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Attraction of a steel rod with a solenoid

  1. Jan 22, 2013 #1
    1. The problem statement, all variables and given/known data
    From this video: http://www.youtube.com/watch?feature=player_detailpage&v=mdZo_keUoEs#t=77s it shows that the steel rod will eventually stay in the middle of the solenoid. At first the magnetic field of the solenoid would look like this:http://postimage.org/image/amsg16gon/full/

    It looks similar to 2 magnets with opposite poles and hence they attract each other. Once the steel rod reaches the middle of the solenoid, the magnetic field would look like this: http://postimage.org/image/um9vlmjo3/

    When this happens according to the video there is no more attraction.

    So if we look at the first image where there is attraction, actually what is the cause of the attraction? And in the second image what is the cause for it to not be attracted?

    The main question of this would be, how does a magnetic field apply a force on another magnetic object?

    2. Relevant equations

    none

    3. The attempt at a solution
    I'm thinking for the first image, attraction occurs due to the magnetic field lines concentrating in the steel rod. However, I don't think that is the right answer as when two magnets of the same size and strength with opposite poles are placed together they are still attracted. So my reasoning is wrong.

    For the second image. I think it's because the magnetic field around it is constant so there is will be no force applied.

    However, for the main question i have no idea how forces will arise from magnetic fields. The answer to this can probably be used to answer the first 2 questions as well.

    Please help me to find a solution to this thanks :smile:
     
  2. jcsd
  3. Jan 22, 2013 #2

    Andrew Mason

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    The coil produces a magnetic field when current flows. This causes the iron molecules in the steel rod to align with the magnetic field. The iron molecules act like little magnets and when they are all aligned the same way, the steel becomes a magnet that is attracted to the coil. This attraction moves the rod toward the coil. When the rod is in the middle of the coil, the attraction ends. If it moves past the middle, repulsion begins so it remains in the middle of the coil.

    AM
     
  4. Jan 22, 2013 #3
    Hi Andrew thanks for the reply. However what do you mean by repulsion? Why would it stop attracting in the middle? Is it possible to explain this using the magnetic field?

    It's quite hard for me to understand this because the steel rod is actually sort of inside a magnet. If we assume that the steel doesn't produce its own magnetic field then how can there be repulsion? Repulsion requires 2 different magnetic fields to do so however attraction only requires one (if I'm wrong here please correct me and explain so). Since it doesn't produce its own magnetic field then how would it be repelled?

    So in the end the main question is still how the magnetic field will give rise to the force.

    Thanks :smile:
     
  5. Jan 22, 2013 #4

    Andrew Mason

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    You have to carefully read my earlier post. The steel rod becomes a magnet in the presence of a strong external magnetic field. The field of the coil causes iron molecules, which are little magnetic dipoles (ie. little bar magnets) to turn and align with the magnetic field. This phenomenon is referred to as induced magnetism. This turns the steel rod into a bar magnet while there is current flowing in the coil.

    AM
     
  6. Jan 22, 2013 #5
    Oh then what if i use a material that is very difficult to magnetize? Or is the reasoning this: If it gets attracted then some of it's dipoles have been rearranged such that it would produce it's own magnetic field (even if it's a tiny amount). Then in that case, why would the magnet stop in the middle? Since the repulsion can occur when it is partially inside as well (now assuming that it has it's own magnetic field).

    Thanks :smile:
     
  7. Jan 22, 2013 #6

    Andrew Mason

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    If you use a material that is difficult to magnetize you will not get magnetic attraction. This is why fridge magnets do not stick to aluminum fridge panels.

    In order for a material to be attracted to a magnet it has to have atoms with magnetic dipoles that are free to align with the applied magnetic field. The applied field and the material's dipole moment both have to be strong enough so that the magnetic force is sufficient to overcome interatomic forces and thermal energies that operate between the atoms. Iron works, but very few other elements have these qualities.

    AM
     
  8. Jan 22, 2013 #7
    Oh in that case, we can treat the the steel rod as another magnet when it is near it? So how will the magnetic field of the solenoid be like? Initially will the magnetic field look like this http://postimage.org/image/amsg16gon/full/ or like this

    I think it should be the second one as it should start to have it's own magnetic field when it's dipoles gets rearranged. However, once part of it is inside the solenoid, besides the pull of the solenoid won't there be repulsion by the steel rod's own magnetic field?
     
  9. Jan 23, 2013 #8

    Andrew Mason

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    Don't worry about what the interacting magnetic field looks like. It is complicated.

    Just think of two magnets, one hollow and the other able to fit inside the hollow magnet. As they approach each other the approaching poles are opposite (eg. bar's north pole is approaching coil south pole). So the bar is pulled into the coil. As the bar goes out the other side the bar's north pole is met by the coil's north pole, and the bar's south pole meets the coil's south pole so the repulsion slows down. If the is pulled out the other end of the coil, the bar is attracted back into the coil just as it was on the other side.

    AM
     
    Last edited: Jan 23, 2013
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