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Proton and Electric Field

  1. Mar 2, 2005 #1
    For a proton moving in the direction of the electric field

    a. its potential energy increases and its electric potential decreases
    b. its potential energy increases and its electric potential increases
    c. its potential energy decreases and it electric potential increases
    d. its potential energy dcreases and its electric potential decreases


    Originally, I thought the answer was b, but this is wrong. Exactly why is b incorrect and what exactly is the correct choice?

    Thanks.
     
  2. jcsd
  3. Mar 2, 2005 #2
    electric potential energy [tex] U = k \frac{q_{1} q_{2}}{r} [/tex]
    electric potential [tex] V = k \frac{q_{not proton}}{r} [/tex]

    since the proton moves in line with the field, it is moving + to - or vice versa??
    Now that you knwo the direction you can see waht happens in the initial and final conditions
    calculate the electric potential energy and electric potential of the proton when it close to the positive end and the negative end, assuming that there are TWO charges of opposite sign creating this field. You know the direction which is bigger than which??
    if the values went down then it obviously decreased and if the values went up then increase
     
  4. Mar 2, 2005 #3

    DB

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    Hey stunner. A couple questions:
    What's "k"?
    And what does the charge in the electric potential equation stand for if its not that of the p+?
     
  5. Mar 3, 2005 #4

    Doc Al

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    Staff: Mentor

    Here's a hint: Positive charges accelerate toward regions of lower potential. (Assuming there are no other forces acting on them.) How do potential and potential energy relate?

    Compare this to a ball falling in a gravitational field. Which way does the field point? Where is the PE greater?
     
  6. Mar 3, 2005 #5
    [tex]k = \frac{1} {4 \pi \epsilon_{0}}[/tex]
     
  7. Mar 3, 2005 #6

    DB

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    Ahh, so electrical potential energy is the same as coulombs law?
     
  8. Mar 3, 2005 #7
    No, Coulomb's law is for the force between two charges.
     
  9. Mar 4, 2005 #8
    Well, since that hint is true, is it d where both decrease?
     
  10. Mar 5, 2005 #9

    Doc Al

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    Staff: Mentor

    That's right. The field points toward the region of lower potential. The proton accelerates as it moves through the field, losing potential energy and gaining kinetic energy. (Assuming no other forces act.)
     
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