Fucking magnets, how do they work, etc.
November 8, 2017 2:41 PM   Subscribe

OK so I used the concept of "when you were in 3rd grade and you laid a non-magnetic metal next to a known magnet next to each other and you looked in your desk the next day and they were both magnets" in a facebook comment recently and then realized:

I might be the one wrong on the internet?!?

Can this be done? Because all the googling I just did, frantically, for 10 minutes, only reveals making magnets by rubbing the magnetic & non-magnetic pieces together, or by using batteries or whatever.

Does it happen? WAS I WRONG?

Please help.

(N.b. while science is fascinating, I'm not actually interested for purposes of this question about "why"; I mostly just want to know if I'm misremembering a thing that we did (or didn't) do in science class.
posted by tivalasvegas to Science & Nature (12 answers total) 1 user marked this as a favorite
 
Proximity won’t do it. If they are in contact, the magnetism can be transmitted through a steel part. But when the magnet is removed, so is the magnetism.
posted by Huffy Puffy at 2:51 PM on November 8, 2017 [1 favorite]


you laid a non-magnetic metal next to a known magnet next to each other should read: "you laid a ferrous metal and a known magnet next to each other" and it may work a little bit. Add the rubbing and it should work better. On preview, "next to" should more precisely read "touching."
posted by JimN2TAW at 2:55 PM on November 8, 2017 [3 favorites]


I wish it didn't work at all. I use steel calipers when machining metal, the tips somehow get magnetized leading to chips sticking to them and bad measurements. It's a common problem. I have no magnets in my shop, but do have electric motors.
posted by rudd135 at 3:02 PM on November 8, 2017 [5 favorites]


I seem to remember magnetizing paper clips with horseshoe magnets in third grade. The paper clips would then seem to stick to other metal objects. Not with much power, mind you.

But, since this is a memory from when I was nine years old, it may be that we simply convinced ourselves that the paper clips had become magnetized. It was certainly more fun than learning subtraction, which was what we were supposed to be doing.
posted by Crystal Fox at 3:10 PM on November 8, 2017 [1 favorite]


The key thing is you can't (permanently) magnetise metals that aren't ferromagnetic. So the odds are that the non-magnetic metal was iron or steel (I doubt it was nickel or cobalt). Magnetism isn't transmitted by contact - it's via a magnetic field that surrounds the magnet. The closer they are, the more strongly the field from the magnetic steel will interact with the non-magnetic steel. But to permanently magnetise the second piece of steel there needs to be some relative motion involved, as only a moving field will line up the dipoles in the metal (imagine lots of very tiny compass needles) and cause the steel to remain magnetic after the field from the magnet is taken away.

So yes, the second piece of metal may have become magnetised, but probably only weakly, so might support a few paperclips (and even slightly magnetise them). The 'rubbing together' (or more accurately stroking in one direction, with a sort of circular motion) would help the process of lining up the dipoles and making the second piece of steel into a permanent magnet.

A lot of my steel tools have become a bit magnetised over the years, due to magnetic fields from other magnets and also electromagnets and motors.

There are plenty of resources online that you can use to bring yourself up to speed with magnets. They're fascinating things - one of those things in science that seem a lot like I imagine magic would be.
posted by pipeski at 3:20 PM on November 8, 2017 [11 favorites]




The scientific term you're looking for here is "magnetic hysteresis."

Ferromagnetic materials like iron have the weird property that when you put them in a magnetic field, they also become a magnet aligned with that field. When that field is taken away, they generally go back towards being unmagnetized, but not quite all the way. That effect is called hysteresis.

I can't say I really know what the relative benefits of repeatedly applying and removing a magnetic field (as you would by rubbing) and just letting a ferromagnetic sit in a magnetic field for a long period of time. But I mean, it's science - the best way to figure it out is to do an experiment. Get two similar ferromagnetic things, leave one with a magnet overnight, then rub one with a magnet and see how they compare at picking up paperclips or something like that.
posted by Zalzidrax at 3:33 PM on November 8, 2017 [5 favorites]


You can whack some ferromagnetic metal with a hammer and it will align with whatever field it's currently in. My first-year college physics teacher demonstrated this after first reading this chapter from Moby Dick. (Read the whole chapter, it's short. For context, there was a terrible lightning storm the day before. The binnacle is the housing for the compass.)

