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Electric potential energy

  1. Aug 10, 2007 #1
    1. The problem statement, all variables and given/known data

    In a model for the hydrogen atom, an electron is in circular orbit at mean distance Ro from the nucleus.

    Sketch U as a function of seperation distance r. Write an algebraic expression for the electric potential energy U for a model of this system, i.e. charges +e and -e seperated by distance Ro. Be sure to state the reference you have used for zero potential energy.

    The attempt at a solution

    The only relevant (?) formula I could find is this one:
    V=q/(4pi*εo*r)

    How I can sketch the function or write the algebraic expression from that is beyond me though..:bugeye::confused:
     
  2. jcsd
  3. Aug 10, 2007 #2
    "charges +e and -e seperated by distance Ro"
    so, you know q, and you have the algebraic expresion.
    "V=q/(4pi*εo*r)"
    And the independent variable is R(or = x).

    and put V on the y axis.
     
  4. Aug 10, 2007 #3
    for q do I insert the charge on an electron, 1.60217646 × 10-19.

    Edit: figured it out using this method...

    [​IMG]
     
    Last edited: Aug 10, 2007
  5. Aug 10, 2007 #4
    huh, so i forgot to square radius?
     
  6. Aug 10, 2007 #5

    learningphysics

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    No no never mind... I totally messed. You did evernthing right tnp, I'm messing up here. sorry lol.
     
  7. Aug 10, 2007 #6
    ?
    I did?

    charges +e and -e, hence q1 = -1.6*10^-19 and q2 = 1.6*10^-19
     
  8. Aug 10, 2007 #7

    learningphysics

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    Yeah... that's what you did in the diagram... you used q, and Q... I didn't notice that...
     
  9. Aug 10, 2007 #8
    lol, had my confused there for a moment!
     
  10. Aug 10, 2007 #9

    learningphysics

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    lol. sorry.

    one thing though, I don't think a negative r makes sense here since r is the distance... the potential energy between a positive and negative particle is always negative...
     
  11. Aug 10, 2007 #10
    So I wipe the negative x-axis. Thanks for pointing that out!
     
  12. Aug 10, 2007 #11
    Got another Q,

    Points A,B,C and D are at corners of a square of sides 2*2μm. Charges of +1nC and -1nC are placed at corners A and B respectively. What is the change in the electric potential from C to D.

    A------B
    | |
    | |
    C------D
     
  13. Aug 10, 2007 #12

    learningphysics

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    Find the potential at D... find the potential at C... subtract the two.
     
  14. Aug 10, 2007 #13
    I guess you don't need to do any calculations, just analyze the diagram carefully!
     
  15. Aug 10, 2007 #14
    Using this formula?
    [​IMG]

    not sure what you're getting at :bugeye:
     
  16. Aug 11, 2007 #15

    learningphysics

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    If there were two different r's r1 and r2 for q1 and q2, then it would be right...

    The potential due to a point charge at a point r away is kq/r or q/4(pi*e0*r)

    What is the potential at C due to the charge at A?
    What is the potential at C due to the charge at B?
     
  17. Aug 11, 2007 #16
    Does the sign of the charge matter?

    For potential at C due to charge at A, I get
    (8.99*10^9)(1*10^-9)/(2*10^-6) = 4495000V
    Quite a large figure!

    For potential at C due to charge at B, I get
    (8.99*10^9)(1*10^-9)/(2*root(2)*10^-6) = 3178445V

    Total potential at c = 3178445V + 4495000V = 7673444V
     
  18. Aug 11, 2007 #17

    learningphysics

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    Yes, sign matters... keep the sign of the charge.
     
  19. Aug 11, 2007 #18
    In that case, total potential at C = 1316555V

    and

    total potential at D = -1316555V

    I want the change in the electric potential from C to D, so I subtract C from D?
     
  20. Aug 11, 2007 #19

    learningphysics

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    Yup that's right.
     
  21. Aug 11, 2007 #20
    cool,
    change in electric potential from c to d = -1316555 -1316555 = -2633110V

    Final part of the question,
    A 3rd charge of +2nC is initially placed at C and then moved to D. What is the change in the potential energy of the system? Take care with sign
     
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