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Practicality of building large size magnets

  1. Mar 27, 2016 #1
    I am curious about the possibility of building a large permanent magnet that could exert a huge pushing/pulling force of about 135 million newtons. In answering the questions, I wanted to have these following points addressed:

    1. The magnet is 30m wide by 30m long....would have to push a plastic container (that is 30m wide by 30m long and 20meter high) that has a steel lined bottom, down in a body of water about 15 meters. (thus displacing down a force of about 135,000,000 Newtons? Thus...how high would the magnet dimension be to achieve this push? (i.e. 30m * 30m * ?m)

    2. For the first case no use of natural expensive rare magnetic elements such as Neodyium or Samirum cobalt -- just the use of permanent magnets made using regular industrial methods (such as high temperature heating etc...)

    3. For the second case how thick would the magnet have to be if it was made of Neodyium or Samirium Cobalt?
  2. jcsd
  3. Mar 27, 2016 #2

    Simon Bridge

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    Neat questions - the magnet would attract the steel of the object, so you are thinking of putting it on the bottom and using to hold the object to the bottom.
    Very big magnets are usually electromagnets - basically so you can switch them off. There does not seem to be any practical advantage in using a permanent magnet for the purpose suggested... so the answer to your question is "not practical at all". Much simpler to hold something underwater using hooks and chains and very heavy weights.

    Have you seen:
    ... can't seem to find a similar calculator for an old-fashioned iron magnet.
    They tend to be very weak compared with modern ones ... I wonder if you'd be better just to put the iron on the container to sink by dead weight.
  4. Mar 27, 2016 #3


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    Beginning with 15m initial separation, over a 30m base, really limits you to one pole only.
    You need some geometry that reduces magnetic path length continuously.

    There are probably better materials than steel to line the base of the box.

    The magnet assembly must not lift off the bottom and rise to stick to the box base, so they will need to have a weight, less their buoyancy, of 135 MN.

    Magnetic forces are used in industry to shape relatively thin metal sheet. Will the steel lined bottom of the box be shaped by your magnets from below.

    Magnetic clamps used to hold steel in grinding machines are made with permanent magnets that can effectively be turned on and off by the movement of an internal gang of keepers.

    You have not considered superconducting electromagnets.

    The solution to the original problem is almost certainly not magnetic. Too much damage will be caused by the many old bicycles and anchors that will get caught in the gap as it closes. Any straight steel rods will stand on end and so puncture the hull.

    You might consider flooding the plastic box. That will prevent collapse of the plastic wall by maintaining the hydrostatic pressure on both sides. An alternative might be to build an open caisson, then pump the water out.

    Or you could place a rotating impeller, or many small propellers, on the floor that cause a circulation of water. The pressure in the centre will fall as rotation of the fluid increases. That is like being sucked down a whirlpool.
  5. Mar 29, 2016 #4
    Yeah my idea is to not use any external energy input to do so --- that is why I chose the permanent magnet idea. My idea also is to push down on the object so I guess I would be trying to use 2 ends of the same pole to repel each other and hold the container down in the water. With this I have 2 further questions which I can't find on google:

    1. I think the answer is no...but if 2 identical magnets of the same strength are lined up on the same poles, would the repel force be doubled (i.e. - the force of each is X, and thus the 2 magnets push each other with 2X?

    2. For the situation above, would the effect repelling force be twice as far as the attraction force distance? For example, if the pulling force for a single magnet is Y, would the repelling distance for 2 magnets facing each other of the same pole be 2Y?
  6. Mar 30, 2016 #5
    Baluncore, thank you for all the great input and thought into my question!! I really appreciate the detailed thought when answering my question. Here is some of my response to some of your suggestions (some are questions to your responses :) )

    1. Can you direct me to a website that describes how these "gang of keepers" work?

    2. I am not considering any superconducting electromagnets due to efficiency of energy used -- I do not want to use much external energy for this experiment.

