4 ms·

>As far as I know, that’s possible with permanent magnets (and it would be weird, but not impossible, if the group instead synthesized a novel ferromagnet and d

>As far as I know, that’s possible with permanent magnets (and it would be weird, but not impossible, if the group instead synthesized a novel ferromagnet and didn’t notice) As far as I know a stable arrangement of permanent magnets levitating is impossible without a baring surface to keep them aligned. (i.e. free floating levitation is not possible without active control)

floxy 3y ago
Just so everyone is on the same page, static passive diamagnetic levitation is possible with materials like pyrolytic graphite. https://en.wikipedia.org/wiki/Diamagnetism https://en.wikipedia.org/wiki/Diamagnetism https://www.kjmagnetics.com/blog.asp?p=diamagnetic-levitation https://www.kjmagnetics.com/blog.asp?p=diamagnetic-levitatio... ...and superconductors are usually perfectly diamagnetic.
- colechristensen 3y ago
  Every substance is one of * ferromagnetic - attracted to one pole of a magnet but not the other (in a given orientation), this is what everybody thinks of when they think of "magnets" * paramagnetic - attracted to both poles, i.e. stuff that sticks to magnets * diamagnetic - repelled by both poles, except in superconductors, this effect is very weak compared to the forces experienced involving ferro-ferro or fero-paramagnetic materials. There isn't another category, everything fits in to one of those buckets. Saying >Just so everyone is on the same page, static passive diamagnetic levitation is possible with materials like pyrolytic graphite. is a bit deceptive, as what people know as "magnetic" materials are ferromagnetic.
  - CMScientist 3y ago
    You literally missed the most common magnetic phase - antiferromagnetism
jacquesm 3y ago
That's not quite true. There is a Halbach array with a bunch of compensation coils that will nicely center as long as it is moving, no active control or bearing required. https://www.sciencedirect.com/science/article/abs/pii/S0304885311001181 https://www.sciencedirect.com/science/article/abs/pii/S03048... And many others besides. Halbach arrays are fascinating.
- floxy 3y ago
  https://en.wikipedia.org/wiki/Earnshaw%27s_theorem https://en.wikipedia.org/wiki/Earnshaw%27s_theorem
  - jacquesm 3y ago
    stationary. Hence the 'as long as it is moving' bit above. Because the motion allows for the coils to generate enough of a current to drive the compensation. So you need a support system to bring the assembly up to a certain minimum speed above which it will stably levitate.
    - floxy 3y ago
      Right. Everyone should just read up on Earnshaw's theorem to know what all the boundary conditions are. https://en.wikipedia.org/wiki/Levitron https://en.wikipedia.org/wiki/Levitron
      - jacquesm 3y ago
        I wonder how long you could get one of those to spin in a vacuum. Halbach arrays with compensating coils have been proposed for some interesting applications, such as low loss flywheels for electrical storage. I don't know if that ever got commercialized but I do recall that some prototypes were made by a US company. I can't find a reference to it though.
        floxy 3y ago
        >I wonder how long you could get one of those to spin in a vacuum. Not too much longer apparently... https://www.youtube.com/watch?v=mn7IedCgva0 https://www.youtube.com/watch?v=mn7IedCgva0

floxy 3y ago

Just so everyone is on the same page, static passive diamagnetic levitation is possible with materials like pyrolytic graphite. https://en.wikipedia.org/wiki/Diamagnetism https://en.wikipedia.org/wiki/Diamagnetism https://www.kjmagnetics.com/blog.asp?p=diamagnetic-levitation https://www.kjmagnetics.com/blog.asp?p=diamagnetic-levitatio... ...and superconductors are usually perfectly diamagnetic.

colechristensen 3y ago
Every substance is one of * ferromagnetic - attracted to one pole of a magnet but not the other (in a given orientation), this is what everybody thinks of when they think of "magnets" * paramagnetic - attracted to both poles, i.e. stuff that sticks to magnets * diamagnetic - repelled by both poles, except in superconductors, this effect is very weak compared to the forces experienced involving ferro-ferro or fero-paramagnetic materials. There isn't another category, everything fits in to one of those buckets. Saying >Just so everyone is on the same page, static passive diamagnetic levitation is possible with materials like pyrolytic graphite. is a bit deceptive, as what people know as "magnetic" materials are ferromagnetic.
- CMScientist 3y ago
  You literally missed the most common magnetic phase - antiferromagnetism

jacquesm 3y ago

That's not quite true. There is a Halbach array with a bunch of compensation coils that will nicely center as long as it is moving, no active control or bearing required. https://www.sciencedirect.com/science/article/abs/pii/S0304885311001181 https://www.sciencedirect.com/science/article/abs/pii/S03048... And many others besides. Halbach arrays are fascinating.

floxy 3y ago
https://en.wikipedia.org/wiki/Earnshaw%27s_theorem https://en.wikipedia.org/wiki/Earnshaw%27s_theorem
- jacquesm 3y ago
  stationary. Hence the 'as long as it is moving' bit above. Because the motion allows for the coils to generate enough of a current to drive the compensation. So you need a support system to bring the assembly up to a certain minimum speed above which it will stably levitate.
  - floxy 3y ago
    Right. Everyone should just read up on Earnshaw's theorem to know what all the boundary conditions are. https://en.wikipedia.org/wiki/Levitron https://en.wikipedia.org/wiki/Levitron
    - jacquesm 3y ago
      I wonder how long you could get one of those to spin in a vacuum. Halbach arrays with compensating coils have been proposed for some interesting applications, such as low loss flywheels for electrical storage. I don't know if that ever got commercialized but I do recall that some prototypes were made by a US company. I can't find a reference to it though.
      - floxy 3y ago
        >I wonder how long you could get one of those to spin in a vacuum. Not too much longer apparently... https://www.youtube.com/watch?v=mn7IedCgva0 https://www.youtube.com/watch?v=mn7IedCgva0