Electromagnetic Induction

Discussion in 'General Electronics Chat' started by mentaaal, Jul 28, 2009.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Hey guys, in order to understand electromagnetic induction I have been pondering a hypothetical question.

    This may sound quite silly but here goes:

    If a coil were emersed in a UNIFORM magnetic field, i.e. the magnitude and direction of the magnetic field were exactly the same at all points around the coil, with the hollow of the coil orientated to be in the same direction of the magnetic field. If the magnetic flux density of this field were made to change in density (but still keep its uniformity) is it possible that there will be an induced emf in the coil?

    Naturally, if a magnet were moved toward a solenoid, a potential will be created but my question replaces the magnet with a completely uniform field.

    My instinct is to say that no, no induction will occur...

    Could somone give me substantiated answer to this question?
     
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    My though would be:
    Changing the strength of the field induces a voltage.

    It does not matter if this change is due to moving the coil or changing the field strength. Think of a transformer; no physical movement, just changing field strength.
     
  3. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Hello and thank you for your answer. I take your point but my posed question is somewhat different that to what occurrs in a transformer. In a transformer, the flux density in the core is much greater that that outside the solenoid. Further, the direction of flux is completely different, and for this reason I cannot say conclusively that the same result will occur.

    Remember, in my question the flux was uniform and had the same density inside and all around the inductor.
     
  4. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Would your explanation be equivalent to introduce/remove a core into the coil held steady in the unmoved uniform field ? That would change the density of the field in the core...

    Or do you mean to increase/decrease only the magnitude of the field (thus its density) where the coild rests unmoved, without altering its orientation ?
     
  5. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Precisely, the coil's spacial orientation doesnt change whatsover and it remains motionless. ONLY the flux density across the entire coil changes, such that the density of the flux at any instant in time is the same across the coil.
     
  6. davebee

    Well-Known Member

    Oct 22, 2008
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    Why do you think the answer would be no?

    Faraday's law says that if the magnetic field through a surface bounded by a path changes, then an EMF will be induced in that path.

    Each turn of your coil seems to qualify as a path that bounds a surface through which a magnetic field passes, so if the field changes, it seems to me that an EMF will be induced in the coil.

    I don't think that uniformity of the field has anything to do with this; anything that changes the overall field strength across the bounded surface should produce the EMF - field changing, coil twisting, etc.

    This would be a special case in that coil motion along the direction of the field would produce no EMF, because with a uniform field, this motion would not cause any field strength change within the bounded surface.
     
  7. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Thank you for a more substantial answer. To answer your question I am not sure why i think it. I guess I always envisaged the model in my in a fundamentally flawed way. What you have said has answered more than one of my questons. :)
     
  8. davebee

    Well-Known Member

    Oct 22, 2008
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    You're welcome.

    Thinking more about it, I guess that the coil could also move sideways without generating any EMF, as long as it doesn't twist, because in a uniform field that also won't change the overall field through the coil.

    (Edit: or will it? Now I'm confused. This motion would "cut the lines of flux".)
     
  9. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Well if what you say is true, then surely twisting would not cause any problems? Surely any motion at all would not induce an emf as long as the coil does not leave the confines of the uniform magnetic field?
     
  10. davebee

    Well-Known Member

    Oct 22, 2008
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    I don't think that rotating about the axis would generate EMF, as no field lines would be cut by the conductior, but twisting the coil so its axis changes it's angle to the direction of the magnetic field should generate EMF since that would vary the field across the coils.

    Digging a bit into Faraday's law, I see that it's sometimes separated into two distinct cases - one, varying the magnetic field, and two, moving a conductor in a non-varying field. The second case would apply here. A number of physicists - Maxwell, Faraday, Feynman, Einstein, have commented on the unusual aspects of this law, according to the wikipedia page on Faraday's law http://en.wikipedia.org/wiki/Faraday's_law_of_induction , so maybe this isn't really such a simple question after all!
     
  11. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Well its obvious that you have put some logical thought to it. This is what I wanted to gain a better feel for the subject. And what you have said makes perfect sense as the orientation of the coil surely does have an impact.

    You know what I would love to know? How induction actually occurrs, I mean, if the coil is open circuited, no current will flow so you only get an emf, so the electrons arent moving (well of course they are due to heat and interference but you know what i mean) so they wont generate a magnetic field so I wonder how the changing magnetic field induces an emf in the coil...

    I guess that a good grasp of quantum mechanics would be a good start which i really dont have :-(

    Would anyone like to give me a hand waving explanation of this phenomenon... I am not looking for detailed stuff here just a rough explanation would be great
     
  12. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Hi.
    Depending how do you envision the movement of the coil in the fixed field, it may or may not produce and EMF. If the core aims NS and the motion is NS, probably not.
    If the core is inserted and removed, I would say it will generate EMF, as the lines of force that were passing outside the coil are bent towards the metallic core when it is inserted.

    I cannot help you with the quantum mechanics stuff, but can put another variable in your mind.... if the coil being affected by the uniformly/moving/changing magnetic field is shorted, it will produce a voltage, and current. Thus, such coil creates its own magnetic field inside the field that made its EMF to start with...

    Miguel:rolleyes:
     
  13. davebee

    Well-Known Member

    Oct 22, 2008
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    I don't think quantum mechanics has anything to do with this. Quantum mechanics would deal more with photon absorbtion/emission. I think relativity is what you might need, because one of the puzzles that relativity explains is that what looks like a magnetic field in one frame of reference can look like an electric field in another frame of reference, but the mechanics of the system being studied works out the same way, although different "fields" may be involved depending on the observer.

    Actually, even if the coil is open-circuited, if it moves in a magnetic field (or the magnetic field changes) then the induced EMF will create a momentary current. Charges will move until the electric field due to the realigned charges is just sufficient to balance the induced EMF. After that, there won't be any current, but there will be a state of unbalanced charges.
     
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