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Magnetic Field vs Magnetic Force

  1. Jul 2, 2003 #1
    I got sucked into the page on a website about magnets. It got into stuff about magnetic force on a moving charge. It said that the force on the charge is perpendicular to the magnetic field and the velocity of the charge. I got very confused because it called this force the "magnetic force" and it was clearly discerning it from the magnetic field. I had always considered the magnetic force to be what I felt when I brought two magnets together. Yet the force I feel doesn’t appear to be perpendicular to the magnetic field between the two magnets. So anyway... is there a difference between force and field?

    Here's a link to the page I found this stuff on

  2. jcsd
  3. Jul 2, 2003 #2
    They're describing the force

    on a single moving charged particle, the force between two magnets is much more complicated than that. And yes, the field and the force are different constructs.

    However you can model the two magnets as being made with moving charges, such as in two solenoids, and the behavior of the individual charges and the two magnets will agree. So their description is correct.
  4. Jul 3, 2003 #3
    If I could just understand that I'd be satisfied, but that’s really what I'm not getting. Isn’t it true that when you place a north near a south pole you get flux lines running basically straight to each other (like the illustration on that page). If those flux lines represent the magnetic field and if you believe the description on that page how could you have a force running straight along those flux lines. The force I feel when I place a north near a south surely isn’t perpendicular to the illustrated flux lines.

    Help me understand this and I'll be happy.
  5. Jul 3, 2003 #4
    Because with your two magnet example, you have no moving charges. There is a current moving through the wire and the moving charge has a force applied to it from the magnetic field it is moving through.
  6. Jul 3, 2003 #5
    Aren't magnets made up of moving charges? The atoms are aligned and the electrons circle somewhat uniformly...?
    Last edited: Jul 3, 2003
  7. Jul 3, 2003 #6


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    Ultimately the "moving charges" in a magnet are electrons.

    The iron in the magnet is composed of small regions called domains. And there are free electrons moving in there forming what is called a Fermi gas. And in iron (but not in most other metals), they tend to move the same direction within the domein and reinforce the induced magnetic fields.
  8. Jul 4, 2003 #7


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    Force Perpendicular to Magnetic Field...

    Consider a magnet on its own. A North Pole and South pole creates a magnetic field between the two poles. Consider the magnetic field lines moving out of the North Pole and then sharply directing towards the south pole. (This is hard to illustrate with words) But, many diagrams present this clearly.
    When two magnets are brought together, head on there will be either an opposing or attractive force.
    At the point where you experience this force the magnetic field lines (magnetic field lines are not actual lines, but are merely used as an effective means of demonstrating magnetic fields) are perpendicular to the plane of the magnet.

    It helps to draw yourself a diagram.
  9. Jul 4, 2003 #8
    The concept of a field in general does not make any sense without an associated force. It is just a formal means to translate an 'action-at-distance' into a pseudo-local interaction.
    Both the electric and magnetic forces are caused by interacting charges, the electric force by a static interaction, the magnetic force by a dynamic interaction (associated with a motion). The force between two magnets is in fact also a dynamic interaction associated not with moving but spinning electrons.
  10. Jul 4, 2003 #9
    The magnetic force and magnetic field are always at right angles to one another.

    This is different to the electric field and force which are parallel to one another. The electric field is multiplied by a charge at that point to give the electric force.

    Similarly the magnetic field is 'cross-producted' by an electric current at that point to give the magnetic force.
  11. Jul 8, 2003 #10
    I've been doing a bit more reading on this topic, and I've got a new question:

    What constitutes for a 'moving' charge? Let's there is an electron floating out in space. Let's also say we have an electromagnet here with us on earth, and the electron is still relative to the magnet. As soon as I turn the electromagnet on, the magnetic field propagates out at c, correct? When it reaches the electron, could you say the electron is moving through the magnetic field even though it is not moving relative to the magnet?
  12. Jul 8, 2003 #11

    As long as the field and electron are moving relative to one another there will be a force applied to the electron. But the force will always be at right angles to the magnetic field lines. So if you make the motion along the field lines the electron won't feel any force.
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