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Dir. of field vs. dir. of force, what's up with that?

  1. Mar 24, 2004 #1
    An electron is moving in the plane of a page in my physics textbook up towards the top. What do you know? A magnetic field is also in the plane of the page, directed towards the right. What is the direction of the magnetic force on the electron?

    According to the right hand rule, the magnetic field should point IN to the page. However, the correct answer is OUT of the page? Why? Does it have something to do with the particle being an electron???

    Also, I'm not quite clear on how a magnetic FORCE can act in a direction other than the same direction the magnetic FIELD is acting in? Gravitational and electric fields act in the same direction as their force! I thought the direction of fields and forces were inseperable--two and the same--and I'm not sure why it differs with magnetic fields, and more so, what does the fact that they operate in different directions MEAN anyways, in laymens terms???
  2. jcsd
  3. Mar 24, 2004 #2
    Yes, remember the force is defined as:
    [tex]F_m = qvB\sin \alpha[/tex]
    If [tex]q[/tex], the charge of the particle, is negative, the force operates in the opposite direction. More information:

    Someone else will have to answer your second question, as I can't explain it very well (primarily because I don't quite understand it msyelf :smile:).
  4. Mar 24, 2004 #3


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    A "field" is not a "force". a field is just a quantity used to show how to calculate a force. Since the magnetic force on exactly the same charge moving in different directions will have different directions (which is not the case for electric field or gravitational field), it can't show the direction of the force- that depends not only upon the field but also upon the properties of the object.
    Last edited by a moderator: Mar 24, 2004
  5. Mar 24, 2004 #4
    Huh? I got the first two sentences. But here I'm clueless. Can you reiterate?
  6. Mar 25, 2004 #5
    He is saying that a magnetic field cannot show the direction of the force because the force direction depends on the direction the charged particle is travelling in the field.

    Electric fields do not share this problem because the electric force is velocity-independent.
  7. Mar 25, 2004 #6
    Ahhhhh!!!!! Thanks! :smile:
  8. Mar 25, 2004 #7

    Regarding your second question:

    The 'magnetic field' is a physical phenomenon of the magnetic object just like gravitational and electrical fields. However, the term 'magnetic force' is a bit misleading because it refers to the force effect on a moving charge and not the force of attraction/repulsion between another magnetic object. Both gravitational fields and electric fields are utilized to show relations between masses (gravity) and charge (electric), both of which line up with the forces exerted between two or more objects with mass and/or charge.

    If you take two magnetics and place them near each other, the magnetic will align themselves along the field lines, or what is sometimes called the lines of force. The force of attraction between the two magnetic will act along the field lines pulling the magnetic towards each other instead of perpendicular to the field in the case of the effect on a charge.

    Unfortunately, the term 'magnetic force' is really not correctly applied in physics in my opinion. 'Magnetic force' should apply to the interactions between two magnetic objects and another term should have been coined for the force interactions between magnetic objects and moving charges.

    Of course, this is just my opinion, but it may help you to reconcile your confusion expressed in your second question. If you take a look at typical diagram showing the relationships between a magnetic field and a charge, there are two fixed magnetic poles shown (north and south) between which the magnetic field established. Now imagine if the two magnetic poles where release so that they can move freely. What would happen?
  9. Mar 25, 2004 #8
    Two words: Lorentz force
  10. Mar 25, 2004 #9

    My reply was geared toward helping PCake understand and reconcile the confusion that the accepted terminology creates. Discussion of the Lorentz Force Law would not explain why the 'magnetic force' terminology creates the confusion expressed in the second question. :smile:
  11. Mar 25, 2004 #10
    Thanks Heorman! Things are (conceptually) a lot more clear now. I hope you will be around to check out some of my homework problems I'll be posting later on tonight!!!
  12. Mar 25, 2004 #11


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    Well, I'll say that that is fair enough, but can you define a "magnetic object?" I don't recal the term 'magnetic force' being applied in physics seriously, but I'll go ahead and assume that it has been. There is an appologetic explanation for this; that is, historically there was a stone discovered on an island named Magnesia that was recognized to induce a force on certain materials such as iron. The term "magnetism" was coined, and has stuck with us. Does that mean we are always referring to stones found on the island of Magnesia when we talk about sources of magnetism. Certainly not; no more so than we literally mean "backwards" every time we refer to electricity. These are just etymologies that some people may decide to deam unfortunate because they have been endowed with that dangerous marginal level of intellectual privelage.

    A couple of supplementary comments:
    - Magnetic monopoles have yet to be discovered.
    - Magnetism and Electricity were originally considered distinct, but have since been united (along with the weak interaction, but I don't know anything about that, so I'll leave its mention in this parenthetical).

    There is nothing wrong with calling the force a charge experiences by virtue of its relative velocity a "magnetic force." In fact, that is less confusing than calling it an "electric force," because this term is already reserved for the Coulomb style force. The best thing one can do to understand this peculiarity of nature, IMO, is to understand that the electromagnetic field is more than just a vector field in three dimensions; it is a second rank tensor field. If one is not so inclined to accept this notion, or is not so ambitious to appreciate it, then one could alternatively accept that the magnetic force is a consequence of an assymetric length contraction between the source and test.
    Last edited: Mar 25, 2004
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