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Electromagnetic induction of a wire in a complete circuit

  1. Apr 17, 2015 #1
    Say for you had a wire in a complete circuit inside a magnetic field (pointing inwards) perpendicular to the wire.

    You move the wire across (to the right) , cutting lines of flux, this induces a current in the wire.

    The induced current acts upwards using the dynamo rule (thumb is motion, middle finger is flow of electrons, first finger is field)

    But since an induced current is created upwards in a magnetic field this means that a force is also created.
    Using Fleming's left hand rule (thumb is motion, middle finger is conventional current and first finger is field), the motion of the wire is to the right.

    More lines of flux are then cut, current increases, speed increases ect you get energy out of nothing?

    How is this possible?
  2. jcsd
  3. Apr 17, 2015 #2
    You are only considering one half of the picture. The magnetic field is also affected here too. If you point your left hand towards your right hand your fore-fingers point in opposite directions, can you guess what happens to the magnetic field?
  4. Apr 17, 2015 #3


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    It's not possible. The wire will experience a force that slows it down, not speeds it up. I believe you've mixed conventional and non-conventional current together, which will give you conflicting results.

    Hold out your hands. Per the right hand rule the current (conventional current, not flow of electrons) is induced upwards. (Thumb to the right, middle finger upwards)
    Per the left hand rule this upward current produces a force to the left. (When middle finger is upwards, thumb is to the left)
  5. Apr 17, 2015 #4
    I believe you've got conventional current and actual current mixed up mate...
  6. Apr 17, 2015 #5
    ahh so the current creats a magnetic field to move the wire to the left, this makes a lot more sense now thanks :)

    so two forces are created, one to the left and one to the right, but th onr to the right is much smaller than that to the left?
  7. Apr 17, 2015 #6
    You can guarantee it will be sufficient to prevent the creation of energy : )
  8. Apr 17, 2015 #7
    thanks for the help :)
  9. Apr 18, 2015 #8


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    I'm not sure where this came from!
    In the original case, you provided the force to the right, which moved the wire to the right through the magnetic field into the page. That created an emf upwards, which could cause a conventional current upwards.
    Now that upward conventional current in the into-the-page magnetic field causes a force on the wire directed to the left.
    Only one force is created by the current - to the left.
    As to the magnitude of the force, that depends on the field and the motion, so is not obviously related to the original force you applied to the wire.
    If you took a stationary wire and applied a constant force, the wire would accelerate, producing an increasing emf and (depending on the circuit the wire was in) an increasing current. So the force would be increasing in opposition to the motion, reducing the acceleration, until there was equilibrium with the wire moving at a constant speed with the electromagnetic force exactly balancing the constant applied force.
    (In other circumstances, with a varying applied force, the electromagnetic force could exceed the applied force and even be in the same direction as the applied force at some times.)

    I was puzzled by your comment that you had been taught that the LHR used conventional current and the RHR used electron flow. This seemed incredible to me. Although I've never used either rule (I would get mixed up which was which), I remember being taught that thuMb was motion, First finger field and seCond finger current and to me current has always meant conventional current. A conventional current may be caused by flow of electrons, holes, ions or whatever, but always is from positive to negative.
    In fairness to you I thought I'd search the web to see what other people said and was utterly amazed to find a website which said,
    http://www.education.com/science-fair/article/effect-magnet-electron-beam-right/ (there may be others, but having found one, I stopped looking) so it is understandable that people get confused! All I can say is that, after more than half a century of doing electronics, I've never used or thought of current as anything other than the conventional flow from positive to negative and never come across anyone seriously using any other convention. Generally it is only mentioned to explain that it is an arbitrary convention (or in wierd discussions like this!) If I need to talk about electrons, holes or ions, I am careful to say electron flow, ion flow or hole movement or something like that and reserve the word current for the associated conventional current.

    As a general point, there are other situations in science where such arbitrary conventions are used, some much less widely accepted than this one. It never matters which one you choose to follow (except that you may confuse other people), but you should always stick to the one you've chosen. Changing conventions for different parts of a calculation or argument, is a recipe for errors. If you ever choose to use a convention which is not as widely accepted as this one, it would be a good idea to state your chosen convention clearly at the start.
  10. Apr 18, 2015 #9


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    I'm pretty certain I do not.
    From wiki: http://en.wikipedia.org/wiki/Electric_current#Current

    A flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention is needed for the direction of current that is independent of the type of charge carriers. The direction of conventional current is arbitrarily defined so that a positive current flows in the same direction as positive charges and vice versa.

    The consequence of this convention is that electrons, being negatively charged, flow in the opposite direction to the direction of conventional current flow in an electrical circuit.
  11. Apr 18, 2015 #10


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    The magnetic field created by the current of the wire is to be taken into account only if you care for self induction effects, however the latter are not big enough in a straight wire, you need some spiraling of the wire for self induction to become noticeable.

    The induction current will be such that the resulting Laplace force opposes the motion of wire. When you using the right hand rule to find the direction of the laplace force, thumb is current , first finger is field, and second finger is direction of force.
    Last edited: Apr 18, 2015
  12. Apr 18, 2015 #11
    I think I get it now, so right hand rule is also conventional current, this makes a lot more sense and I see now that because of my example right hand HAS to be conventional current as well. Thanks for the help :)

    I think I was confused because I've been taught to use the right hand rule to work out the motion of a beam of electrons in a magnetic field, but this is for motion and in this instance the current is actual current not emf.

    But then when you're working out the induced current from motion, the current is conventional.

    But it's all good now
  13. Apr 18, 2015 #12
    The current from my example also produces a magnetic field, how would I work out which way the field acts, and would this field cause motion in another direction?
  14. Apr 18, 2015 #13


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    See diagrams in http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html
    The general rule for the field caused by a current in a straight conductor is that, if the current (conventional + to -) is away from you, the field is clockwise around the conductor.
    In your case; field into the paper, movement to the right, wire vertical, induced emf and resulting current (if any) upwards, then the magnetic field caused by the induced current is out of the paper at the left of the wire, into the paper at the right of the wire, left to right near the observer and right to left on the far side of the wire from the observer. So the overall field is strengthened to the right of the wire, weakened to the left of the wire and bent in front and behind the wire (from the observer's view.)
    conductor_in_field_annotated.gif This diagram is from http://www.fastonline.org/CD3WD_40/CD3WD/ELECTRIC/GTZ021E/EN/B309_4.HTM and shows your situation viewed from below.
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