1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Work required to bring charge from infinity

  1. Apr 16, 2015 #1
    1. The problem statement, all variables and given/known data
    Two point charges are located on the x-axis, q1 = -e at x=0 and q2 = +e at x=a. Find the work that must be done by an external force to bring a third point charge q3 = +e from infinity to x = 2a.

    2. Relevant equations
    W = -ΔU

    3. The attempt at a solution
    W = -(Ufinal - Uinitial)
    Uinitial = 0 because it is at infinity.
    So W = -Ufinal = -kq3(q1/r1 + q2/r2). After doing this I get:
    -(e^2)/(8πεa) which has the correct magnitude but wrong sign. My book uses W = U instead of W = -U. Is there a reason for this? Work is equal to change in NEGATIVE potential energy, so why would it be positive?
  2. jcsd
  3. Apr 16, 2015 #2

    H Smith 94

    User Avatar
    Gold Member

    This is a issue with conventions stemming from the fact you've used a negative Coulomb potential. By convention the Coulomb force ##\vec{F}(\vec{r})## is negative to imply attraction: [tex] \vec{F}(\vec{r}) = -\frac{q_1q_2}{4\pi\epsilon_0}\frac{1}{r}\cdot \hat{r} [/tex] which gives rise to the Coulomb (or electrostatic) potential ##U(\vec{r})## found by, [tex] U(r) = \int F(r) \, dr [/tex] and so [tex] U(r) = -\frac{q_1q_2}{4\pi\epsilon_0} \int \frac{1}{r^2} dr = -\frac{q_1q_2}{4\pi\epsilon_0} \left( - \frac{1}{r} \right) = \frac{q_1q_2}{4\pi\epsilon_0} \frac{1}{r}.[/tex]
    Either that, or [tex] \Delta U = U_\text{final}-U_\text{initial}.[/tex]
  4. Apr 17, 2015 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    You know from the basic charge layout that it will take positive work to bring the positive charge to x=2a. Try to think that way in general instead of relying on the math to give you the correct signs.
  5. Apr 17, 2015 #4


    User Avatar
    Homework Helper

    The work of the electric field is equal to change in negative potential energy, but the work that must be done by an external force is opposite.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted