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Force of repulsion

  1. Aug 18, 2011 #1
    1. The problem statement, all variables and given/known data

    An alpha particle has a mass m = 6.64x10^-27 kg and a charge q = +2e.
    Compare the force of electric repulsion between two alpha
    particles and the force of gravitational attraction between them. Explain briefly
    why the gravitational force is ever significant, given its seeming insignificance
    here.

    2. Relevant equations
    F=qE, F=kqq/r, F=ma


    3. The attempt at a solution
    coloumbs law cant be applied here since im not given r
    and i cant think of a way to obtain the E_field or a in f=ma when i equate qE=ma
    (maybe Electric flux can be found using gauss's, but i dont think thats the right way to go)

    Some guidance would be much appreciated

    thanks
     
  2. jcsd
  3. Aug 18, 2011 #2

    fzero

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    Both the magnitude of the electromagnetic force and that of the gravitational force depend on the distance between sources. Can you come up with a mathematical combination of the two that is independent of the distance?
     
  4. Aug 18, 2011 #3
    im not sure how eliminate the distance

    F_e=kqq/r
    F_g=Gmm/r^2

    equating those 2 will still leave me with distance
    I cant think of any other formula for F_e that doesnt require finding the electric field or using the distance
     
  5. Aug 18, 2011 #4

    fzero

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    First, go back to your text or notes and find the correct expression for the Coulomb force (what you have is the potential energy). Second, you don't really want to equate the EM and gravitational forces, but the correct expression should be clearer once you can compare the correct dependences on the separation.
     
  6. Aug 18, 2011 #5
    oh sorry i forgot its squared...
    ahh divide both the expressions to find the ratio?
     
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