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Beginner question about magnetism

  1. May 14, 2015 #1
    Hi all,

    I read the FAQ so please take into account that this is NOT a perpetual motion (free energy or whatever) question but a theorical case that puzzles me.

    I read that magnetic fields have an infinite (but decreasing) range. What bothers me is this :

    let's put a magnet and a iron far way from each other (in space far from any influence, for the sake of the reasoning let's say no gravity nor other magnetic fields). Then the iron should be attracted by the magnet no matter how far they are (even if it takes a very long time to attract each other).

    If I demagnitize the magnet after the iron has been accelerated towards the magnet, then we get an object with cinetic energy ? If we let the iron ball pass the magnet and magnetize the magnet again, then the ball and the magnet will attract each otehr again.

    My problem here is : the energy necessary to (de/re)magnetize the magnet is not proportional to the distance between the iron and the magnet. Meaning, if we wait a very long time in order to have the iron very far from the magnet, then we'll get a bigger cinetic energy once we magnetize the magnet again.
    Of course, this process would take a very long time since magnetism's strength decrease very quickly with distance but since range is infinite, it would be only a question of time before the iron would come back.

    Far from me to say it's a free energy concept, I just would like to know what is wrong in the reasoning above in order to better understand magnetic principles. Thanks in advance
  2. jcsd
  3. May 14, 2015 #2


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    Science Advisor

    You intuition is wrong. If I let the iron ball accelerate towards the magnet, then demagnetize the iron ball, let the iron ball pass the magnet, and then remagnetize it again some distance away, the iron ball will just keep going. The magnetic force will be too small to bring the iron ball to rest and re-accelerate it in the other direction. Just because the force extends to infinity does not mean that it is large enough to bring the iron ball to rest.
  4. May 14, 2015 #3
    Oh ok, thanks for the answer. So this rises another question about how forces work.

    Is your answer also valid for the following case : two objects attracted to each other by their gravity. I understood that two objects alone would attract each other no matter how far they are. If one of the object has a speed (moving away from the immobile object), should it not progressively slow down and finally be attracted again towards the immobile object ? How can an object be constantly submitted to a force (even tiny) without losing progressively his impetus ?

    Since magnetism also follows an inversely proportional relation with the square of the distance, I assumed that the iron would come back in the same way that any object would fall on a planet, no matter how far the object is (just a question of time) as long as it does not enter orbiting of course.
  5. May 14, 2015 #4


    Staff: Mentor

    As long as the velocity of the one object is greater than the escape velocity for the immobile object then the object will not return. It will progressively lose impetus, but never to zero or backwards, always to some finite separation speed.

    Also, I believe that a magnet falls off faster, something like 1/r^3 or 1/r^4
  6. May 14, 2015 #5
    Ok, now I understand where was the misconception... but that raises the question of minimal speed. I mean, how slow can an object be compared to another one ? I know this sounds like one of Zeno's paradoxes but has not modern physic solved that question already ?
  7. May 14, 2015 #6


    Staff: Mentor

    At rest. Relative velocity = 0.

    Zeno's paradox is irrelevant here (besides being resolved for centuries). I don't know why you are bringing it up.
  8. May 14, 2015 #7
    because you said "It will progressively lose impetus, but never to zero or backwards, always to some finite separation speed.". So actually it is me who does not know why you're bringing up speed at rest.
    I said "it sounds like zeno's paradox" but the question is "what is the minimal speed" and not "what is speed at rest" (which would be silly).
  9. May 14, 2015 #8


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    Staff Emeritus
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    Your question is unclear. Are you asking how slow the metal ball can be relative to the magnet in your scenario without being brought back to the magnet after passing it? Or are you asking a general question?
  10. May 14, 2015 #9
    I have already read several articles where they clearly state that they don't know yet if energy is of a discontinuous nature. As far as I know, quantum physics does not work beyond a certain density of energy.

    It is clear... if you read the question I initially asked (and not the answer to the answer of my question). I just didn't repeat the context of the initial thought-experience because that would have been, well, repetitive.

    So to be clear, it's your first assumption : "how slow the metal ball can be relative to the magnet in your scenario without being brought back to the magnet after passing it". The other option would be senseless since it would mean "what's the slowest speed" and the answer would be zero, duh :)
  11. May 14, 2015 #10


    Staff: Mentor

    The thread is reopened, but the digression about Zenos paradox is removed. It is not considered a real paradox.

    @Incredible_bulk discussion of PMM is not permitted here. You expressly claimed that you are not proposing a PMM but your argumentative stance seems to contradict your claims of wanting to learn. We are willing to explain this stuff, but not debate it.

    It will always travel at some finite separation speed IF it is traveling faster than the escape velocity initially. That speed is an asymptotic speed, not a minimum speed. The value of that speed can be found through conservation of energy.
    Last edited: May 14, 2015
  12. May 15, 2015 #11
    I never did such a thing, I accepted the answer and claimed nothing else. I expected more from a supposedly scientific moderator. You had not answered the question "what is the minimal speed" hence me asking that. It is a fair and interesting question. Zeno's paradox is called like that, it's the name and I used the name everyone use. The fact that you made a wrong statement (before erasing it) tells a lot about the your level of self-question here and the ego that it implies.
    You pretend you want people to learn.. but you don't want to learn yourself.
  13. May 15, 2015 #12


    Staff: Mentor

    I misunderstood your follow up question, but there were no wrong statements in my response. Also, I didn't delete any of my statements. I deleted the Zenos paradox digression which I was not part of.

    Anyway, I think this thread is done. For gravity and the asymptotic speed you should read about escape velocity and gravitational potential energy. For the force law between the magnets you should read about magnetic dipole dipole interaction. For energy conservation in EM you should read about Poyntings theorem.
    Last edited: May 15, 2015
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