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Thread: donutting, simple questions about gravity and magnetics

  1. #1 donutting, simple questions about gravity and magnetics 
    Forum Professor marcusclayman's Avatar
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    I'm not very well versed, so bear with my simple ideas please. This is probably child stuff compared to what you guys are interested in. This is not homework.

    Lets take a liquid iron bubble in zero gravity for our subject

    It is rotating on all 3 axis equally, making it into a perfect sphere. This wouldn't have a magnetic field would it? If it does where would it's poles be.

    Now lets stop it's rotation completely. Lets slowly start to spin it along one axis.

    The faster it spins the more it flattens out along it's equator, and the stronger/larger it's magnetic field becomes.

    Keep accelerating the molten iron until it is more of a disk than an ellipse. Eventually the outward momentum of the equatorial regions would be greater than the gravity and magnetic pull towards the center and little balls of molten iron would be shot out into space, right?

    Now the magnetic field would be weakened as it lost mass, but it would be strengthened as it sped up. So lets find a happy medium of mass and speed, where the magnetic field is as strong as it gets. This would be a disk spinning as fast as it can without loosing any of it's equator.

    Would the magnetic field be strong enough to make the disk into a donuts shape?

    would the magnetic field be different between a disk or a donut of the same mass and rotation?


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  3. #2  
    Forum Radioactive Isotope MagiMaster's Avatar
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    I can't answer your question completely, but I do have something to say about your first part. A 3 dimensional object can only have 1 axis of rotation, so the first part of your question doesn't make sense. The rest of your questions will have to wait for someone who knows a bit more about this stuff though.


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  4. #3  
    Forum Professor marcusclayman's Avatar
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    Alright, you can't have something rotating on because rotation needs 2 dimensions.

    But you can have something spinning on two axis.

    Take the earth's pivot for example. It is about 21 degrees right? If it was 180 degrees we would not call it a pivot, we would be rotating in two ways. One direction of rotation would still take a day and one direction of rotation would take a year. I'm assuming this would have a profound effect on our magnetic field, assuming the liquid core also changes direction.
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  5. #4  
    Forum Radioactive Isotope MagiMaster's Avatar
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    The yearly 'rotation' isn't, it's revolution. The Earth revolves around the sun. It could do this without any rotation whatsoever. Mathematically, at any one point in time, a 3D object can only be rotating around one axis. It might be easier to visualize this as a composite rotation, but the object is still rotating around some single axis between the two.
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  6. #5 Re: donutting, simple questions about gravity and magnetics 
    Forum Junior c186282's Avatar
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    Quote Originally Posted by marcusclayman
    Lets take a liquid iron bubble in zero gravity for our subject

    It is rotating on all 3 axis equally, making it into a perfect sphere. This wouldn't have a magnetic field would it? If it does where would it's poles be.
    This is a very complicated way to rotate something. So I will concentrate on the other questions.

    Quote Originally Posted by marcusclayman
    Now lets stop it's rotation completely. Lets slowly start to spin it along one axis.

    The faster it spins the more it flattens out along it's equator, and the stronger/larger it's magnetic field becomes.
    Not quite. Fe or Iron is Ferromagnetic which means that an iron atom all by itself is a little magnet because of how all of the magnetic moments of the orbitals and electrons add up. So the liquid iron ball is made up of lots of little magnets that are pointing in a random direction because the iron is so hot that it is a liquid. Spinning this will not change the overall magnetic field.

    Spinning a ball of electrical charge would make a magnetic field. To know which way the field is pointing one could use Lenz's law

    Quote Originally Posted by marcusclayman
    Keep accelerating the molten iron until it is more of a disk than an ellipse. Eventually the outward momentum of the equatorial regions would be greater than the gravity and magnetic pull towards the center and little balls of molten iron would be shot out into space, right?
    Unless this ball of molten iron is real big (like 10's of km wide or bigger) then gravity really has little to do with it. Also it takes force to hold magnetic field lines close to one another. There is an energy density in a magnetic field. The only thing that would be holding your small ball of molten iron together would be the surface tension and cohesion of liquid iron. I have no idea how liquid iron behaves I have never played with it.

    Quote Originally Posted by marcusclayman
    Now the magnetic field would be weakened as it lost mass, but it would be strengthened as it sped up. So lets find a happy medium of mass and speed, where the magnetic field is as strong as it gets. This would be a disk spinning as fast as it can without loosing any of it's equator.

    Would the magnetic field be strong enough to make the disk into a donuts shape?

    would the magnetic field be different between a disk or a donut of the same mass and rotation?
    I really can't answer this because, well I do not think it would get to this.

