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      CommentAuthorAngus
    • CommentTimeMar 5th 2014
     
    Never mind that. More usefully: to improve the clarity of the English language we must now introduce a distinction between the transitive and intransitive forms of other verbs. Presently we have the useful distinction between "I lie down" and "I lay (it) down" . We can add

    "I tie up" - "I tay (the boat) up".
    "I die" - "I day (him)".
    "Oh fie!" - "Fay on it!"
    "Hi there!" - "I hayed him yesterday".
    "I am jiewalking" - "I jaywalked that street"


    and so on. A vast improvement, don't you think?
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    Not in a thread called "Physics", no.
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    Posted By: Andrew PalfreymanThe question to which I seek an answer is this: how much net lift can be obtained by exploiting the changes in magnetic field on or near to the surface of the Earth?


    I don't know but I would wager that it is smaller than a breadbox, mineral as opposed to vegetable or animal, and costs more than a car but less than a house.
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    Posted By: Andrew PalfreymanNot in a thread called "Physics", no.

    By now you will have learnt that the warp and woof of the fabric of the Moletrap is wildly nonlinear and even of fractal dimension.
    • CommentAuthortinker
    • CommentTimeMar 5th 2014
     
    How much nett lift could you catch in a net anyway?
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      CommentAuthorAngus
    • CommentTimeMar 5th 2014
     
    Oh all right. Just to get that out of the way - as much as you want if you can generate sufficient current. Now can we talk about Grammar?
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    Fine. How is she doing these days?
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      CommentAuthoralsetalokin
    • CommentTimeMar 5th 2014 edited
     
    I'm not sure that's right. I thought we could only get a torque that would line the thing up with the ambient field, but nothing that would keep it from sliding sideways, orthogonal to the ambient field, and nothing to make it "seek the pole" by actually moving along the ambient field lines. So it would fall to the ground under the influence of that other pesky force, pointing North all the way down, like a dropped magnetic compass. If you think you can "pin" field lines in the superconductor, well you can but they stretch, the silly things, and if you stretch them far enough they will disconnect and reconnect elsewhere and you still fall down go boom.

    Maybe.
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    Yes. First let dB/dx = dB/dy = 0. Uniform field. No forces can result, only at most restoring torques. The twin horizontal coils have a force of +2 down in the middle, and -1 up at each end, and so stay put. No torque. Gravity is all that remains, and so it falls to the ground, as you suggest.

    Now make dB/dx finite. Now we get -1, +2+d, -1 + 2d for a field difference of 2d over a distance dx = the beam length. Now there's a restoring torque and the beam ends up vertical, and the field runs through the loops. Maybe. Because one of the loops opposes the prevailing field direction, it may end up at an angle something like 45 degrees to the horizontal. But gravity still pulls it to the ground. It doesn't travel anywhere either.

    So go back to a single current loop. With finite dB/dx, it ends up vertical for sure. What determines differential lift is not dB/dx but dB/dy. There are 2 possibilities, assuming dB/dy <0 (field weakens with height; I haven't yet checked):
    1. The top current segment produces more force than the bottom one, and it's upwards. We get net lift
    2. The top current segment produces more force than the bottom one, and it's downwards. Fail.

    We have no choice about the sign of dB/dy (it's how the Earth's made).
    We have no choice about the orientation of the loop (it must align in both direction and sign with the prevailing field).
    So the answer is in there. Two succeed and two fail, and only one is real.
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      CommentAuthorAngus
    • CommentTimeMar 5th 2014
     
    I said current, not current loop.
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    Did you invent current that doesn't run in a circuit? That would be something. Including very nasty charge build-ups. I don't think you'd want that. All the electrons end up lying on the floor.
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    Posted By: Andrew PalfreymanSo the answer is in there. Two succeed and two fail, and only one is real.
    Sorted - and fail. Just my luck (he said for the umpteenth time, goddammit).
    Based on the vector rules for force and field, a stable (in the compass or aligned dipole sense) current loop, vertical at the equator, and thus threaded by the prevailing horizontal field, experiences an upward force on the top end and a downward force on the bottom end. So in order to get lift, we require dB/dy > 0; i.e. that the field increases with height.

    But it doesn't. A dipole fit to the earth's field isn't bad, and for that Br at the equator goes as 1/r3, so dB/dr = dB/dy goes as 1/r4. That is, the higher up, the weaker the field.

    This is particularly clear at the poles. All the field lines converge close to the ground and are more spread out further up in the air. So this is even worse there than at the equator.

    There's one remaining possible loophole. Normally, the shape of a coil makes no difference to the forces acting on it. But in a gradient, it might be possible to beat the rap.
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      CommentAuthorAngus
    • CommentTimeMar 5th 2014
     
    Posted By: Andrew PalfreymanDid you invent current that doesn't run in a circuit? That would be something. Including very nasty charge build-ups. I don't think you'd want that. All the electrons end up lying on the floor.


    I didn't invent it. Nature invented it. Lightning for example.

    For your purposes you just charge up a walloping big capacitor and arc it across when you're ready. I'm sorry you don't like the nasty stored energy, but what do you think rocket fuel is?
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    Posted By: Andrew PalfreymanThere's one remaining possible loophole. Normally, the shape of a coil makes no difference to the forces acting on it. But in a gradient, it might be possible to beat the rap.
    Fail again. I tried a sloping line geometry to minimise the down force by minimising the lower horizontal length. It did help, but the net force is always still down. Perhaps there's a better geometry...

    Posted By: AngusFor your purposes you just charge up a walloping big capacitor and arc it across when you're ready.
    Short-lived (very). I am after sustained lift.
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      CommentAuthorAngus
    • CommentTimeMar 5th 2014
     
    You have to store the energy somehere-why not in a big tank of charged capacitors?

    (That is - unless you want to transfer it into kinetic energy all at once á la Professor Cavor.)

    Why are we even discussing drives based on winding yourself up on the gossamer thread of Earth's minuscule magnetic field?
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    Posted By: Andrew PalfreymanPerhaps there's a better geometry...
    Well, yes there is, and it uses a quadrant of a circle. There may be an even better curve, but at least I can get positive lift out of that, provided I did the calculus right. Now I need to figure out the maximum tolerable mass density.
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    Posted By: AngusYou have to store the energy somehere-why not in a big tank of charged capacitors?
    Because a) they are heavy, and b) the pulse is short-lived.
    I don't know where the energy comes from though. Maybe you can advise. I made this remark earlier re. Meissner.

    (That is - unless you want to transfer it into kinetic energy all at once á la Professor Cavor.)

    Why are we even discussing drives based on winding yourself up on the gossamer thread of Earth's minuscule magnetic field?
    It's worse than that. I'm discussing winding up on the gossamer thread of the change in the Earth's magnetic field.
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      CommentAuthorAngus
    • CommentTimeMar 5th 2014 edited
     
    Yes, I know. I do recall having a discussion on precisely this kind of drive right here on the 'Trap several years ago. I shall have to ask myself if I can be arsed to go look for it.
    BTW it's so bad that I was trying to help you out with the capacitors.
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    Posted By: AngusYes, I know. I do recall having a discussion on precisely this kind of drive right here on the 'Trap several years ago. I shall have to ask myself if I can be arsed to go look for it.
    BTW it's so bad that I was trying to help you out with the capacitors.
    You lost me - I like being helped! Do you mean that you recall that discussion ended in a bust for this idea?
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      CommentAuthorAngus
    • CommentTimeMar 5th 2014
     
    It was woo. It may be possible to get some thrust in fact, I don't remember, but it was never going to be useful.

    Here is something off Arxiv