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Homework Help: Inserting a loop of copper wire between 2 magnets, what happens?

  1. Sep 7, 2010 #1
    1. The problem statement, all variables and given/known data

    You have 2 magnets separated by a distance. The top magnet has its south end facing the north end of the other magnet thus forming a magnetic field. Finally you insert a loop of copper wire (the pic shows a copper tube though) between the two magnets with the open ends perpendicular to the surfaces of the magnets. Is there an attractive magnetic force that tends to pull the loop in, like a magnet pulls on a paper clip? Or do you need to push the loop in against a repulsive force? Explain.

    2. Relevant equations

    no equations but eddy currents may be relevant.

    3. The attempt at a solution

    Ok, so my best guess is that as you put the loop between the two magnets an eddy current is formed and just wants to hold the piece of copper loop in place so you need to push it into the field. Does this sound right or am I way off base?
  2. jcsd
  3. Sep 7, 2010 #2
    You will generate an emf that will induce a magnetic field. I'll ask you, what will the induced magnetic field look like, where will it point, etc.?
  4. Sep 7, 2010 #3


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    Hint: There are 3 kinds of magnetism, Ferromagnetism, paramagnetism and diamagnetism. Which does copper usually fall under? Does that type create a field opposed or aligned with an external field (like the one from your two magnets)? Does that result n an attractive or repulsive force?
  5. Sep 7, 2010 #4
    [PLAIN]http://img837.imageshack.us/img837/5986/diagram1j.jpg [Broken]

    Ok, well from my scrappy paint made image above there will be a clockwise current through the loop. i base this on the right hand rule because B is flowing from north to south, v is to the left so the positive charges will move into the page and the negative towards the side we can see and since electricity flows from positive to negative there you have it.

    Hopefully that's right otherwise my entire nights studying went very wrong. Thanks anyways for the help.
    Last edited by a moderator: May 4, 2017
  6. Sep 7, 2010 #5


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    I don't think that's what the questioner is looking for here. Macroscopic current loops are not the only source of magnetic field that can be induced (not only that, but I don't think you've got the current induced macroscopic current). An external magnetic field will also have some effect on the orbital angular momentum of electrons, as well as extering a force on each electron and proton due to their intrinsic magnetic moments. Theoretically sorting out the net effect of these 3 mechanisms is often extremely complicated, but copper is an often-used, well-known material and its net magnetic properties (experimentally determined) are easy to look up.
    Last edited by a moderator: May 4, 2017
  7. Sep 7, 2010 #6
    The question is a concept question for a chapter in my text book and we are supposed to be able to explain it using the concepts covered in that chapter. The topics covered are:

    -Induced currents
    -Motional emf (eddy currents come in here and this is the only part of the chapter that discusses there being a resistive force between 2 magnets)
    -Magnetic flux
    -Lenz's Law
    -Faraday's Law
    -Induced Fields
    -Induced currents:Three applications
    -LC Circuits
    -LR Circuits

    I appreciate the information and quick reply gabbagabbahey but what you've explained to me isn't covered in the chapter, nor have we covered it in lectures yet.
  8. Sep 7, 2010 #7


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    You haven't touched on paramagnetism, or diamagnetism at all? Usually there is at least a paragraph or two early on in an introductory EM text that tells you that some materials are paramegnetic, others are diamagnetic and which one result in an attractive/reulsive force when placed in aq magnetic field.

    Considering that the copper wire has open ends and isn't bent into a closed loop, I'm pretty sure the questioner is extecting you to identify whether copper is paramagnetic or diamagnetic and what type of force it will experience in an external field (atrractive or repulsive).
  9. Sep 7, 2010 #8
    The original poster's question said the wire was bent into a loop. I actually even discounted the fact that the metal was copper, and simply treated it as a conductor, when I originally read the question. I think you could, and probably should, get into magnetism, but I think you should be able to use faraday's law to figure out the emf of the loop, then use the emf to figure out the induced magnetic field, which finally tells you what magnet it will go towards.
  10. Sep 7, 2010 #9
    I remember those terms from back in yr 12 but I don't think they apply here since they are not mentioned in this chapter.

    I did however come across some additional information in the section on eddy currents. It says that wire, typically copper, is not a magnetic material. A piece of wire held near a magnet would feel no force. It is the induced current in the complete loop that causes the wire to experience a retarding force.

    It then goes on to explain about the current whirlpools which are the eddy currents and says that it is these that will cause a non magnetic material being pulled between two magnets (like in my diagram) to quickly decelerate until it stops. It also says the same applies for a material being pushed between two magnets and gives an example of magnetic brakes on a train, it then talks a little about heat, etc.

    So I think that as the loop is pushed between the two magnets there will be an induced current. It is this induced current that will interact with the magnetic forces to create a repulsive force that will oppose the direction of movement that created the current.

    oh and to add to your last post mindscrape, the original question just asks if the loop will be pulled in between the two magnets or if it will experience a repulsive force.
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