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Homework Help: Magnetic Fields

  1. May 20, 2007 #1


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    Hi. Im having a little trouble grasping the concept of a magnetic field. My book doesnt explain it very well.

    I understand that any magnet has a magnetic field, which is the space around it in which other magnets are subject to a force due to the magnet.

    What im having trouble with is concepts of magnetic flux and magnetic flux density. From what ive read, magnetic flux density is a number of field lines per unit area (where the field lines are perpendicular to the area). What im not understanding is how a number can be assigned for the number of field lines passing through an area. I thought the idea of field lines were invoked for the simplicity in graphically representing magnetic fields. I would have though that in an actual magnetic field, the number of actual field lines would be infinite for any given area.

    Is somebody able to clear up this misconception?

    Thanks in advance,
  2. jcsd
  3. May 20, 2007 #2
    Hi Dan,

    You're right. The concept of field lines and flux density is all really abstract. Its not meant to be literal when they say flux density B is the number of field lines per unit area. (Since you don't really measure field lines in numbers; its measured with units tesla). Its just so that when we're looking at a diagram, we can say that a particular spot has greater B when the density of lines is greater. Sort of like the closer the lines are together, the stronger the field.

    Does that help?
  4. May 20, 2007 #3


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    Of course you are correct that field lines are just a way of graphically representing the field. That is also true of magnetic flux and magnetic flux density. The concept of counting lines passing through an area is just a way of graphically representing these concepts to make them easier to grasp.

    I think you also have to remember how field lines are used. So when the magnetic field is strong field lines appear closer together and thus the magnetic flux through a unit area for example will have more field lines passing through it than the flux through an area in a weaker field.

    EDIT: Kenny beat me but hopefully two points of view might help.
  5. May 20, 2007 #4


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    That helps alot, thanks guys :smile:
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