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Homework Help: Determining the magnitude of the charge (Coulombs Law)

  1. Oct 21, 2008 #1
    1. The problem statement, all variables and given/known data

    Two equal charges exert an electric force on each other of 0.0385N when positioned 0.15 m apart. Determine the magnitude of the charge on each point.

    2. Relevant equations


    3. The attempt at a solution

    I narrowed it down to



    I squared this and I get q1q2=310.4143415C

    This is not the answer to the question. Im close as far as the numbers go but im not sure what im missing here.
  2. jcsd
  3. Oct 21, 2008 #2


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    Could be the number of significant digits.
    Could be that the charges could either both be plus or minus, but they do only ask for magnitude.
  4. Oct 21, 2008 #3
    The answer given is 3.1 x 10-7 C

    The answer I got was 3.1 x 10^2C right? So I figure the numbers are correct, just something missing, or maybe even the answer in the book is wrong.
  5. Oct 21, 2008 #4


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    Recalculate it again then. (K is 9*109 And you are dividing by it.)
  6. Oct 21, 2008 #5
    yeah, thats been done.
  7. Oct 21, 2008 #6


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    If you divided by 109 and you got 102 ... that's a lot of 10's to be off.

    Why don't you type out your whole calculation?

    On the face of it you have 10-2*10-1*10-1/109

    102 seems like an unlikely result.
  8. Oct 22, 2008 #7
    I have here

    qq=0.0385N x (0.15)(0.15) / 8.99x10^9

    So I get

    8.66x10-4 / 8.99x10^9
    Last edited: Oct 22, 2008
  9. Oct 22, 2008 #8


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    And that gives 9.62 x 10-14

    And taking the square root ...
  10. Oct 22, 2008 #9
    "And that gives 9.62 x 10-14"

    I wish I could copy and paste what my calculator is saying because I just dont get that answer.
  11. Oct 22, 2008 #10
    I tried it with an oline calculator and I got what you got, so im not sure if the settings on my calculator are just messed up.
  12. Oct 22, 2008 #11


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    Nothing like pencil and paper.

    If in doubt, work it out.
  13. Oct 22, 2008 #12
    Its been a while since ive done that. How do you divide exponents? Can you give me a quick crash course?
  14. Oct 22, 2008 #13


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    Just look at post 6:

    Dividing by 10 makes it 10-1

    Dividing by 109 makes it 10-9

    Dividing by exponents subtracts. Multiplying adds.

    Don't let your teacher know you didn't know.
  15. Oct 22, 2008 #14
    OK see there we go.

    I have 8.66x10-4 / 8.99x10^9 right?

    Im taking 8.66x10^+4 and dividing it by 8.99x10^+9

    But if im dividing an exponent im supposed to make the +9 a negative 9?
  16. Oct 22, 2008 #15


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    That should be -4 and dividing by 109 makes that 10-13 and the numerator being less than denominator makes it 9.xx * 10-14.
  17. Oct 22, 2008 #16
    That was a typo on my part.

    That makes perfect sense, I still dont get though why my calculator spits out 96329.25473

    This is on a TI83 sci. calulator


  18. Oct 22, 2008 #17


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    I'd recommend not using that calculator to figure orbital trajectories for a Mars landing then.

    Maybe use it for grocery shopping?
  19. Oct 22, 2008 #18
    its going to be used as a stool for my radio here if it doesnt smarten up.
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