Author Topic: why does heat affect magnets?  (Read 1035 times)

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
why does heat affect magnets?
« on: November 21, 2018, 05:43:08 pm »
I stuck something on the side of my boiler, in my office, using a white board magnet.

When it got hot, it fell off.

Why?
“Procrastination is the thief of time, collar him.” –Charles Dickens

Kim

  • Timelord
Re: why does heat affect magnets?
« Reply #1 on: November 21, 2018, 05:56:23 pm »
AIUI there's a temperature where the dipoles become unstuck and your permanent magnet becomes more like a big ball of wibbly wobbly timey wimey paramagneticy ...stuff.

https://en.wikipedia.org/wiki/Curie_temperature

Might be some other more boring effect, of course.  Thermal expansion of plastic casing moving the magnetic bit further from the surface so it doesn't quite have enough force to overcome friction, for example.  Or Niceday manufacturing tolerances.
To ride the Windcheetah, first, you must embrace the cantilever...

Re: why does heat affect magnets?
« Reply #2 on: November 21, 2018, 06:05:08 pm »
I stuck something on the side of my boiler, in my office, using a white board magnet.

When it got hot, it fell off.

Why?

I've recently bought some 10mm dia neodymium magnets to hold some spanners onto a router table I'm making.  Before I opened the envelope that they came in via the post, they were attracting various loose screws on my workbench.  They came in a stack of 10, like a cylindrical magnet, and boy oh boy did they take some getting apart.  It's a wonder they got here without getting stuck to some sorting machine in the entrails of Royal Mail.

Try one of those rather than a whiteboard magnet! 

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: why does heat affect magnets?
« Reply #3 on: November 21, 2018, 06:27:43 pm »
Heat always reduces magnetism. Personally, I believe magnetism is a fundamental property of owls (not directly, but anything they look at becomes magnetic), but more conventional scientists (if they are to be believed) will state that the putting more energy in the system will increase atomic jigginess and thus disorder those once orderly magnetic domains. Cooling reduces jigginess and gets everything back in order makes magnets stronger. Putting an owl in the fridge is liable to get you pecked, so just don't.

Of course, if you heat ferri- and ferromagnetic materials beyond the Curie limit, the magnetism is lost as they lose their magnetic alignment and everything becomes disordered.
!nataS pihsroW

nicknack

  • Hornblower
Re: why does heat affect magnets?
« Reply #4 on: November 21, 2018, 06:38:13 pm »
Back in the dim and distant when I used to deal with largish sound systems I occasionally had to repair loudspeakers. The worst cases were when the speaker had caught fire (usual cause was 500W Crown amp going down and dumping 90VDC across the output terminals) because everything had soot on it. I usually dealt only with JBL stuff cos they'd put up with this and were easily repairable. But they had to be dismantled which was a problem since they were only held together by magnetism. I had to trundle down the M4 to ATC who had a de/remagnetising machine, demag (whereupon it fell apart), clean, reassemble and remag. Then take it back to base and stick a new coil and cone in it. The heat of the fire was nowhere near enough to affect the magnetism.
There's no vibrations, but wait.

Re: why does heat affect magnets?
« Reply #5 on: November 21, 2018, 06:55:44 pm »
fundamentally what exactly is happening depends on what kind of magnetic material you are dealing with.  Most of the things described above can happen but not with all different ('permanent') magnetic materials.

However whilst the exact nature of the effect may vary,  very many permanent magnet materials simply become (temporarily or permanently) weaker at high temperatures.

 This kind of  effect was (is) used in weller soldering irons; a special material in the hot part of the soldering iron has a carefully selected curie temperature and forms part of a magnetic circuit that also controls the heater current; the temperature is thus automatically controlled; different materials are used in soldering bits that are intended to run at different temperatures (eg for different types of solder). 

cheers

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: why does heat affect magnets?
« Reply #6 on: November 21, 2018, 07:36:40 pm »
Heat (below the Curie temperature) won't have a permanent effect on magnets (it will on owls though, keep them warm but not too warm is my advice). Speakers are big chunks of strong ferromagnet anyway.

