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Where does inductive reactance go?

  1. Jan 28, 2015 #1
    Lenz law states the induced emf in any circuit is always in a direction to oppose the effect that produced it. So my question is,
    1. Does this mean xl creates a difference in potential and actual electrons flowing in the opposite direction or just oppose the current direction that created it, resist it?
    2.If it does mean electrons actually flow in the opposite direction in the conductor, where do they end up?

    Im just trying to get a picture of what physically is happening with the electron flow. Any help would be appreciated. Thanks
  2. jcsd
  3. Jan 29, 2015 #2

    jim hardy

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    Golly - at first i thought that you had mis-quoted Lenz.
    But a cursory search shows that wording is common now.

    I was taught "Lenz's law states that the induced EMF will oppose the change in magnetic flux"

    Teacher shortened that to "it'll try to keep flux constant" which to me seems easier to remember.

    , emphasis mine jh

  4. Jan 29, 2015 #3
    So what is the opposing emf that was created? is it a difference in potential, current flow or just opposing flux lines?
  5. Jan 29, 2015 #4

    jim hardy

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    It's an EMF in the wire , see Faraday's Law at the same link
    The induced EMF will show itself as a potential difference between the ends of the wire as shown by the + & -
    which in the pictures above will cause a current to flow around the coil as shown
    and current always produces a magnetic flux, that's Ampere's law http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/amplaw.html#c1
    Lenz tells us the direction of that new flux - it'll try to keep total flux constant

    It really works

    you can try it yourself with an old loudspeaker
  6. Jan 29, 2015 #5
    Hi jim,
    First of all thank you for taking the time to answer my question. I really appreciate it.

    Now the drawing you posted with the 4 coils and magnets helped. I had to study it for quit a while and also figure out the direction of the current flow was based on the right hand rule and not the left hand rule like I am learning. t

    I think I got it. Tell me if this sounds correct.

    When you cross a conductor with a magnetic field, a current will be induced in that conductor. Now I think this part is key. The current will always be in a specific direction that depends on the polarity of the magnetic field and the direction in which the conductor enters that field. Now the current traveling in the conductor produces its own magnetic field also called "flux". This flux encircles the wire and travels in a direction that opposes the force that created it. In this case that force is the magnetic field from the magnet and its polarity.

    I think that happens like this. The magnet approaches or enters the , lets use coil here more specifically than conductor, perpendicularly. Lets say its north pole is closest to the coil. The coil has its natural reaction of forming a current flow and flux. Again because of variables of polarity of the magnet and direction of the conductor/coil entering the magnetic field, the polarity the flux will create in this case a north pole of coil to north pole of magnet. Resisting the change of the magnet entering its area. When that magnet is pulled away it will change one of the aforementioned variables and the coil will become south pole to north pole of magnet resisting the change of the magnet leaving its area.

    Now if all that is correct. In the case of a coil with a voltage applied to it, will there be current flowing in two directions. One from the applied voltage source and one from the induced emf? Or will the induced emf just be a resistance to the applied voltage source because maybe the voltage source is stronger or happens before self induction occurs? And is this similar to how Xl in a typical residential transformer or electric motor might work?

    Again thanks million
  7. Jan 29, 2015 #6

    jim hardy

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    yes textbook current flows in opposite direction from electron drift.
    Look up when the electron was discovered versus when Ampere lived and you'll see why....
    i suggest that you become conversant in both "conventional current" and "electron current" .
    Engineers learn the former but some technical training still uses the latter.
    If you can swap back and forth effortlessly it will help you in your career.
    If you ever work with older Navy techs you will find that the Navy training based on electron current produced exceedingly competent people.
    Where i worked, my technicians had great fun teasing us about "Engineer's Current - it goes backward ". When teaching i learned pretty quick to draw my mesh and loop currents with two sets of arrows, green for the technicians and brown for the engineers.

    That is perhaps not the best way to think of it, though it will work. Keep in mind there's only one value for current, in your case the sum of your two individual currents.
    Myself i prefer to think of it as two opposing EMF's , net is again the sum of the two.
    That mental model of EMF's i think will help when you get to the topic of counter-EMF in motors and coils.

    There's no separation in time....
    there's just different mental models one can use..

    This all stems from the force exerted on an electric charge in a magnetic field whenever there's relative motion between the charge and the field.
    Since you mentioned left hand rule i guess you're studying electron current?
    Sadly most reference material is written for conventional current, but it's easy to swap between the two.

    Force on a single unit of charge is F = Q X relative velocity X magnetic field strength
    and they're related by the right(or left) hand rule
    study this page or one similar .

    You'll find engineering texts use the equation F = Q V cross B which means there's a vector calculus operation that properly calculates the result, it's called "cross product" which means your calculated result will agree with the right hand rule pictured above.
    Those texts always assume Q is positive conventional charge not negative electrons,
    so to solve for electrons you'd just swap hands and the engineers would just insert a negative value for Q .

    Do you begin to see why i recommend becoming fluent in both dialects?

    Yes we had some discussions on that a while ago
    Xl and motor action both result from Q V cross B
    so it's a very important concept to grasp

    see page 2 of this old thread


    old jim
    Last edited: Jan 29, 2015
  8. Jan 29, 2015 #7

    jim hardy

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  9. Jan 29, 2015 #8
    Thanks for conformation and extra input and tip to learn the conventional hand rules. I have a better understanding of xl now. Im an not and engineer or tech but an electrician. I'm just not satisfied not knowing how some of the basic components of an electrical system like a transformer internally operate. I don't really know anyone personally that I can discuss this stuff so I really appreciate the input.

  10. Jan 31, 2015 #9
    I certainly agree regarding the Navy guys. I worked with any number of these guys who got their training in the 60's and 70's and were absolutely amazing.

    Re current flow:
    My sophomore circuits instructor was quick to point out very early that physical charge movement was not our concern. Anyone who was overly concerned with electron flow could go on to include ion flow in gas tubes, ion flow in chemistry, and the behavior of "holes" in semiconductors. I guess that would include protons in accelerators.
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