    3. I did not consider the flooding of inside the box because my original idea was to repel the container bottom -- I guess that means I must line the bottom with a series of magnets and have the same poles facing

    4. So...a magnet this powerful that can pull or push at 135MN is not really plausible cause other objects will get caught? How close will object be before they experience a significant pull? What if we clear the area as much as possible?
  7. Mar 30, 2016 #6

    Simon Bridge

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    Well, the thing is that repelling an equal magnet means you have two very large heavy magnets ... one of them on the container you want to be able to float when not pushed down.
    I still think the magnet would be so large you'd be better off just using it to sink the container.
    What is the problem you are trying to solve? i.e. what is it for?

    Pushing down into water with no external power source sounds like a buoyancy generator is being attempted.

    Force between magnets:
  8. Apr 23, 2016 #7
    I am not exactly trying to build a buoyancy generator but without getting more into details it is does involve the buoyancy of water combined with magnetism to generate a potential energy situation...I guess like a million other people. My friends are all calling me a nutcase...but hey, at least I am thinking about it. I am not totally naive and understand all the cases against the laws of thermodynamics and creating some kind of perpetual motion machine (which I think can't work).

    Using the magnets to sink the container is not an option for the idea but thanks for the idea.

    Thanks for the input on this Simon. I am floating around with other ideas for now will be posting other questions soon.

  9. Apr 23, 2016 #8


    Staff: Mentor

    I believe that the video below shows what Baluncore called "keepers".

    It also demonstrates very well how short range magnetic forces can be. That's not what you want to hear.

  10. Apr 23, 2016 #9
    Thanks for the video Anorlunda! But do you know how it turns it off or on? Do they just bring the magnet closer to the front of the switch where the pull is exponentially greater? Or is it blocking the magnetism with some thin metal plate?
  11. Apr 23, 2016 #10


    Staff: Mentor

    It is just distance. If you watch the video very carefully, you see the magnet move as he turns the knob.
  12. Apr 25, 2016 #11


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    Sorry about not finding an example. I looked for a good diagram of a “magnetic work-holding clamp” but the manufacturers do not provide good diagrams or details of their solutions. I ignore electrically switched electromagnet systems and consider only mechanically switched systems. There are two types. Both operate by re-arranging the magnetic flux. Some work must be done when re-arranging the field pattern.

    Firstly; Most commonly, by moving magnets. A line of magnets arranged on a magnetic base plate, say all with N pole up, S pole down. Non-magnetic spaces are kept between the magnets. The top surface is then N-N-N-N. If every second magnet was then rotated by 180°, half of each surface would becomes S. The upper surface is then a N-S-N-S pattern which will clamp close magnetic materials.

    Secondly; By moving the flux path. Consider a line of fixed magnets, with alternating polarity, on a magnetically conductive base. A magnetic keeper resting at the bottom of the space between magnets makes little difference to the flux at the upper surface. But if the keeper is raised to be between the upper poles it will “short circuit” the flux between the N-S-N-S upper surface poles, and so reduce but not completely remove the attraction of magnetic materials to the upper surface.

    The short range of attraction is a fundamental problem that this topic's model needs to overcome. If the horizontal separation of poles in the surfaces is less than the gap between the surfaces that need to be attracted, then the majority of the flux will take the shorter path and so not attract the other object. This reduces the number of poles on each surface to about 2. The pole pattern then becomes, say a peripheral N pole with a central S pole.

    The pole areas would need to be separated by a non-magnetic material such as manganese steel. https://en.wikipedia.org/wiki/Mangalloy
    Take a look at the design of scrap lifting magnets. They are rarely able to pick things up until the range is about half the magnet dimension.
  13. Apr 25, 2016 #12

    Simon Bridge

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    I think the details are important here.
    I think the details are important here. Without know what you are trying to solve it is unlikely we can help you much. Best advise at this stage is to consult an engineer. I'm not sure that helping people with confidential projects is what we do here.

