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Work Done in an Electric Field

  1. Oct 6, 2015 #1
    Suppose we have an electron in a uniform electric field by two parallel plates A and B. (Plate A is higher in potential than B), and we have an electron on plate B moving to A, am I right to say that the electron is losing its own EPE to convert it to KE and there is no work done by electric field on it as it is only moving there with its own energy?
    Then if its moving from A to B, work is done on the electron by an external force?
  2. jcsd
  3. Oct 6, 2015 #2


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    Staff: Mentor

    The change in the electron's potential energy, ΔPE, is just another way of talking about the work WE done by the electric field on the electron (with a minus sign thrown in). So if you're talking about the situation in terms of PE and KE, you don't also talk about the WE done by the field, because that would be at least redundant, and at worst, counting the energy involved twice. You talk only about the W done by whatever other (non-conservative) forces are involved in the situation.
  4. Oct 6, 2015 #3
    In both cases there is work done by the electric force on the electron. The signs of this work will be different.
  5. Oct 6, 2015 #4
    So if work is done by electric for on the electron ( aka electron moving from a lower to a higher potential plate ) it is positive while the other way round ( electron moving from higher to lower potential plate ) will have a negative work. Am I right ? Then what about it's EPE and KE , is the EPE/KE the work done ?
  6. Oct 12, 2015 #5
    This confusion arises because the "system" and the "surroundings are not clearly identified. There are two ways of looking at it the problem
    1. If the electron is the system, then:
    (a) it has kinetic energy
    (b) the surroundings do work on it, positive or negative. This is the work done by the electric field
    (c) the kinetic energy of the electron changes by an amount equal to the work.
    (d) There is no other potential energy in this description.
    2. If the system is the electron + the field of the capacitor, then:
    (a) there is kinetic energy of the electron
    (b) there is potential energy of interaction between the electron and the field, which depends on the position of the electron
    (c) the total mechanical energy, the sum of item (a) and item (b), is constant
    (d) so as the electron moves, the potential energy changes, and the kinetic energy changes in a way that keeps the total energy constant.
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