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!

Potential Difference in case of electrons

  1. Mar 17, 2014 #1
    If the potential difference between point A and point B is 10 Volts, then when a unit positive charge passes from A to B, the charge loses 10 J of energy.

    But when an electron passes from A to B does it gain energy, because in W = Q . V , Q is negative.

    I am really confused in potential difference and in the formula W = Q . V.

    I have seen in some books that when electron is said in the question the calculation use the value of positive elementary charge, why is it so?
  2. jcsd
  3. Mar 18, 2014 #2


    User Avatar
    Science Advisor
    Gold Member

    In physics the current is defined in terms of a positive flow of charge; in ordinary wires and batteries this positive current is the negative of the actual flow of electron charge.

    In some engineering contexts they concentrate on the electron flow; in this case they use a different convention: the positive current is the flow of electrons. Hence they are treating the electrons as "positive" in some sense. This is useful when you are studying electronic devices.

    The question of "work done by" or "work done on" requires a change of sign. But the absolute values are the same.

    So you just have to understand the conventions being used by the text.
  4. Mar 18, 2014 #3
    You must be confused (or may be the book is confused). The electron has negative charge so, as you suspected, an increase of its potential V corresponds to a decrease of its potential energy U because U = q V and q is negative for an electron.
  5. Mar 18, 2014 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    When an electron is accelerated across a gap with a PD across it, it ends up with KE. When it passes through a resistance, the small amount of KE that it gains, is negligible. In both cases, eV energy is supplied (and transferred) so the work done (force times displacement, in the end) on it is positive.
    You can be confident that there is no self contradiction in the system. All you need to do is to go over your problem and take strict account of the signs. This will give you the right answer, which will be that work is done on the electron in both cases. The resulting energy of the electron is either passed on to the material of the resistor (heating) or on the material of the Anode (heat, also).
    If your book seems to be getting it wrong, look at another book and that may resolve your difficulty.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook