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!

Why is electric potential energy U defined negative in this example?

  1. Oct 19, 2009 #1
    1. The problem statement, all variables and given/known data
    From Young and Freedman's book University Physics, ch 23, problem 23.54:
    In the Bohr model of the hydrogen atom, a single electron revolves around a single proton in a circle of radius r. Assume the proton remains at rest. ++ (the rest is irrelevant to my question).

    2. Relevant equations
    In the solution of this problem, U is defined as
    Code (Text):
    U = -k e[SUP]^2[/SUP] / r
    .

    3. The attempt at a solution
    I've been staring at this equation for a long time. I'm probably just too tired. But why a minus in front of the equation? Why not just
    Code (Text):
    U = k e[SUP]^2[/SUP] / r
    ?

    I'm feeling stupid right now! ;)

    Thank you in advance:)
     
  2. jcsd
  3. Oct 19, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The force is attractive--it requires work to move the electron away from the proton, thus the potential energy increases as r increases. Evaluate each expression at a few radii and see how the minus sign changes things.

    (Compare a similar expression for gravitational potential energy.)
     
  4. Oct 19, 2009 #3
    Thank you very much for a fast and clear explanation! All confusion is gone:)
     
  5. Oct 19, 2009 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Note that U = 0 at r = ∞.
     
  6. Oct 19, 2009 #5
    Right right, so this electric potential energy would be analogous to a topographical "hole"... (?)
     
  7. Oct 19, 2009 #6
    Yup, a potential well is the common analogy to such systems (gravitational etc.)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why is electric potential energy U defined negative in this example?
Loading...