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Potential difference question

  1. May 25, 2007 #1
    Potential difference question....

    1. The problem statement, all variables and given/known data

    A proton (mass = 1.67 10-27 kg, charge = 1.60 10-19 C) moves from point A to point B under the influence of an electrostatic force only. At point A the proton moves with a speed of 60 km/s. At point B the speed of the proton is 80 km/s. Determine the potential difference Image .

    2. Relevant equations
    Vb-Va = 0

    3. The attempt at a solution
    a) +15v
    b) -15V
    c) -33V
    d) +33V
    e) -20V

    im stumped with this one....any help would be appreciated.
  2. jcsd
  3. May 25, 2007 #2


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    delE = q(delV)


    Change in E(energy) = charge*change in V(potential)

    for starters
  4. May 25, 2007 #3


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    actually, I don't get near any of the answers there. Don't take my word for it, let me look into it more
  5. May 25, 2007 #4


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    ok, I did get within .4 of one of the answers there (i wasn't paying attention to units when I calculated last time)

    so that way should work, methinx.
  6. May 25, 2007 #5
    i get within .4 of the answers a well

    i guess they want you to round them off

    now decide which is right... a or b
  7. May 25, 2007 #6


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    if you did it right, you should get either a or b, not both. They are 30 units apart.
  8. May 25, 2007 #7
    I get point b about -14.6 volts relative to point a. Can't tell from the question whether that means answer a or b is correct. Probably b :uhh:
  9. May 27, 2007 #8


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    I didn't bother doing any calculations, but I can tell you the sign of the answer. Think of it this way: the final velocity was greater than the initial velocity, right? So the particle's kinetic energy increased. It moved under the influence of a conservative force solely, so the increase in kinetic energy came at the expense of a(n) ______________(increase/decrease) in potential energy. As a result, considering that the charge is positive, its final electric potential is ______________ (larger/smaller) than its initial electric potential. Therefore, the change in electric potential between the two points (final minus initial) is ______________ (positive/negative).

    Fill in the blanks....:tongue2: :devil:
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