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Electric potential and potential difference

  1. Jul 8, 2006 #1
    hiya


    1)
    Some quotes from physics tutorial site:
    The above all makes sense. But this is where I get lost:

    Wouldn't it be more correct if instead we said:

    *The kinetic energy charge has was transformed from charge's potential energy, and this kinetic energy gets lost as charge goes trough light bulb?*



    The way I understand it, as electron moves trough electric field its potential energy is transformed into kinetic energy ( same way as potential energy of object falling towards the ground is transformed into kinetic energy W[k] ). And when this W[k] goes trough lightbulb, W[k] gets transformed into other types of energy. So, if two charged parallel plates are 1 meter apart and electron is put inside their EF at point A(near negatively charged plate), then once electron travels half the distance between the plates it will loose half of its potential energy

    In short, loss of potential energy depends on how much closer a charge is to its final destination ( compared to its initial position ) and not how much of this charge's kinetic energy a device like lightbulb has transformed into other kinds of energies ( light etc ). Correct?




    2)

    The following text

    also makes it sound as if potential energy is not location dependent, but instead it solely depends on where the devices such as ligh bulb are located!



    Let me explain what I mean:

    Note - green thingy represents a light bulb and red is wire. The two boxes are positive and negative terminals.

    [​IMG]


    If potential difference is defined as how much closer charge is at point B ( compared to point A ) to its final destination ( final destination being positive terminal ), then how can there be potential loss resulting from current going trough the light bulb? I know there is loss of energy due to electrons passing trough the lightbulb, but since charge in point B is not at all closer to positive terminal then in point A, the kinetic energy ( the one electrons lost while going trough light ball ) was not created from potential energy. Actually, I'd say that no potential energy charge posseses has yet been transformed into kinetic energy?


    thank you
     
  2. jcsd
  3. Jul 8, 2006 #2

    Andrew Mason

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    No. You are oversimplifying what happens in a conductor. In a cathode ray tube electrical potential is converted into the kinetic energy of electrons. But in a conductor it is altogether different and quite complex. Electrical energy moves through a conductor at virtually the speed of light. Electrons don't move nearly that fast. In fact, the drift velocity of the electrons in a conductor is actually rather slow.

    In a conductor, current flow has to do with the properties of a conductor including quantum effects. See other discussion on PF such as this recent one or here or here or try reading a good text on Electricity and Magnetism on "Conductivity in Metals".

    In conductors, loss of energy or energy per unit charge (electrical potential) does not depend on distance. Potential loss depends on resistance, which in itself is rather complicated.

    AM
     
  4. Jul 11, 2006 #3
    I'm confused why my text book would explain how loss of potential energy depends on the distance the force acts on a charge, and then in next chapter it would without any explanations ignore all of what it was said in the previous chapter and and declare that loss of potential energy does not depend on the distance traveled by this object inside EF? In one chapter it was said that potential energy is location dependent, and in very next chapter it kinda negate that.

    This simply doesn't make sense. It's kinda like if object at some height above the ground would turn its potential energy into another type of energy without falling to the ground. You can't turn potential energy into anything but kinetic energy.It doesn't make sense.

    If loss potential energy indeed does depend on the distance and force acting on charge during this distance, then why in conductors potential energy won't follow natural laws and instead "act against its nature" ?
     
  5. Jul 11, 2006 #4

    Andrew Mason

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    Well, free electrons do gain kinetic energy that is exactly equal to the electric field x electron charge x distance. The problem is that they only do this for the distance between collisions - they accelerate until they collide with other atoms in the metal. Conductors do not provide large volumes of free space for electrons to move freely in.

    What you are saying is something like this, in an analogy to gravity: "the chapter on gravity says that a ball of mass m falling a distance h due to a gravitational field g will gain kinetic energy of mgh. But the chapter on fluid flow says that when I drop that ball through a fluid of viscosity [itex]\nu[/itex], it does not gain kinetic energy based on the height fallen - it moves at a constant speed related to [itex]\nu[/itex] even if I increase the mass." See the point?

    AM
     
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