Here's the most relevant part: With a blow from the top-maul Ahab knocked off the steel head of the lance, and then handing to the mate the long iron rod remaining, bade him hold it upright, without its touching the deck. Then, with the maul, after repeatedly smiting the upper end of this iron rod, he placed the blunted needle endwise on the top of it, and less strongly hammered that, several times, the mate still holding the rod as before. Then going through some small strange motions with it- whether indispensable to the magnetizing of the steel, or merely intended to augment the awe of the crew, is uncertain- he called for linen thread; and moving to the binnacle, slipped out the two reversed needles there, and horizontally suspended the sail-needle by its middle, over one of the compass cards. At first, the steel went round and round, quivering and vibrating at either end; but at last it settled to its place, when Ahab, who had been intently watching for this result, stepped frankly back from the binnacle, and pointing his stretched arm towards it, exclaimed,- “Look ye, for yourselves, if Ahab be not lord of the level loadstone! The sun is East, and that compass swears it!”

Hammering on the long iron rod, part of a harpoon, Ahab aligned the rod's magnetic domains with the Earth's magnetic field (which is more or less up and down on most of the surface of the Earth, with the top end of the rod angled slightly towards the equator) by hammering it. Then, he used the newly magnetized rod to magnetize, by rubbing, the compass needle, which had lost the alignment of its own magnetic field in a lightning storm the previous night.

My professor then demonstrated that exact thing, though he had to substitute a regular rod because he was fresh out of harpoons, right in the classroom.

So, it does take some motivation for the metal object you're talking about to do any re-alignment. One thing you learn is that magnetic fields don't do any work; rather, moving or changing magnetic fields do.

Other facts: When you build a ship or other large metal object, you've got giant chunks of ferrous metal being hammered and riveted for months on end in the same position. Every ship that launches has a magnetic field that's aligned with the Earth's. Before the ship goes into service in the military, at least, you degauss the ship. Tom Scott explains a bit here. My dad was in the Navy, retiring in the 90s, and he said that they would degauss the whole ship by wrapping it in giant cable coils, and A) you had to remove every cassette tape, floppy disk, TV and computer from the ship to do it, and B) the coils sit on the bottom of a bay somewhere all the time except when in use, and so the whole procedure was exceptionally foul as all the gunk that builds up on the cable starts to rot in the air and sun.

So... kind of eventually it may take on an alignment, but it still takes the action of all those hammers, saws, and every bit of mechanical energy to re-align the field in the metal.
posted by Sunburnt at 4:15 PM on November 8, 2017 [24 favorites]


It seems to take some time, but I had disassembled a home theater speaker which has a magnet on the back of the internal speaker and accidentally left a cheap iron/steel screwdriver on the magnet for many months and after that the screwdriver was magnetic, good for falling computer case screws.
posted by TheAdamist at 7:56 PM on November 8, 2017 [1 favorite]


to permanently magnetise the second piece of steel there needs to be some relative motion involved, as only a moving field will line up the dipoles in the metal (imagine lots of very tiny compass needles) and cause the steel to remain magnetic after the field from the magnet is taken away.

It doesn't take very much motion at all to establish quite distinctly persistent magnetization in materials like steels that are particularly prone to it. Just the initial movement of a strong magnet approaching the steel can do it, especially when combined with the shock induced by the CLACK as the steel suddenly finds itself unable to get any closer to the magnet it's just become enamoured of. Drawing a magnet along the length of the steel works better, but a quick stinging thwack works well enough.

If I'm trying to repair an iPhone and I need a magnetized screwdriver to stop those ridiculously small internal screws escaping irretrievably into the carpet, and the screwdriver I have is not already magnetized, all it takes to impart enough grabbitude to be useful is pointing it roughly north or south and giving the shaft a good smack with a bigger screwdriver or the side of the long nose pliers.

accidentally left a cheap iron/steel screwdriver on the [speaker] magnet for many months and after that the screwdriver was magnetic, good for falling computer case screws.

I've done the same thing to a screwdriver just by allowing a magnet scavenged from a dead hard drive to clack onto it once, then removing it straight away.
posted by flabdablet at 7:15 AM on November 9, 2017


Also worth noting: the simple fact that a metal is attracted by magnets does not guarantee that it will retain a persistently magnetized state on its own. Various grades of steel exhibit different degrees of willingness to retain magnetization.

As a rule of thumb, the harder the steel the better it stays magnetized; some stainless steels are a notable exception.
posted by flabdablet at 7:34 AM on November 9, 2017


Magnetizer-demagnetizer
posted by TedW at 11:54 AM on November 9, 2017


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