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Trying to understand magnetic pump

  1. Oct 26, 2009 #1
    I'm confused how a magnetic pump works as described in my text. I'll quote:

    """Liquid metals, such as the molten sodium used in certain nuclear reactors, can be moved through pipes using an electromagnetic pump that has no moving parts. If the metal has to be moved in a pipe that is orientated north-south, for example, a large electric current can be sent across the pipe --east to west perhaps. Then, if a strong magnetic field is directed downward through the same section of pipe, the current-carrying metal will be forced to move southward"""

    This appears to be two magnetic fields, one crossing the pipe also in a horizontal plane to the current, and the other magnetic field directed downward perpendicular to the pipe direction. But I don't understand why you need two.

    My understanding so far:

    First Field: If the current is across the pipe east to west, right to left, and the magnetic fields are circular moving north-south along the pipe, the force is already downwards in a vertical sense and not down the pipe.

    Second Field: A second field vertically downward apparently moves the molten sodium if the force is also southward.

    I thought that magnetic fields did not interfere with charge, so I'm not following how the molten sodium itself actually moves at all. Why does this need two magnetic fields?

    Thanks for any understanding.
    Last edited: Oct 26, 2009
  2. jcsd
  3. Oct 26, 2009 #2
    Hi sf2k-
    The force is called the Lorentz force. The force F is the vector cross product of the current I and the magnetic field B.

    F = I x B

    The force is perpenducular to the plane containing the current and the magnetic field.
    Here is a nice web site with a visual java applet. Click on the knife switch to make it close and open:


    Bob S
    Last edited: Oct 26, 2009
  4. Oct 26, 2009 #3
    First off thanks for the link and a very nice reply!

    Ok, so in one case the first magnetic field has a Lorentz force vertically down and the other Lorentz force is in the plane of direction to move the molten sodium. Where I'm not following is why there needs to be two fields to move the sodium? And how can it move at all if there are two forces in perpendicular to each other?
  5. Oct 26, 2009 #4
    The "magic" in the Lorentz force is that the force is perpendicular to the plane in which both the current and magnetic field vectors lie. The current and magnetic field vectors cannot be parallel. Only the mutually perpendicular component of B and I is important (vector cross product). You need only one current and one magnetic field to create a force. By reversing the current, you can reverse the direction of the force.
    Bob S
    Last edited: Oct 26, 2009
  6. Oct 26, 2009 #5
    yes thank you. I"m sorry if I'm not clear.

    I know that both the current and the magnetic field are in the same plane but 90 degrees apart while the force is perpendicular 90 degrees to their plane. like x,y, and z.

    however the magnetic pump needs two such magnetic fields and not one. With only one's force used to move the molten sodium. So it's really an issue of understanding the forces that move the sodium along and why I need two magnetic fields to accomplish this in a magnetic pump.
    Last edited: Oct 26, 2009
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