Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electric potential- equations

  1. Apr 27, 2013 #1
    We have two equations

    delta EP = -W

    V = -Ed

    Now, to my understanding for the second equation (E = -V/d) since its a vector quantity, we need to make it have direction

    Therefore, if we substitute values E = -50V / 5 m = -10 N/C

    We add in the negative sign to the equation because generally:

    A proton would move DOWN, therefore the negative sign represents down

    -------------------(+)


    -------------------(-)

    But the confusion stems from the second equation

    -------------------(+)
    A

    B
    -------------------(-)

    If we have a proton that moves from A to B delta EP would be negative

    So -d EP = -W

    Work done is positive

    But if we move it from point B to A, we exert force on it and therefore work done is negative (according to the equation)

    This is extremely weird for me though, because in the first equation we're assuming down is negative and up in positive (like we have been for the rest of the units we've learned)

    Yet for the second equation, we assume the exact opposite! Can someone clear up my confusion?

    Thanks
     
  2. jcsd
  3. Apr 27, 2013 #2

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    There is no contradiction. Electrical potential is defined for a unit Positive charge. The energy for a negative charge would have the opposite sign. To move a proton in a given situation would require equal and opposite work compared with the same operation with an electron. Put the appropriate signs in your equations and the right answer will emerge. Do it for yourself and adhere scrupulously to the signs, without jumping ahead in your mind (which it can be all too easy to do). You will find it works out. No 'verbal' explanation should benecessary.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook