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Relating Movement of Electrons in Currents to Wire Movement/Energy/Power

  1. Sep 4, 2012 #1
    Off the Wikipedia:

    "...in a copper wire of cross-section 0.5 mm2, carrying a current of 5 A, the drift velocity of the electrons is on the order of a millimetre per second."

    Suppose we have a setup with a very very light wire which can transmit very very high currents. Once the current is switched on, can we detect a visible movement in the wire due to the initial acceleration of the electron flow (due to F=MA) within it?

    Another question is taking into account electron density in a conducting copper wire, can we relate the electron speed to the energy/power provided by the wire taking into account E= 0.5mv^2?

    IH
     
    Last edited: Sep 4, 2012
  2. jcsd
  3. Sep 4, 2012 #2

    Dale

    Staff: Mentor

    I would have to work it out to be sure, but my guess would be yes.

    No. I have worked this out before, the KE of the electrons is an insignificant fraction of the power delivered to a wire.
     
  4. Sep 4, 2012 #3
    Then in what form is the (bulk of the) energy/power of electrons transmitted in an electric current? Vibration/wiggling of electrons or something else?

    IH
     
  5. Sep 4, 2012 #4

    russ_watters

    User Avatar

    Staff: Mentor

    Electrical energy is electrical and magnetic. It depends on voltage and charge movement. You can see that in the equations.

    Also, judging by the mass difference between an electron and a proton/neutron pair, I'd say the kinetic energy/force would be negligible.
     
  6. Sep 5, 2012 #5
    Ok, understood.

    Can we however say that electrical energy can only be extracted from a system via movement of electrons or changes in an electric field? That a static electical system may well contain electrical energy but that this can only 'flow' in a dynamic system?

    Kinetic energy of electrons may be too weak to matter much, but I think that a dynamic system is essential to transmission of electrical energy.

    IH
     
  7. Sep 5, 2012 #6

    Dale

    Staff: Mentor

    Yes. The movement of charges is called a current and is represented by the symbol j. In Poynting's theorem, the energy transfered to matter from the EM fields is given by E.j, so as you say, energy can only be extracted via movement of charges in the presence of an electric field.
     
  8. Sep 5, 2012 #7

    russ_watters

    User Avatar

    Staff: Mentor

    Sure, but note again that it is the charge that matters. Ie, a moving proton and a moving electron carry the same charge even though the proton is 1800 times more massive.
     
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