams to kilograms before doing anything else.
     
  4. Sep 10, 2013 #3
    I converted mass to kg and mastering physics still said I was wrong but now it was close enough for them to just think it was a rounding or sig fig error. It was my last attempt so they showed the correct answer was 1.02e13 and I got 1.71e12. I'm sure I probably typed something wrong into my calculator, thanks for you help though :)
     
  5. Sep 11, 2013 #4
    You should repeat your initial calculations using kg instead of g (as pointed out in post #2).
    You will then get the correct answer.
     
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