Seeing those new “floating globe” desk thingys, I thought of getting one but I see that they all seem to require electricity for the magnets to work. I hesitate to have to use more energy for mere show.
Why can’t simple magnets keep the globe suspended?
Because it’s dynamically controlled. The magnetic pull upward on the globe has to just balance the downward pull of gravity. This requires sensors and circuitry.
Enlarging: Yes, for any given magnet there is a point where a given mass of iron below it is experiencing exactly as much upwards as downwards pull. But the system is not self-correcting; if the ball is a hairsbreadth too low, there’s too little magnetic pull; if too high, too much. Either way, once it’s out of whack it gets more out of whack, until it’s terminally so. You can only address this problem by having some kind of variable magnet, and some means of deciding whether to increase or decrease the pull.
On the other hand, if the floating body is ABOVE a fixed magnetic body, arranged to have a magnetic force pushing away (up,) and braced against any side-to-side drift or change of orientation, then it CAN float without electricity. There the forces will tend to stabilize to equilibrium, because the higher you are, the less the force pushing you up.
Earnshaw’s Theorem says that you can’t levitate a magnet with any number of fixed, permanent magnets. You can do it with variable electromagnets or superconductors, though.
Expanding. Once room temperature superconductors are created and become affordable, there’s going to be some really nifty desk gadgets hitting the market.
But you do need some sort of side-to-side bracing, because otherwise you’ll sort of roll off the top. The stability is only for the vertical forces.
Couldn’t you use a three dimensional system of magnets to create an energy well at the middle? I’m thinking of a static field that pushes the globe toward the center. It might not be as pretty or compact, but could what I’m imagining be made?
If it’s any consolation, the thing might not need to consume much electricity at all–even with ordinary electromagnets. If the setup starts with a neutral balance, all the electromagnet has to do is correct for variations introduced by wind currents, etc. These might be very small in a quiet room.
Does anybody here have a floating globe? If so, tell us what the input power rating on the AC adapter is. This will be a label on the “brick” that says something like “Input: 120VAC 60Hz xxW” or 110-240VAC, 50-60Hz, xxA." That’ll be a worst-case power estimate–I’d divide input Watts by two to estimate typical usage, and it’s probably even less than that.
(BTW: For energy consumption estimates ignore the brick’s output rating.)
If I had the cash and desk space, I’d buy one of these things and hook a wattmeter up to it. I’d bet a trivial sum that most folks will use more electricity to light the globe than it takes to keep it balanced.
Compact fluorescent lighting, people! Lots of green in a small package.
There is no way to make a stable levitating system out of just magnets, whether you have things arranged to make a well or do any other trick. I think there’s a way to demonstrate this with the calculus but can’t remember it.
However, you can create a stable levitating system of magnets plus diamagnetic materials. I have one set in a wooden frame on my windowsill. It uses two plates of pyrolitic graphite as horizontal repellers to create stability, and a neodymium iron boron magnet on an adjusting screw some distance above these to provide the levitating force. It can create maybe 3 mm or so of space above and below the tiny magnets it can levitate.
That would be Earnshaw’s Theorem, linked by ricksummon.
The weird part is, I actually understood that explanation. I think though, that it assumes that the only force is the field (electric or magnetic). If you had a cup of magnets though, wouldn’t gravity provide enough of an energy well to keep it from leaping over the sides. I do see though, that the magnets cannot do it on their own.
Since the gravitational force has the same functional form as the electrostatic force, you could apply the same mathematical arguments to it (in practice, of course, it’s much easier, since for most purposes we can approximate the gravitational field as uniform). So you can’t have a stable system using just magnetostatic, electrostatic, and gravitational forces, either.
You can make a stable system using electromagnetodynamic effects, though, which is a good thing. My butt is currently stably supported by my chair via electromagnetodynamic forces.
I was going to try to explain how the combination of forces might work, but I think I inevitably realized that the best I could do was a globe that levitated but stuck to the wall. It’s not a really magical phenomenon though. Earnshaws theorem is pretty cool if I understand it right.
Ok – but does anyone actually have one of these globes? Do they work as advertised?
I don’t own one, but I’ve seen them in stores. They work as advertised.
For something that possibly even more cool, try this:
http://www.levitron.com/
I have one, and I’ve gotten pretty good at it. It does take a lot of practice to do correctly, although the new ones are easier.
The commercial ones I’ve seen are all stabilized using an electric eye, and if anything at all happens to it (the globe gets dropped on the floor, someone bumps the table it’s sitting on, there’s a large iron object nearby, whatever) they get out of alignment and don’t work any more. But last year one of the undergrads here made one based on using the electromagnet itself as the sensor, and it worked every time, for any magnet. The only problem was it had an oscillating mode that sometimes tended to grow, but you could counteract that by attaching a piece of paper or something to the hanging object to give it more air resistance. I don’t know if any of the commercial ones have picked up on the idea.
Someone posted recently that they got this for themselves. Not sure who it was. It apparently moves but…it doesn’t plug in.
I recently got one of these floating pens as a gift from a science museum. It really does “float” if you put the pen in the right position on top of the magnetic field. You can spin it in place, too.
So why don’t either of those things have to be plugged in?
I’m sure the physicists who invented the laser were similarly dismayed when people started using them as cat toys.
The globe is floating in fluid.
The pen’s point is being forced against the wall. It’s not really “floating.”