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Forgotten force: The other great cosmic attractor

  1. Nov 9, 2013 #1
    I have just read an article in the new scientist "Forgotten force: The other great cosmic attractor"

    Would anyone be kind enough to help me understand this paragraph please?

    "Now we have a pretty good idea how the sun and Earth generate their fields. As molten iron in Earth's outer core (or plasma in the sun's case) moves across magnetic field lines, the effect is to induce electric currents. These give rise to a magnetic field that supplements the existing one. Thanks to this dynamo action, a small "seed" field can grow into a much larger one."

    How does that work?

    I had always assumed wrongly it was molten iron ions moving that caused the field.
    So with the mechanism proposed above I would have thought that (from Lenz's Law) that the induced current's field would oppose the original field, rather than enhance it. How does this fit with conservation of energy?

    Any thoughts gratefully received.
  2. jcsd
  3. Nov 9, 2013 #2


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    The intergalactic magnetic fields are quite weak (as stated in the article) - much weaker than the earth's magnetic field.

    So does Iron Mountain, Michigan move under the influence of our magnetic field, or does it stay put due to gravity?

    BTW, the earth's magnetic field is explained here: http://en.wikipedia.org/wiki/Earth's_magnetic_field#Physical_origin
  4. Nov 9, 2013 #3
    Hi UltrafastPED,
    Thanks for the link,
    This is what I have understood so far.

    Three conditions are necessary for the generation of the magnetic field.

    • An electrically conductive fluid medium (The liquid iron in the outer core)
    • Kinetic energy provided by planetary rotation (Coriolis effect)
    • An internal energy source to drive convective motions within the fluid (Convection in the liquid iron)
    In a perfect conductor Lenz’s law would ensure that any change in magnetic field would be opposed. It is the convective currents that deform the liquid that generate a changing field. This process is limited by the competing action of Lenz’s law.

    Do we have any idea of origin of the magnetic field the seeded the process originally as the Earth was formed. Was the Earth's field formed with it?
    Thank you for any links or information
  5. Nov 9, 2013 #4


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    Here is a more interesting presentation:
    http://science1.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield/ [Broken]

    which contains this link for the dynamo effect:
    Last edited by a moderator: May 6, 2017
  6. Nov 10, 2013 #5
    Thanks for your help - I'm still digesting the links you gave me.
    So once the fluid has interacted with the seed field, it then generates its own magnetic flux, which due to the flow of a conducting liquid iron, creates a feedback loop, which increases the flux density of the field, limited only by Lenz's law and the magnetic and mechanical properties of the fluid.
    Surely this field would be orientated in opposition to the seed field due to Lenz’s law,
    I still don't really see why the seed field is needed - surely the flow itself would create a magnetic field anyway? Moving delocalised electrons constitute a current, and they are moving in a helical motion, like d.c. in a solenoid.

    Thanks for all the help
    ain't physics brilliant!
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