Neodynium is weird. It's actually antiferromagnetic in practice as it has a Curie point of (googles) -254 degrees. Only in combination with transition metals does it become ferromagnetic at room temperatures. Its strong magnetism is down to very specific orientation of magnetic domains and the fact that it has an extra unpaired electron hanging around compared with iron. Its magnetic homogeneity means its difficult (near impossible in practice) to demagnetise. There's a couple of kilos of neodymium in the average electric cars (in the motors) and a large wind turbine probably uses a couple of hundred kilos. Pretty much if Ironman tries to fly past one, he's fucked. Same for owls, though through a different mechanism.

The effects of heat on magnetism are important to spinny hard drives with high capacities (because the grain size is so small). They use high-coercitivity ferromagnetic films (the write heads, on the other hand are low coercivity – this being a measure of how easy it is to demagnetise) that won't flip at room temperatures so they can use ultra-small grain size (smaller grains, more bits). That means, however, that they have to zap them with teeny frikkin' lasers (to heat grains individually) when writing. Another method is to layer ferro- and antimagnetic molecules.

Paramagnets act like magnets while in the presence of an externally applied magnetic field (diamagnets repulse). Antiferromagnets are magnetically ordered but the domains cancel each other out. Some owls are attracted to other owls, others are repulsed. It's a lot more unpredictable.

I really don't get invited to many parties. I don't know why.
!nataS pihsroW

nicknack

  • Hornblower
Re: why does heat affect magnets?
« Reply #7 on: November 21, 2018, 07:46:20 pm »
I seem to recall that the speaker magnets I usually dealt with were Alnico. I think it was commonly used in high power applications.
There's no vibrations, but wait.

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: why does heat affect magnets?
« Reply #8 on: November 21, 2018, 07:52:08 pm »
Apparently, Alnico (iron plus aluminium, nickel, cobalt) is the original material used in loudspeakers and what you use affects the way they sound. Which I suppose I knew without actually knowing.
!nataS pihsroW

Feanor

  • It's mostly downhill from here.
Re: why does heat affect magnets?
« Reply #9 on: November 21, 2018, 08:07:04 pm »
This is how the temperature is regulated in older Temperature-Controlled soldering irons.

Magnets on the back of the interchangeable tips hold a contact closed turning the element on.
Once it gets to the correct temperature, the magnet lets go of the contact, and it switches the element off.

Re: why does heat affect magnets?
« Reply #10 on: November 21, 2018, 08:41:32 pm »
Not quite correct. The magnet is in the switch and is attracted or not by the cap on the back of the tip.  Can have selection of curie points and so tip temperatures. I have several. (Good party topic)

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: why does heat affect magnets?
« Reply #11 on: November 21, 2018, 08:46:42 pm »
For those interested in magnet porn (ok, don't actually go there, rule 34 applies), Los Alamos can offer a destructive pulsed magnet field in excess of 1,000 Tesla (it's basically a bomb). That's a serious magnetic field. The Ruskies managed a 2.8 kT field before their equipment understandably blew up. Basically, anything above a couple of hundred Tesla is going to blow up so (a) get an adult's permission and (b) do it outside.

That said, for a proper magnetic field, you need a neutron star.
!nataS pihsroW

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: why does heat affect magnets?
« Reply #12 on: November 21, 2018, 08:57:55 pm »
I don't think my boiler is in competition with a neutron star, much as my wife might try
“Procrastination is the thief of time, collar him.” –Charles Dickens

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: why does heat affect magnets?
« Reply #13 on: November 21, 2018, 09:17:19 pm »
You know, when my wife gets back later and asks me how I spent my evening and I tell her I was googling the strongest magnetic fields, she'll naturally just assume a rule 34 scenario.

I now know it takes 16 Tesla field to levitate a frog. Oddly, it takes 0 Tesla to levitate an owl. Proof, if it were needed, of my contention that owls are responsible for magnetism.

