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Voltage drop and EMF

  1. Oct 28, 2013 #1
    Please i have a question.When we have an inductor in a circuit we say that we have 2 electric field : conservative and non-conservative. so who is doing work on the charges?if we draw an rl circuit with a DC source the conservative electric field across the inductor has the same direction of the current thus the voltage drop but the non conservative electric field has the opposite direction thus doing work against the electric field thus the rise of the voltage. this is confusing me a lot
    thank you
  2. jcsd
  3. Oct 28, 2013 #2
    Can we say that the charges inside the inductor do not move?
  4. Oct 28, 2013 #3


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    1mm per second is not moving much, imo. If you concern yourself with the distances involved in a circuit then the speed of propagation of the electric and magnetic fields is just a tiny bit greater than the drift velocity of the electrons ( by a factor of at least 10^10).

    There are occasions when the motion of charge carriers becomes relevant - for instance, in some semiconductors - but that tends to be considered separately and that approach gets good results.
  5. Oct 29, 2013 #4
    but who's driving the current ? the conservative electric field or the non conservative one? and from where the conservative electric field is coming? and in an RL AC circuit there's a part of time where the inductance is giving energy in this part who's driving the current?
    Last edited: Oct 29, 2013
  6. Oct 29, 2013 #5


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    Where's the force coming from that makes the link in a bicycle chain move? The direction of the 'original' force is the direction of the cyclist's foot and that could be in totally the opposite direction to the direction that the link is moving. The thing that's "driving the current" can be the steam in the turbine that's driving the alternator back at the Power Station or it can be the charge that's immediately to the left or right of the charge in question. The answer to the question is that it's irrelevant, once you have accepted that the charges move to lower potential. What is important is the Energy transfer and this is why we discuss the behaviour of circuits in terms of Potential Differences. Now, Field is the gradient of Potential, so we are not totally rejecting the idea of Fields. They are just less relevant than Potentials.

    Perhaps you are trying to analyse the RL circuit in an 'unfruitful' way. If you want to talk about fields then would you not also be needing to consider the Capacitance which must surely be there? Any such circuit boils down to an RLC circuit, considerably (possibly) off resonance.
  7. Oct 30, 2013 #6


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    The non-conservative field in the inductor's coils resist any changes in current. The conservative field is the external forces being applied to the inductor that are being resisted.

    Perhaps this can help:
    https://njctl.org/courses/science/ap-physics-c/induction/inductance-presentation/?download&token=933f0f0ca62599c37e09e83ca6b24c60&attachment_version=220110728142937&format=pdf-1-slide-per-page [Broken]

    Does that help? If not, ask differently based on what's been said.
    Last edited by a moderator: May 6, 2017
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