    However, there is a spinning ball of liquid iron below our feet. But the magnetic field of the earth is not made because of spinning iron or because iron is ferromagnetic but because of moving electrical charges this is called the dynamo effect
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    Forum Professor marcusclayman's Avatar
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    i enjoy being wrong almost as much as enjoy doing research... thanks for the civility
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  8. #7 Spinning disc of molten iron 
    Forum Freshman vistotutti's Avatar
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    What a great question!

    First, on the rotation:
    Only two axis of rotation is possible for a 3 dimensional object.
    In the case of this spinning disc there is spin, and tilt.

    Let's keep it simple with just spin.

    With a magneto-ferrous object spinning so fast I think there would be a magnetic field.
    As to if the field would be greater from a donut or a disc... well my engineering background tells me that the difference would be slight.
    Engineers often use a donut shaped magnet where a disc could be used, as it is just as effective and costs less in materials.(The slowly moving inner part of a disc contributes little to the velocity/Field strength equation of electro-magnetic energy.)

    Would a spinning ferrous disc, or donut, have a magnetic field in a perfect vacuum?
    I don't know.
    But in any familiar part of the galaxy, where magnetic and electric fields abound, I think that such a large and fast spinning object would show electric and magnetic effects that would boggle the mind.
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  9. #8  
    Forum Radioactive Isotope MagiMaster's Avatar
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    Again, a three dimensional object can only be spinning around one axis at one time. You can composite multiple spins together, but in the end you'll get a single axis. This axis can move over time, and the object can move around while spinning, but neither is the same as spinning around another axis.
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  10. #9  
    Forum Professor marcusclayman's Avatar
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    if a disk is spinning both like a record and like a coin that makes 1 axis?

    I kind of understand, but it just doesn't seem right you know? I will have to see it and be shown in order to understand, I am a visual-tactile learner.
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  11. #10  
    Forum Junior c186282's Avatar
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    if a disk is spinning both like a record and like a coin that makes 1 axis?
    Yes, seams rather odd, but lets slow things down and think about this complex rotation.

    A very high speed camera watching this object spin will produce many frames where the object has only moved a very little bit between frames. Looking at two neighboring frames we can determine what axis is it spinning around during that instance. We can do this by noticing that every part of the object has moved a small amount around one axis. We can say that, at that time instance the object was spinning around just one axis. This is a first order statement. I have only used two frames of the film to come to this conclusion. I could do this with any two neighboring frames of the film. However, I would then find out that the axis of rotation is moving. This is a second order effect and takes, at minimum three frames to see.

    This concept generalizes to rotations in -space.

    A rotation in -space is a mixing of two perpendicular dimensions.
    It is not a rotation about an axis. We only feel comfortable with the idea of rotation about an axis because we spend all our time in 3-space.
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  12. #11  
    Forum Radioactive Isotope MagiMaster's Avatar
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    What c186282 said, but also it feels like you might be misunderstanding the meaning of axis (though I could be wrong, but for completeness, I'll explain a bit either way). Axis doesn't mean just up (spinning like a record), or left (spinning like a coin), or forward (also like a coin, only to the side). Axis means any pole of rotation. A good example of this is to take a book or cereal box and hold it with one finger on two opposite corners. The box will rotate around the axis made by the line between those corners. Having something with three different side lengths (like a book or cereal box) makes this easier to see.

    It's hard to imagine, but think about how a disk would spin if you spun it around an axis about halfway between that of a coin and a record. In some ways, it'd be spinning both like a record and like a coin.
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  13. #12 Axis of rotation 
    Forum Freshman vistotutti's Avatar
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    Quote Originally Posted by MagiMaster
    Again, a three dimensional object can only be spinning around one axis at one time. You can composite multiple spins together, but in the end you'll get a single axis. This axis can move over time, and the object can move around while spinning, but neither is the same as spinning around another axis.
    I understand what you mean.
    It is a matter of of perspective, an N dimensional object has N-1 freedoms of rotations, but in the end, an outside observer will see just one axis of rotation. A single vector.

    But if one lives on track 1 of side A of a spinning and also tumbling cosmic vinyl record, one would see the stars surrounding both swirl and tumble, both.

    It makes me wonder about the single spin of electrons in quantum mechanics.
    How many axises of motion do electrons have?
    And what perspective are we looking at them from?
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  14. #13  
    Forum Radioactive Isotope MagiMaster's Avatar
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    No, an N dimensional object has N-2 degrees of rotational freedom. Imagine rotating a 3D object slightly. Now take all the points that didn't move. This is, by definition, the axis, and will be a line, a 1 dimensional object. Similarly a 2 dimensional object rotates around a point, and a 4D object rotates around a plane.

    In the absence of any outside forces, an object will only spin around a constant axis. If the axis is moving, it's due to some outside force moving it. Again, this isn't rotation though, it's an acceleration. It's the change in rotation over time.
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