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Electrons acceleration in a wire

  1. Jul 31, 2014 #1
    I have some questions about electrons flow.
    1)Lets say we have a simple circuit with a metal wire and a battery supplier.Firstly if we can imagine an electrons flow in a conductive material like metal but with zero resistance and no atoms for the electrons to collide , the Fc1 force from the negative pole which repels electrons and the Fc2 of the positive pole that attracts them are both non constant and distance depended forces.So,Fsum=Fc1+Fc2.Ι know that electrons in this case would do an accelerative motion , but is Fsum as a sum force constant and thus electrons acceleration same at each point in this case or they do accelerate till the end but with bigger and smaller accelerations at each point?
    I put that this way in order to understand if in a metal wire after each collision with atoms the electric field acceleration gained by an electron at different points throughout the wire is different or is constant.
    2)The second question is depended on the answer of the first.According to my thinking electrons would do an accelerative motion throughout a metal wire (let's say an 1 meter wire example) but the acceleration would change prices at each point as it moves.So,how is it possible if the net accelerations are different at each point throughout the wire in a series circuit the current to be the same at each point? Different accelerations would accumulate in every wire point different amounts of electrons.If we suppose net accelerations change either increase or decrease, net velocity will continuesly getting higher so current would getting higher as electrons move further and further.I am not speaking for drift velocity just the net velocity(It is very different).So distribution would change very slightly.So,how come current is the same in every point?

    Thanks in advance for the try and your time but please i don't want an answer with just a link to a physics law or a mathematical formula.I am student in university and i know the basics.I would just like a descriptive and serious answer.The meaning.
    Last edited: Jul 31, 2014
  2. jcsd
  3. Jul 31, 2014 #2

    Andrew Mason

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    If there is no resistance, such as in a vacuum tube or cathode ray tube, the electrons accelerate from - to +.

    In a circuit with resistive loads in series, the potential difference across those loads is distributed in proportion to the resistance. The end result is a steady state flow of current with the forces on the electrons at each point being just enough so that, on average, the charges move through the circuit at a constant rate. At the molecular level there are electrons slamming into atoms and picking up speed and losing speed all the time, but at the macroscopic level, Ohm's law applies.

  4. Aug 1, 2014 #3


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    I'd like to point out that electrons in a conductor move in random directions. When no voltage is applied these random motions equal out to zero net current in either direction of the circuit. But when you apply a voltage this is no longer the case. The directions are still random, but when you add up all the directions there is a small preference for one direction, leading to current flow. This is why the drift velocity is so small in conductors. The current isn't composed of electrons moving in solely one direction, but instead is composed of a large number of electrons moving randomly throughout the conductor with a small preference for the direction of current flow.
  5. Aug 2, 2014 #4


    Staff: Mentor

    This is not metal, this is vacuum.

    In vacuum Fsum would not be constant. You can calculate or measure the E field and find that it is not constant.

    What you describe above will not help in understanding the behavior in a metal, just in vacuum.

    It is not possible. Therefore the net accelerations are not different.

    Because the E field is uniform inside a wire with a uniform current, due to Ohm's law. This is the key difference. In the vacuum the E-field is not uniform, it is determined by Coulomb's law. In an Ohmic material the E-field depends on the current, it is uniform for a constant current.
  6. Aug 2, 2014 #5
    Thank you all for yours answers.Dalespam your answer especially helped me very much.Let me ask you something though,is current always constant in an Ohmic material for a specific price of voltage?
  7. Aug 2, 2014 #6


    Staff: Mentor

    Yes, that is essentially the defining characteristic of Ohmic materials.
  8. Aug 2, 2014 #7


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    More specifically, in an Ohmic material with uniform resistivity ##\rho## and conductivity ##\sigma = 1/\rho##, ##\vec E## is uniform if the current density ##\vec J## is uniform:

    $$\vec J = \sigma \vec E$$
  9. Aug 2, 2014 #8
    The discussion above, with moving electrons, colliding with one another and atoms, having particular positions, velocities and accelerations, is a classical model of conductivity. We know now that it is seriously wrong. Conductivity in metals is a quantum phenomenon and can only be fully explained as such.
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