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Homework Help: Force Due to Magnetic Fields

  1. May 20, 2014 #1
    Hi, I am a year 12 student and I have some some questions about magnetic fields.

    1. The problem statement, all variables and given/known data

    Do all magnetic fields (in particular ones produced by electromagnets) have distinct poles?

    How does a force act an object carrying current under presence of a magnetic field.

    2. Relevant equations


    (force on a current carrying conductor in a magnetic field)

    3. The attempt at a solution

    I was talking to a physics teacher today, and I asked in the case of a wire carrying current, does it have poles like a solenoid? My teacher said the solenoid acts like a bar magnet and the magnetic field lines touch the conductor, giving it north and south poles on the ends. In the case of a wire, the magnetic field lines are circular around it, and as the magnetic field lines never touch the wire, no poles are produced.

    I was then thinking, if the wire has no poles, why then is a force experienced on the wire when placed under presence of a magnetic field? Aren't all magnetic field forces caused by north and south poles interacting? In a way similar to how electric fields are affected by positive and negative charges?

    I would appreciate a push in the right direction, I feel like our physics dosen't go very much in depth. Thank you in advance.
  2. jcsd
  3. May 20, 2014 #2

    Simon Bridge

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    Welcome to PF;
    It is magnets that have poles, not the fields.
    Magnetic fields are caused by the movement of charges - electricity and magnetism are aspects of the same phenomena.

    The magnetic poles are an effect caused by fields of a particular shape.

    What you are noticing is that the y12 curriculum is not giving you a complete picture of magnetism.
  4. May 20, 2014 #3
    Magnetic monopoles are not known to exist.

    Even in a bar magnet/solenoid, there are no monopoles. Your teacher is oversimplifying things. If you pick any good physics textbook then the derivation for magnetic field due to a bar magnet is done by considering it to be similar to a solenoid and integrating the field due to rings of vanishing thickness. The books which derive expressions by considering magnetic dipoles are making unwarranted assumptions.

    Searching for north/south poles in a wire will not get you anywhere. If you want to know more buy a book which teaches physics at physics at college level.
  5. May 20, 2014 #4
    Thank you for the replies, much appreciated. May I just ask what causes the force, on a wire carrying current under the presence of a magnetic field? Is it to do with the magnetic field from the current carrying wire, interacting with the magnetic field of the other magnet? If that is the case I find it hard to understand as most things attract/repel along a straight line, wheras I believe the force on a current carrying wire acts perpendicular to the magnetic field.

    I would be happy to be linked any reading material if available, Thanks.
  6. May 20, 2014 #5

    Simon Bridge

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    The deflection of a current-carrying wire in the presence of a permanent magnet, or another current, can be understood at your level in terms of an interaction between magnetic fields.

    That is not the whole picture.

    In the standard model, it is understood in terms of electric charges interacting via photons.

    I think the next step to your understanding is to look at the force on a charge moving in an electro-magnetic field. The equation describing that is: $$\vec F = q(\vec E + \vec v \times \vec B)$$... where v is the velocity of the charge and q is the amount of charge. E and B are the electric and magnetic field vectors - which will usually differ at different points in space. The cross product means that the magnetic force acts perpendicularly to the field and the velocity.

    An electric current, in this picture, is a whole lot of charges moving along the wire at some speed - so this should help you understand the relationship between the force on the current and individual magnetic fields.

    The relationship between electric and magnetic fields gets clearer when you learn about relativity.

    Later on you'll also learn about intrinsic magnetic dipole moments and how atoms can act as small magnets... this is what gives rise to the usual magnetism of magnets that you are used to. But right now, you are still exploring the phenomena.
    Last edited: May 20, 2014
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