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Build two electromagnets

  1. Jan 29, 2004 #1
    build two electromagnets, stack one on top of the other so that when they receive current they will repel each other. how far can you separate them? how much current will it take for maximum separation ? what difference does it make if they each way one gram or ten tons?
     
  2. jcsd
  3. Jan 29, 2004 #2

    turin

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    Re: magnets

    That is virtually unlimited, unless the magnetic field doesn't extend out to infinity.




    Maximum current.




    Gravity. There are several latent forces going on here. First, let's get some perspective. If these two inactive coils are all alone out in empty space, then it will take some fixed, and extremely small, amount of energy for them to escape each other. In this case, they would seperate to infinity (which is a large separation) without even turning the magnets on. OK, perspective established. Now, on Earth, sitting on a table, these two magnets are accelerating at 9.8 m/s2. The "bottom" magnet (the one closer to the center of the earth) is being accelerated by the table top. It in turn accelerates the other magnet in front of/on top of it by pushing on it. How does this pushing occur? Electrostatics, so they say. What they also say is that there is no such thing as contact because of this mechanism. What it seems you want to do is to exagerate this lack of contact as much as possible. The reason there is close proximity is that the bottom magnet is accelerating the top magnet. Less force is required to accelerate a smaller weight for a given amount of force. The magnetism will provide a given amount of force. So, the mass matters.
     
  4. Jan 29, 2004 #3
    Lets say the magnets are stacked vertically at 1,000 feet above sea level on a New-Mexico lime stone Plato. Each of the one ton magnets coils receive an equal amount of current simultaneously. How much power would lift the upper magnet? How high could the average lightning bolt lift the magnet if at all? (Approximately)
     
  5. Jan 30, 2004 #4

    turin

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    You need to be more specific than this. The issue of altitude and location on the Earth is pretty insignificant.

    You can think of the magnetic force between the coils two ways: 1) use the biot-savart law, 2) treat them as bar magnets to first order, or something else to higher order. Either way, though, you should specify your geometry.

    1 ton magnet will require 1 ton of force to overcome the acceleration of gravity that is already present. Any extra force will lift the magnet. The magnet will be lifted until the force from the magnetic field drops down to 1 ton (roughly). Without specifics, this is too complicated to generalize in more detail.
     
  6. Jan 30, 2004 #5
    I am unfamiliar with the biot-savart law , for that mater unfamiliar with the particular behaviors and formulas needed for the precise forecasting of magnetic behavior beyond the very rudimentary . I am though quite curious about the feasibility of introducing the current of a lightning bolt to these two magnets ( simultaneously ) what ever their dimensions. What do you think I might witness upon the introduction and in the aftermath?
     
  7. Jan 31, 2004 #6
    Well, if you are close enough to it, you would witness your own death or severe injury.
    Lightning redirected to travel the circuit of an electromagnet would likely cause serious overheating resulting in fused wires, and possibly even a small explosion.
    Assuming, for the sake of argument, that one could construct a pair of electromagnets durable enough to survive the lightning voltage/current, then of course one hell-of-a repulsion would occur between the two electromagnets properly situated.
     
  8. Jan 31, 2004 #7
    Actually, there's a much shorter way to go about this, that doesn't involve Biot-Savart or vector diagrams: just use conservation of energy.

    Your lightning strike contains 'enough energy to power a home for two weeks', according to a website near you. My aunt says her power bill for last month was 3265 kWh, but the previous year was 1649 kWh. We'll say her family uses 2000 kWh per month or 1000 per two weeks; so a lightning strike contains 1000*3600000 = 3.6E9 Joules.

    By V = mgh, that's enough energy to throw a 1000 kg magnet under earth gravity (g = 10 m/s^2) to a height of 360 kilometers. (That's high enough that g would be more like 9 than 10 m/s^2, but...)

    But the process would not be one hundred percent efficient so you just multiply by a fudge factor: how efficient do you imagine the process might be? I estimate less than ten percent, probably more like one percent. That gives you a height of somewhere between 3 kilometers and 36 kilometers.

    Good enough for a rough estimate, yes?

    Then you get into stuff like, wouldn't it better to channel the lightning into a large capacitor and bleed it off into the electromagnet slowly? If'n you did that, you could feasibly 'levitate' the one magnet at any height up to 36 klicks for a reasonably long amount of time. I put levitate in air quotes because you'd need to run a flag pole up the center of the magnet to keep it from flying off to one side. It's an unstable equilibrium, as you know if you've ever tried to get a permanent magnet to hang in the air above another one.

