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Homework Help: Work done by an External force

  1. Jun 6, 2014 #1
    1. The problem statement, all variables and given/known data

    Screen Shot 2014-06-06 at 5.05.49 PM.png

    Point charges, Q1 = +56 nC and Q2 = -98 nC, are placed as shown. In Figure 18.1a, an external force transports an electron from point A to point B. The work done by the external force is closest to:
    A) +620 eV
    B) -920 eV
    C) -620 eV
    D) +920 eV
    E) +770 eV

    2. Relevant equations

    v= kq/r
    W= e (Vb-Va)

    3. The attempt at a solution

    i know i'm doing this completely wrong. i tried to get the electric potential at A from charge 1 & 2 and then added them. i did the same for the electric potential at point b from charge 1 & 2 . once i had the summed potentials i found the difference and then multiplied by the charge of an electron .

    for A[(9 x 10^9) x (56 x 10^-9)/.6] + [(9x 10^ 9) x (-98 x 10^-9)/1 ] = -42 V

    for B : [(9 x 10^9) x (56 x 10^-9)/1] + [(9x 10^ 9) x (-98 x 10^-9)/.6] =-966

    Vb-Va = -924 V
    -924 x (1.6 x 10 ^-19 ) = -1.47 x 10 ^ -16 eV which is nowhere close.

    i must be approaching this the wrong way.
    Last edited: Jun 6, 2014
  2. jcsd
  3. Jun 6, 2014 #2


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    Your method seems fine. What do you get for the potential at B and for the work done?
  4. Jun 6, 2014 #3
    i just edited and added the rest of my calculations in the original thread post! i must be doing something wrong .
    the answer is supposed to be D) +920 eV
  5. Jun 6, 2014 #4
    Your calculated energy is measured in Joules. You want it measure in eV's so that you can compare it with the possible choices. Hint: do not plug in the value of the constant e. Leave it symbolically as e.

    EDIT: also keep in mind that the charge of the electron is -e, not e.
  6. Jun 6, 2014 #5
    so rather than multiplying it i would just leave my answer in joules but basically multiply by -1 because of -e which would leave me with about +920eV?
    Last edited: Jun 6, 2014
  7. Jun 6, 2014 #6
    Yes except that you're not leaving your answer in Joules. You're leaving it in eVs. I assume that was a typo.
  8. Jun 6, 2014 #7
    yes it was sorry haha. that makes sense thank you!

    so why exactly don't you multiply it by the e constant?
  9. Jun 6, 2014 #8
    You do multiply by the e constant symbolically at the unit. The unit of volts "V" times the constant "e" equal the unit electron-volts. Symbolically you have
    e * Volt = electron-volt or
    e V = eV.
  10. Jun 6, 2014 #9
    oh wow okay that makes so much more sense ! thank you so much for all your help!
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