A BFO gigahertz NMR spectrometer of the type grubby little biochemists dream of getting their deuterated paws on puts out a 20ish Tesla field. You do not take metal near those. Especially if it's internal and you'd like it to stay that way. I saw one once before they shooed me away. That was even cooler than my matrix-assisted laser desorption/ionization time-of-flight mass spectrometer. Even frikkin' lasers pale against BFO magnets.
!nataS pihsroW

Re: why does heat affect magnets?
« Reply #14 on: November 21, 2018, 09:36:35 pm »
Alnico backed speakers are often used in high end guitar amps to this day. But in reality, it's just a retro thing that means price tags can go stupid high. They do give a distinctive tone, but that doesn't necessarily mean a better tone. If you're playing a pub gig, most people are going to be too pissed to notice anyway....
Those wonderful norks are never far from my thoughts, oh yeah!

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: why does heat affect magnets?
« Reply #15 on: November 21, 2018, 09:38:29 pm »
This is all new to me, my only experience of serious magnets is emptying my pockets to be near an MRI scanner.  I'm much more used to dealing with stuff that either results in varieties of firey death, or machinery that separates you from your limbs.

Owls, they may have a hand in it, personally it's the giraffes that I think are up to no good.
“Procrastination is the thief of time, collar him.” –Charles Dickens

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: why does heat affect magnets?
« Reply #16 on: November 21, 2018, 09:54:05 pm »
Imagine the whiplash force of a giraffe headbutt. I just did. That's how they fight off honeybadgers.

The main dangers of high strength magnets is that they make lumps of metal leap around (I'm not sure they'd genuinely rip metal out of someone's body, but I figure you'd not want to find out for sure – by-the-by, modern MRIs don't mind pacemakers and other implants). There are case studies of people have swallowed a couple of neodymium magnets (around 1 Tesla) and got their guts knotted as a result. The other danger is fluctuating magnetic fields which will, of course, induce a current in any conducting materials. Damn you, Mr Faraday and your fiendish electrickery.
!nataS pihsroW

Re: why does heat affect magnets?
« Reply #17 on: November 22, 2018, 08:53:19 am »
I thought it was wols that did weird things with magnets?
<i>Marmite slave</i>

Cudzoziemiec

  • Solar powered, tea fuelled cycle-wol
Re: why does heat affect magnets?
« Reply #18 on: November 22, 2018, 08:57:17 am »
Don't know about wols but isn't it reckoned that (some) migratory birds sense magnetic fields as a navigation aid?
sideways bounding monkey lounging under fruit tree

Re: why does heat affect magnets?
« Reply #19 on: November 22, 2018, 09:04:11 am »
"Yes please" said Squirrel "biscuits are our favourite things."

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: why does heat affect magnets?
« Reply #20 on: November 22, 2018, 09:15:26 am »
I don't think we actually know how animals detect magnetism – there was a proposed evolutionary conserved mechanism through cows to nematodes though there's some debate that there's not enough iron present to make it work. So it's a bit mysterious. There's still an argument that animals don't detect magnetism at all, but use some other mechanism.
!nataS pihsroW

Re: why does heat affect magnets?
« Reply #21 on: November 22, 2018, 10:09:21 am »
I think it is these that are responsible for the alignment of cows.
Obvs really, when you think about it.

Kim

  • Timelord
Re: why does heat affect magnets?
« Reply #22 on: November 22, 2018, 01:30:47 pm »
I think TimO has previous for discovering that, given a sufficiently sensitive instrument, cows are magnetic.  I don't know if any owls were involved.
To ride the Windcheetah, first, you must embrace the cantilever...

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: why does heat affect magnets?
« Reply #23 on: November 22, 2018, 02:09:10 pm »
Are termites also magnetic?  They arrange their mounds North-South.

Some people say it's for air conditioning, but I bet they all have compasses
“Procrastination is the thief of time, collar him.” –Charles Dickens

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: why does heat affect magnets?
« Reply #24 on: November 22, 2018, 02:38:39 pm »
They may just look at the sun or have hacked GPS. The Earth's magnetic field is very weak, hence it being difficult to perceive through any known mechanism.
!nataS pihsroW