    P
     
  9. Feb 2, 2004 #8

    turin

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    and wronger



    Here is the first introduction of severe ambiguity in your analysis. A better way to consider it would be to consider why lightning happens. Then, realize that it is strongly dependent on the altitude of the origin (or termination) of the strike and the type of grounding you use (i.e. impedence) at your load. In this case, of course, the grounding to which I'm referring is that of the lightning, which would actually be the hot side of the load.




    Here is the second introduction of severe ambiguity (self explanatory).




    If you want to hover the magnet, yes; if you want to launch the magnet, probably not. A capacitor is not a magical lightning container, and in so storing, you would lose some of the more fantastic properties of the lightning (like the power).
     
  10. Feb 2, 2004 #9

    turin

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    Lightning is highly unpredictable, so you would have that going against you. But, if you want to go ahead and assume the case that the assembly does get struck, then your next feasibility issue is the ability of the assembly to withstand the strike (that is, use the energy in the way the assembly is designed). To get energy out of the lightning, you would have to have the lightning activate the hot end WRT the ground end of your assembly. This imposes an efficiency issue in converting the electric energy into magnetic energy. There are electrons flowing between the ground and the sky very violently for a very brief duration. This translates to a lot of power and a lot of current. But, to transfer that power to your assembly, you have to realize that it is highly inductive by its very nature. This means that the lightning isn't really going to want to strike it. Then realize that the close to ground level, the lower the potential difference between hot and ground (which goes along with the impedance issue). Distributing the energy appropriately to the two coils is a lesser issue.
     
    Last edited: Feb 2, 2004
  11. Feb 2, 2004 #10
    I don't have a problem with 'severe ambiguity' or with a method of approximation being 'wronger.' The question, as I understood it, was, 'how high into the air could you launch an electromagnet with a lightning strike?' I seriously doubt the poser of the question was looking for more than an order-of-magnitude calculation.

    Turin, I find quibbling over the validity of arguments based on the conservation of energy and efficiency coefficients to be quite pointless. I certainly wouldn't embark on a more sophisticated analysis of the scenario without first determining the maximum theoretical amount of energy that could be derived from a lightning strike, and the maximum altitude I would expect as a result of this. Briefly considering many of the issues you mentioned and more--loss from the capacitor, less energetic lightning strikes due to low altitude, poor coupling between the electromagnets as distance increases (most of the acceleration would have to happen in the first few meters), and especially losses due to air resistance and the inherent instability involved in balancing one magnet atop the B-field of another--I felt that one percent is a very reasonable scaling coefficient. That a realistic altitude is on the order of single kilometers should suffice for most purposes: it tells you that this would not be a very good method for earth-to-orbit launches, even if it were highly refined and improved, but also that the electromagnets will certainly not stand still when the strike occurs.

    I am more interested in your point that the high-inductance nature of the system is anathema to the lightning; even if a lightning rod attached to the hot end of the assembly, it seems likely that it would sooner make a second arc through the air than travel through such an arduous path. Perhaps this is why schemes to 'harness the power' of lightning strikes are doomed to fail; the energy levels are high, but the system seems unwilling to apply itself to anything save reaching ground and being converted to heat.

    P
     
  12. Feb 6, 2004 #11
    magnets up

    "I am more interested in your point that the high-inductance nature of the system is anathema to the lightning; even if a lightning rod attached to the hot end of the assembly, it seems likely that it would sooner make a second arc through the air than travel through such an arduous path. Perhaps this is why schemes to 'harness the power' of lightning strikes are doomed to fail; " ......doomed?

    consider the plausibility of constraining this very heavy magnet on a short (one hundred meters or less ?) vertical path. at the apex of its travel it is captured. Its decent is controlled and the stored energy is used to spin a turbine. the energy produced can be routed directly to the grid or converted to hydrogen for storage.

    To attract a lightning strike; use a small cheep rocket and a spool of wire, monitor atmospheric conditions, fire when conditions are right and viola! its a beautiful thing, I'v seen it happen.

    leo
     
  13. Feb 6, 2004 #12
    Well, if you are close enough to it, you would witness your own death or severe injury.
    Lightning redirected to travel the circuit of an electromagnet would likely cause serious overheating resulting in fused wires, and possibly even a small explosion.
    Assuming, for the sake of argument, that one could construct a pair of electromagnets durable enough to survive the lightning voltage/current, then of course one hell-of-a repulsion would occur between the two electromagnets properly situated.

    So far this has been my favorite response to this thread.

    leo
     
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