    Well that would probably be silly... which was the point of my observation: i.e. it would be more practical just to use the weight the metal sink the whatsit than to use a permanent magnet. Therefore: if using the weight is not an option, then using a permanent magnet as described is unlikely to help either. It seems to create more problems than it could solve... so any problem it's use is solving must be very special.

    I can see that with a switchable magnet, you can alternately sink and float an object ... but the easiest switchable magnet is an electromagnet.
    Since you are not interested in overunity etc then that is a more practical solution.

    In terms of energy use, you may even be better off using a winch to pull the object under.
    So the puzzle is: why do you need a magnet at all?

    You should also consider doing a proof-of-concept model on a smaller scale.
  14. Apr 25, 2016 #13
    I see that the short range is the fundamental problem now! But is there a theoretical size in which we can make the magnet's size and thickness to effectuate the largest attraction range possible? According to a website [http://www.kjmagnetics.com/blog.asp?p=doubled-forces], the doubling up of the thickness of magnets by stacking them has a diminishing return because the furthest magnet away in the stacking adds little power because of its distance!? Is 3 inches really the thickness that is maximum for the additive effect for the attraction?? What if instead of stacking, a large magnet can be manufactured? How would this theoretically be attempted if one would want to make the largest magnet possible to induce a large effective distance of attraction?

    According to the website, for a 3 inch diameter 3 inch thick magnet, the magnet at a distance of 3 inches will have a pulling force of only 5.71 pounds. How could this be increased significantly?? Could a 12 inch diameter 12 inch thick magnet increase the pull force to around 20 to 30 pounds? If the thickness can't be increased, would the increase in diameter make a larger portion of the pull then?

  15. Apr 26, 2016 #14


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    As your diagrams show, there is a curved 1/x like relationship between force and separation.
    But as the buoyant box with vertical sides is pulled down into the water, the force required is linear, like 1–x.
    Unfortunately the 1/x curve and the 1–x straight line do not fit very well.

    If you can change the side profile of the box you should be able to match the buoyancy force against the magnetic force at the same separation.
  16. May 11, 2016 #15


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    In years gone by you had to submit a model in order to patent a new machine.
    There is a lot to be said for building a model to try out ideas on a more affordable scale.

    Do you need to be able to turn the effect of the magnet off?
  17. May 18, 2016 #16
    Thanks for the suggestion about the side profile of the box -- I viscerally understanding what you are aiming at...but not quite sure of how that would be designed. Any suggestion on what you are thinking?
  18. May 18, 2016 #17
    I have a strange obsession with trying to design things that could work without trying to violate the laws of thermodynamics. I of course am looking at something that makes sense in the physics side and have learned a lot these past few years...but still learning. I don't have a physics degree but I have a rudimentary knowledge about physics and the laws defining force, pressure, and mechanical advantage. Still it's hard to get help without getting dismissed as a dreamer/unreasonable person.

    Yes, I have given thought to the possibility of electromagnets..but didn't want to approach the complicated subject of how much electrical energy is needed to affect the pull/push force because of the need for adding energy (which would reduce or negate any energy gains)...and also because electrical energy and all it's tenants are beyond my knowledge (resistance etc...).
  19. May 18, 2016 #18


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    You know what you are trying to do, and maybe you know the reason why. I have thrown the general idea of a variant buoyancy profile into the pool. You cannot expect an engineered solution to an unspecified magnetic attraction problem. If you do not understand the concept of hull buoyancy and stability, then you are out of your depth trying to invent things in this field.
    Think of the buoyant container / hull as a series of horizontal slabs. At any depth of submergence the buoyant force will be proportional to the submerged volume. To pull the next slab under the water will require a slightly greater plan area for the next slab. The profile of the hull will be something like an inverted cone. It will have a flat bottom rather than a point because it must float level and support the entire mass when high in the water. Mathematically, the integral of the area profile with depth, or should that be height, must match the magnetic attraction curve.
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