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If current increases in an coil, what exactly happens?

  1. Apr 18, 2013 #1

    I understand that when current is increased throughout a coil, the magnetic field (B) increases it's strength. I try to compare this with a permanent magnet. The magnetic field in a magnet is due to the electron spins and the force is due to them, when you increase the size of the magnet there is more mass = more spins = stronger field a more force.

    But what about in a coil? more current means more charges = stronger field and forces?

    I want an explanation as why does the force increase as well.
    If there is a coil in a system, motor,holding unit, etc... We know by increasing current the force is increased greatly.
  2. jcsd
  3. Apr 19, 2013 #2
    That is a really good question, because if you look around you will find a lot of information that says that when a current flows through a wire a magnetic field is created, and that is about it.

    We had a very interesting discussion here some time back about what actually happens in a wire when current flows through it. Often electron flow is pictured as similar to a flow of a liquid through a pipe, but it doesn't quite work that way.

    This is a similar question, and even though I made my living for many years working with electronics I had never gone past the "current flows through wire, makes magnetic field" idea. So after checking it out tonight I think I can propose that current through the wire doesn't produce the magnetic field, but rather the current and the magnetic field are products of the voltage applied. (Talk about sticking one's neck out with a wild statement!)

    Electrons in the wire have magnetic poles. If you put a electric field along the wire (apply a voltage from end to end), the magnetic poles of the electrons should line up in some manner or another. Line them up and you have a polarized magnetic field that varies with the current because the only way you can increase the current on a given wire is to increase the voltage. More voltage, more polarized electrons, stronger magnetic field.

    I can hardly wait to see the comments on this one.

  4. Apr 19, 2013 #3
    The electric fields of the electrons induce magnetic fields. You can have similar magnetic fields from a few electrons moving faster or more electrons moving slower. The induced magnetic field contains energy. Double the magnetic field strength give four times the magnetic field energy. Force is rate of change of energy with separation so for an identical situation doubling the magnetic field strength gives four times the energy, giving four times the force.
  5. Apr 19, 2013 #4
    How does one get the electrons to "move faster?" We are talking about electrical "current" in a wire here. That is an early morning question and my IQ may be running at a low level here. Chuckle. Still a serious question.
    Last edited: Apr 19, 2013
  6. Apr 19, 2013 #5
    Increase the electric field strength that causes them to move. I.E. more volts.
  7. Apr 19, 2013 #6
    Looking back at your second response I see your point about more/less electrons. I do have trouble thinking of it that way as the average "velocity," through the wire, for any given electron is really quite slow if one does the calculations.

    Anyway I think I have to simmer on this one a bit and see what comes up by considering what we have kicked around here about electron flow in a given wire at a given current and looking up some other information. I do like bringing things down to basics. Most interesting.

  8. Apr 29, 2013 #7
    Pumila, are you still around?

    I have been hitting the physics books and doing a bit of online research specifically concerning electron magnetic dipoles. It has been very interesting. I am no ways near finished on this one, but it appears at this point that the electron's magnetic dipoles in the conductor may be aligned by the potential applied. I'm still working on the concept and so far have found some theory on this to be a bit fuzzy.

    Anyone reading this who knows of good applicable references would be doing me a favor by listing a link.

  9. Apr 29, 2013 #8
    If you are talking about conduction electrons in a conductor having aligned magnetic dipoles, these are "free electrons" and as such do not exhibit any magnetic effects. To quote from a paper "Stern-Gerlach experiments: past, present, and future" by Jean-Francois Van Huele and Jared Stenson, "Spin is a non-classical duplicity useful for classifying atomic states but not observable with free electrons." You may find text-books that claim that the Stern-Gerlach experiment works with electrons, but this is an error. Pauli showed that the interaction of the charge with the magnetic field causes an incompressible blurring of the trajectories at least as large as the separation between the spin components. Free electrons' magnetic dipoles are not observable hence do not contribute to a magnetic field.

    The electrons responsible for magnetic fields in permanent magnets are those bound into magnetic atoms; the total field is from the magnetic atomic dipoles. However, in solenoid coils without a magnetic core there are no magnetic atoms and so the mechanism must be different - here the moving electrons' electric fields induce magnetic fields.
  10. Apr 29, 2013 #9
    Pumila: I made a mistake on this one in that coming back I confused you with the originator of the original question. Obviously you are working at a level that is considerably advanced compared to my simplistic treatment of the question.

    I suppose if a radiated electric field has a magnetic component then it is not too much of a stretch to apply a similar characteristic to the electric field associated with the moving electrons in a conductor.

    Then would follow the question as to whether the magnetic field in a conductor is a result of the organization/linearization of the ever present thermal activity of the electrons (that is much larger than the EMF drift) or only from the part of the "flow" due to electrical potential difference?
  11. Apr 29, 2013 #10
  12. May 21, 2015 #11


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    So wait does an increase in current result in an increase in magnetic field?
  13. May 21, 2015 #12
    By the way, did you notice that this topic is 2 years old?
  14. May 22, 2015 #13
    Maxwell's equations:
    $$ \nabla\cdot\mathbf{D} = \rho,\, \nabla\cdot\mathbf{B} = 0,\, \nabla\times\mathbf{E} = -\frac{\partial\mathbf{B}}{\partial{t}},\, \nabla\times\mathbf{H} = \mathbf{J} + \frac{\partial\mathbf{D}}{\partial{t}} $$
    What is the reason for "electron moves" throw a wire? The electric field.
    Electric field is combined with magnetic field even exist much, few or nothing electrons. Magnetic field varies because electric field varies. The fact than different electric field product is different charged particles movement not depends to magnetic field varies directly.
    The way to understand physics is history of physics, not the physics itself.
    Last edited: May 22, 2015
  15. May 22, 2015 #14
    When you run DC current through a coil you build a magnetic field in and around the coil. When the current is broken the field collapses, the magnetic lines of force in the field cut through the coil windings creating a new higher voltage than was induced resulting in higher current for a split second resulting in a larger longer spark than was seen when the connection was made.
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