www.dansdata.com/magnets.htm
Click Here One can academically appreciate the fact that every electric motor in the world works because of magnetism. One can reflect on the fact that ever y transformer uses it, too. CRT monitors, TVs, brushless DC motors, powe r meters, modern welding gear and engine ignition timing systems, variou s vending machines and umpteen other gadgets use permanent magnets. And electromagnetism has a hand somewhere in making practically every other electronic device work. To get a true, visceral appreciation for magnetism, though, you can't bea t playing with magnets. Really terrifyingly freakin' strong magnets, by preference. When I was a kid, I had a bag full of black ferrite magnets of various sh apes and sizes.
"These", in this case, are neodymium iron boron (NIB) magnets. They're co mmonly just referred to as "neodymium" or "rare earth" magnets, and this composition is both more powerful and cheaper than the previous king of the permanent magnet world, samarium cobalt. Well, the smallest magnets in the above picture are little gold-plated cy linders only 64 by 23mm in size. The field extending from these magnet s doesn't reach far with any strength, because the smaller a magnet of a given intrinsic strength is, the smaller will be the volume of space it can fill with a magnetic field of a given strength. Magnetic field stre ngth drops off as the inverse cube of the distance from the magnet, too; get twice as far away and the field strength drops by a factor of about eight. So two of these little tackers barely notice each other over a d istance of more than an inch. But just tossing one of these tiny 'uns in the vague vicinity of another will cause them to click together end-to-end. And they do it strongly en ough that I can hold the one on the end of a full stick of 12 of 'em and twirl the rest around as fast as I can, without them letting go. Spherical magnets, mercifully, tend to push your fingers out of the way as they head for each other, so the se ones aren't too prone to pinching people. But they can certainly do i t, and playing with them without letting them smack together so hard tha t they damage themselves is easier said than done. These magnets are all covered with protective metal plating, but careless play will flake it off quite quickly, and the NIB material itself is quite fragile.
ForceFiel d, who seem to be pretty much the only really serious sellers of a prope r range of cheap surplus rare earth magnets on the Internet at the momen t They're still not dirt cheap, mind you. I bought three $US20 grab bags on Ebay, and got 81 magnets as a result; they'd all easily fit inside a ci garette packet, if not for the fact that they'd roll it up into a ball i f you tried. There's an assortment of rods, discs, rectangular prisms and spheres, and there are only three of the usual flat-banana-shaped surplus hard drive magnets. Those magnets are massively strong, just like any other neodym ium, but their odd shape makes them unusually fragile. The ForceField grab bags seem to be the cheapest way to get hold of these things, short of getting old dead hard drives for free and ripping them to bits. Field strength Any time people talk about super-powerful magnets they have to show pictu res of big metal things dangling, so here some are.
Tool mobile That's a 15 inch spanner hanging there along with the other ironware. The sphere holding the main string of tools is only about two thirds of the way to holding its maximum load.
Magnetic field strength is measured with two units, the Gauss and the Tesla . The earth's natural magnetic field is about 05G, depending on where you are - it's weaker at the equator and stronger at the poles. It's also sl owly declining at the moment, which is something that it does periodical ly; geological evidence shows that it's actually reversed several times over the planet's life.
The strongest cheap ferrite magnets have a field strength at their poles of around 1000G, or 01T. NIB rare earth magnets, on the other hand, hav e surface field strength of about 1T. The size of a magnet has a lot to do with the perceived strength of its f ield, though. None of these magnets are very big, so that inverse-cube-l aw field strength reduction bites into their power quite quickly. Chisel the huge ferrite disc magnet off the back of a large dead speaker (if it wasn't dead before you started chiselling, it sure will be when y ou've finished) and you'll have a magnet with only about 1000G field str ength, measured at the peak strength areas on its poles. But big speaker magnets commonly weigh more than a kilogram and are sever al inches across. The peak strength areas at the poles are thus already a few inches away from the middle of the magnet's field. In this case, y ou can move another few inches away and still have 1/8th field strength. So if you wave one of these big magnets over a pile of nails, they'll lea p up to stick to it from several inches away. Take a 1-Tesla-field-strength neodymium magnet the size of a button, thou gh, and the peak field areas on the outside of the magnet will only be a couple of millimetres away from the middle of the field. Now moving jus t another couple of millimetres away gives you 1/8th field strength.
Magnetic Resonance Imaging (MRI) machines only have about 15 Tesla f ield strength, for comparison. The reason why an MRI machine is a giant contraption that needs liquid ni trogen cooling, rather than a neat little metal-plated lump that you can buy over the Internet, is twofold. It's partly because the MRI machine is also a sensitive radio receiver, detecting the radio-frequency energy emitted by the magnetic nuclei in the patient's body when they interact with a strong magnetic field. But it's mainly because a 15 Tesla MRI m achine is creating a 15 Tesla field over a large enough volume that a p atient can be stuck into said field. By the same token, junkyard car-lifting electromagnets only have about 1T field strength, but they generate that field over a big enough volume t hat their total lifting capacity, for conveniently steel-bodied cars, is massive. The coils under their protective armour draw at least a few ki lowatts, and maybe considerably more - 20kW isn't out of the question fo r a big car-lifter. You're not going to be lifting any Toyotas with a five buck magnet from a nywhere. Nails will hop up only about an inch to hit the strongest of th e magnets in the ForceField grab bags.
Because of their limited field size, small neodymium super-magnets like t hese ones aren't actually much of a problem to deal with, at least as fa r as messing up your monitors and erasing your credit cards and wiping y our video tapes and being hit by flying spanners goes. Yes, when I had one in my back pocket, I at one point found myself unexpe ctedly attached to the washing machine. But the rapid diminution of the field strength means that you can hold the strongest of these magnets - the three spheres end-to-end, for instance - in your hand and wave them around a mere foot and a half from a computer monitor, and notice only s light image distortion and discolouration. Touch those same magnets directly to the screen, mind you, and they'll ma gnetise the heck out of the shadow mask and leave you degaussing until p ractically all of the world's cows have come home, had a nice sleep and gone away again.
but I am not confident enough of my skill with it to deliberately Magn a-Doodle all over a monitor just so you can see what it looks like. Sorr y Quite big rare earth magnets can be had, if you want more field range.
this one, for instance, which only has about 11 times the vol ume of a ping-pong ball, but which ForceField warn you not to buy unless you're confident that you know how not to crush, blind or mangle yourse lf with it. As far as terrestrial magnetic fields go, 1T is quite strong, but it ain' t much by the standards of the universe.
pulsars ( which are spinning neutron stars) have magnetic fields. If they were mad e of nothing but neutrons then they wouldn't, but they've also got super conducting superfluid protons and various other exotic forms of matter, so they have. They get just...
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