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Current Electricity: Help understanding concept of potential energy

  1. Feb 20, 2007 #1
    Current Electricity: Help understanding concept of "potential energy"

    This is not a problem that I need help with. I rambled on and on for 5 minutes with teacher, eventually giving in and pretending that I understood..
    http://i80.photobucket.com/albums/j199/pugfug90/CEPE.gif [Broken]
    Perhaps I'm having more of a hard time than necessary because I keep thinking magnets when I see this diagram..

    Basically, the diagram is representing a "situation" "as is".. When you manipulate the lower case +"q" to go towards +Q, PE increases, but PE decreases.. What? Or when you move +q away from Q-, PE increases but potential energy decreases? Too confused..

    PS Doc Al, I will post a follow up when I get my test back. Your method is easy enough that I'm sure I won't need your verification of your method:smile:
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Feb 20, 2007 #2
    Assume the 2 large Qs are equal in magnitude. The potential along the vertical line equidistant from both charges is zero. It looks like q is on that line now. At a point to the left of that line, the +Q counts for more so the EPE of charge q is increased. At a point to the right of that line, the -Q counts for more so the EPE of charge q is decreased

    If the +q charge is moved to the left, then the electric potential (voltage) at a point to the right is decreased because the +q charge is farther away. But the EPE of charge -Q is increased because the energy is less negative
  4. Feb 20, 2007 #3
    Is there a difference between my teacher's "Potential Energy" and "PE"?
    "the +Q counts for more"
    "the -Q counts for more"

    counts for more what?
    I still don't get why "Potential Energy decrease" yet "PE" (I'm assuming they're the same) increases..
  5. Feb 20, 2007 #4
    I still dont get the left side, but I have a feeling the right side can be cleared up if you differentiate between potential and electric potential energy. Potential is voltage and does not depend on the charge in question (in this case -Q). V=sum of kq/r

    EPE, however, does depend on the amount and sign of charge. It takes more work to lift a heavy object the same distance as a lighter one. EPE= QV = Q[sum of kq/r]. In this case, voltage is decreased but negative charges want voltage to increase. Therefore, the EPE is increased
  6. Feb 20, 2007 #5
    No comprende..

    I'll post more of today's notes..
    "Potential difference is a measure of the magnitude of work required to move a charge from 1 point to another" (this was written before my diagram)
    (After diagram)
    "If charge is moved between these 2 points due to coulombic force of the force field acting upon it, work is done by the electric field and energy is removed from the system. However, if the charge is moved against the coulombic force of the field, work is done on it using energy from an outside source, storing the energy in the system"

    "Therefore, potential difference (V) = Work (W)/Charge (q)"
    "A volt is the potential difference between 2 points in an electric field requiring 1 joule of work to move a charge of one coulomb between these points"
  7. Feb 20, 2007 #6
    This means that Work aka change in EPE = Vq

    But you also need to know that voltage is equal to an electric field times the distance it acts over. V = Ed = kQ/d^2 * d = kQ/d. The potential at any point is the scalar sum of the potentials due to each charge. This makes adding voltages a lot easier than adding fields
  8. Feb 20, 2007 #7
    The diagram you posted doesn't make sense to me. Usually PE stands for potential energy, in the diagram you have contradicting statements, if I am not drunk. Here we are talking about a specific type of PE- electric potential energy. Here is how I think about it.
    Remember gravitational potential energy? If you push a ball up a hill, energy is stored in the ball. (really the ball earth system, since if the earth wasnt there there would be no potential, but anywho..)
    You can imagine that the positive source charge is akin to a hill or mountain and the negative source charge is akin to a valley, of course this is in three dimensions so it is only an analogy. If the positive test charge moves towards the negative source charge, this is like a mass moving down into a valley- it loses potential energy. If the test charge moves towards the positive source charge, this is like a mass moving up a hill-it gains potential energy. Of course the test charge will not do this unless someone or something pushes it up the hill- work must be done by something. In a similar manner, if the test charge moves towards the negative test charge, something did work on it- here it is the field that is already in place due to the source charges.
    The term potential, not potential energy, is like measuring the altitude of any position around the hill or valley. It describes the potential energy per unit charge ( or mass if we are talking gravitational potential, i.e. topographic map)
    The weird thing is when you start looking at negative test charges since the field will "push them up potential hills."
    Hope that helps. :bugeye:
  9. Feb 20, 2007 #8
    Dear Jesus:bugeye: To think that I'll (maybe) have to take this when becoming a Biz major..

    Forget all that other mumbo jumbo..

    Please explain to me..
    Why for both sides.. It says Potential Energy Decreases then says increases at the bottom.. Thanks:smile:
  10. Feb 20, 2007 #9
    magnets aren't so bad a way of thinking about potential energy, at least in the case of repulsion. If you bring a pair of magnets together that are aligned to repel, the closer you get the two together before release the greater the action--in fact with weak magnets they might need to be almost on top of each other before showing any real interest in moving apart. Now for an attractive force in the everyday world, its less obvious. But consider a rock of 100 pounds. Dropped from an inch theres not a lot of potential energy waiting to be unleashed, drop it from an 8 story building on ones head, gonna be a mess. So potential energy is like future profit potential. Its the potential to be a big deal. In some situations the closer the objects are, its greater, in others, further away. Physics is easier than economics in many ways. But if you want to succedd in biz, in the 21'st century and beyond the biz man who has a solid understanding of physics, engr and technology has a huge advantage over his/her less well educated colleague or competitor.
  11. Feb 21, 2007 #10
    Thanks for the insight:)

    Why for both sides.. It says Potential Energy Decreases then says increases at the bottom
  12. Feb 21, 2007 #11
    You diagram say (on the left) PE decreases... what is after that. The only statement that would make sense to follow the ... would be " as the charge is moved away from the positive point charge." On the right, after the ..., it would make sense if the diagram stated "if the charge moves towards the negative point charge."
    hope that helps.
  13. Feb 21, 2007 #12
    It's confusing--the top statements are referring to the situation when the charges are free to move--whether be repelled apart or drawn together, in both cases the system loses PE to kinetic or some other form of energy.
  14. Feb 21, 2007 #13
    Well that makes sense.... So if they are naturally repelled or attracted, they lose potential energy..

    But if I need power to puush + charges together, that gains PE, and if I try to disengage the attraction between - and +, that increases their PE?

    Eh, kind of? Hopefully my teacher can clarify today..
  15. Feb 21, 2007 #14
    Yeah kinda of
  16. Feb 21, 2007 #15
    So.. my teacher said that "Potential Energy decrease" is when it's in the natural state of repulsion and attraction and the second bullet points are for "manipulation"...
    Case closed?
  17. Feb 21, 2007 #16
    I have no idea what you just stated??
  18. Feb 21, 2007 #17
    Notice the diagram..
    The top bullet points say "PE decreases"
    The bottom contradicts..

    My teacher is now saying that "PE decrease" is when the charge naturally goes where it wants to..
    And that "PE increase" is when the +q is forced toward the +Q.
  19. Feb 21, 2007 #18
    Okay that makes more sense.
  20. Feb 22, 2007 #19
    Right, which is what I guessed what he was trying to say.

    After all this is a build up to the topic of current flow. Charges will move if free to do so down a PE gradient and this we call electrical current.
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