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Electromagnetism - The distance from point a to point b

  1. May 31, 2017 #1
    1. The problem statement, all variables and given/known data
    So I want to know the distance to 2. The proton is at v=0 at the 1.

    05403d58dc4248caa422e87412d91150.png


    We know that:

    q=1.602*10^-19 point 1

    L=1mm

    v=1.1*10^6 at point 2

    F=1.44*10^-12 at point 1

    2. Relevant equations

    E=(1/4πε)*(q/r2)

    ΔV=∫E*dr=(1/4πε)*q∫(1/r2)=(1/4πε)*q*(1/r2-1/r1)

    ΔU=ΔK=mv2/2

    ΔK=mv2/2=ΔV*q=q*(1/4πε)*q*(1/r2-1/r1)

    3) The attempt at a solution


    I can't seem to calculate the distance. I don't know where I've gone wrong.
     
    Last edited: May 31, 2017
  2. jcsd
  3. May 31, 2017 #2

    Charles Link

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    L=1 mm. Can you see that this is also ## r_1 ## ?
     
  4. Jun 1, 2017 #3
    Exactly and I'm trying to find out r2.

    According to my calculations r2=2.28*10^-13 but that seems way too little?
     
  5. Jun 1, 2017 #4

    Charles Link

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    What did you use for the mass of the proton? Also, did you convert ## L ## to meters? Also you need to solve for Q. You can do that because itt tells you the force ## F ## at point 1.
     
  6. Jun 1, 2017 #5
    Yes, the mass of the proton is 1.673*10^-27

    Epsilon=8.854*10^-12

    and q=1.602*10^-19
     
  7. Jun 1, 2017 #6

    Charles Link

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    I see one mistake. You assumed the two q's were equal. See also my edited previous post. You need to solve for ## Q ##.
     
  8. Jun 1, 2017 #7
    So Q=F/E ??

    I'll give it a try and calculate it now, thankyou very much.
     
  9. Jun 1, 2017 #8

    Charles Link

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    ## F=\frac{Qq}{4 \pi \epsilon_o r^2} ##. They give you ## F ##, ## q##, and ## r ##. You need to compute ## Q ##.
     
  10. Jun 1, 2017 #9
    5.87*10^-4m

    THANKYOU! Freaking hell I was so confused
     
  11. Jun 1, 2017 #10

    Charles Link

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    Compute ## Q ## in Coulombs. You need this number for the remainder of the calculations. The answer you gave is incorrect.
     
  12. Jun 1, 2017 #11
    How is that possible?

    ΔK=mv2/2=ΔV*q=q*(1/4πε)*Q*(1/r2-1/r1)

    Q=1.00*10^-9


    ΔK=mv2/2=ΔV*q=q*(1/4πε)*q*(1/r2-1/r1)

    (mv^2*epsilon*m*4*pi)/(2*q*Q)=1/r2-1/r1

    (1.673*10^-27)*)((1.1*10^6)^2)*4*pi*(8.854*10^-12)/(2*(1.602*10^-19)*(1.00*10^-9))=1/r2-1000

    1/r2=1702.97

    r2=5.872*10^-4
     
  13. Jun 1, 2017 #12

    Charles Link

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    Close, but your final term should read ## \frac{1}{r_1}-\frac{1}{r_2} =1000-\frac{1}{r_2} ## . ( ## r_2>r_1 ##). ## \\ ## Once you correctly solve for ## r_2 ##, you then need to compute the distance ## D=r_2-r_1 ##.
     
  14. Jun 1, 2017 #13
    In that case r2 should be= 0.00337m

    r2-r1=0.00237
     
  15. Jun 1, 2017 #14

    Charles Link

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    That's what I got also. :) :)
     
  16. Jun 1, 2017 #15

    Charles Link

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    Now solve for ## D ##. See my edited post #12.
     
  17. Jun 1, 2017 #16
    Yep I saw it, and I edited my post and did it ;)

    r2-r1=0.00237
     
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