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Find the distance between the two electrons

  1. Nov 27, 2007 #1
    I have this problem that I'm stuck on



    An electron is released above the Earths surface. A second electron directly below it exerts just enough of an electric force on the first electron to cancel the gravitational force on it. Find the distance between the two electrons.

    The distance is 5.07m......I need to know how to get that. Thanks
     
  2. jcsd
  3. Nov 27, 2007 #2

    rock.freak667

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    The electric force between two charges ,Q[itex]_1[/itex] and Q[itex]_2[/itex] is given by
    [tex]F=\frac{Q_1 Q_2}{4\pi \epsilon_0r^2}[/tex]

    the gravitational force of attraction between 2 masses M[itex]_1[/itex] and M[itex]_2[/itex] is given by
    [tex]F=\frac{GM_1 M_2}{r^2}[/tex]

    you will need to use these equations...try something and tell me what you did
     
    Last edited: Nov 27, 2007
  4. Nov 27, 2007 #3
    Yes but I do not have the force or the distance....so i dont think that equation can allow me to find r....I'm figuring I have to do something with the gravitational force formula also
     
  5. Nov 27, 2007 #4

    rock.freak667

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    Assume the distance between the first election and the earth is the radius of th earth...which is 6x10[itex]^24[/itex]kg
     
  6. Nov 27, 2007 #5
    thats all i did...i pluged in the numbers that we have such as Kc Q1 Q2 G M1 and M2 and got (8.99X10^9) (-1.6X10^-19 * -1.6X10^-19)/(r^2) and the gravitational one is (6.6732X10^-11) (9.109X10^-31 * 9.109X10^-31) /(r^2)...thats as far as I got......I tried setting them equal...and tried finding r that way....but it obviously doesn't work since the r's are the same......I also tired using r=5.07 and working backwards somehow but all of those attempts didn't work out
     
  7. Nov 27, 2007 #6

    rock.freak667

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    For the gravitational force put r as the radius of the earth
     
  8. Nov 27, 2007 #7
    can you explain why?
     
  9. Nov 27, 2007 #8
    because to me that doesnt make sense
     
  10. Nov 27, 2007 #9

    rock.freak667

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    Well I can only guess that above the Earth's surface means that the electron is more or less near to the surface of the earth and if you were to add the radius of the earth and the height above the earth's surface, then the distance between the masses(i.e. 'r') would still be 6x10[itex]^6[/itex]m. The addition of some small distance say 1m added to 6x10[itex]^6[/itex]m would still be 6x10[itex]^6[/itex]m in standard form and wouldnt affect the output very much...
     
  11. Nov 27, 2007 #10
    well i put that in and it still doesnt work
     
  12. Nov 27, 2007 #11

    rock.freak667

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    Are you sure? because i did and got 5.08m but i didnt use the permittivity of air...which the question didnt state if this was done in a vacuum or not
     
  13. Nov 27, 2007 #12
    wait how did you do it then?
     
  14. Nov 27, 2007 #13

    rock.freak667

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    [tex]F_G=F_E[/tex]

    [tex]\frac{GM_1 M_2}{R^2}=\frac{Q_1 Q_2}{4\pi \epsilon_0 r^2}[/tex]

    [tex]\frac{(6.67*10^{-11})*(6*10^24)(9.11*10^{-31}}{(6.4*10^6)^2}=\frac{(1.60*10^{-19})^2}{4\pi(8.85*10^{-12})r^2}[/tex]

    rearrange for r^2
     
  15. Nov 27, 2007 #14
    i think you used the wrong equation for electric force because i used Kc(q1 q2/r^2)....and Kc is 8.99x10^9....and thats what you are supose to use and if you do its not the same answer
     
  16. Nov 27, 2007 #15
    k is just 4pi * 8.85x10^-12 inversed rcs9390.
     
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