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Magnetic Field problem in Griffith's book

  1. Dec 8, 2006 #1
    Griffith's problem 6.14
    For a bar magnet make careful sketches of M(Magnetization) , B (Magnetic Field),and H (Griffith's just calls this H...), assume L = 2a.

    Ok since ther is no free current here
    and [tex] \oint \vec{H} \bullet d \vec{l} = I_{f(enclosed)} [/tex]

    H =0 yes??

    3. The attempt at a solution
    Check out my attached (bad) diagram. I drew the magnetic field. THe magnetization exists ONLY inside the magnet so it points in the direction from S to N. Since H = 0 it would not make an appearance... right?

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  3. Dec 8, 2006 #2


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  4. Dec 8, 2006 #3
  5. Dec 8, 2006 #4

    Meir Achuz

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    Wrong. H=B outside the magnet, but is in the opposite direction inside the magnet. H for the magnet is the same as E would be for two unilformly charged disks at the ends of the magnet. B is the same as for a solenoid.
  6. Dec 15, 2006 #5


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    I didn't want to make a new topic for my question, so i put it here:

    What is the (electro?)magnetic field? Could you say it is like an ocean, since I know that it is the reason for that electromagnetic waves can move, since the energy is "waves" like in the ocean. Is the field stronger some places than others? and does it have anything to do with the electric field that each particle has? (to bind themselves to eachother, like electron (-) and proton (+))
  7. Dec 16, 2006 #6


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    At the first level, an electric field is a way of representing the force one charged object exerts on another. This is the force that is responsible for binding electrons and protons together. A magnetic field is a way of representing an additional force that a moving charge exerts on another moving charge. When the fields associated with these forces are constant in time, we refer to these fields as being "static" fields. However, charges are often in motion in ways that cause both the fields they produce to change with time. Several great minds contributed to the development of a theory of these fields culminating in the work of Maxwell who showed that time varying electric and magnetic fields could propegate as "electromagnetic" waves in the empty space between charged particles.

    As for the "ocean", physicists for a long time speculated about the existance of some ocean in empty space that was called the "ether". In the early 20th century, theoretical and experimental studies led to the conclusion that there is no such ocean needed for the electromagnetic wave to propegate through space.

    The field varies a great deal in both time and space. The study of electromagnetic fields is a huge subject that has evolved from a classical wave perspective to a quantum electrodynamic perspective. There are several books dedicated to the subject, and numerous places on the web where you can find an introduction to the theory.
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