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Completely random questions for anyone bored...

This topic has been archived, and won't accept reply postings.

So, just to take anyone's mind off all the awfulness, let's see if UKC can come up with sensible (or not so sensible) answers to the following random things that have been vexing me:

1) My boss gave me the rather sweet Christmas gift of a reusable coffee mug with nice little pictures of people cycling on it. As well as obviously better than throwing away paper cups, it is made of "bamboo" to make it even more environmentally friendly. On the base it says dishwasher proof but do not microwave. Why? Now I'm one of the many teachers trying to teach from home, my morning coffee can be my weekend luxury of a double espresso onto hot milk, rather than a cup of dodgy instant and only with some milk if my colleagues in the staffroom haven't nicked what I brought in. I normally just warm the milk in the microwave. If I do this in the bamboo cup is it going to explode? Am I going to get some weird bamboo poisoning? Will it melt?

2) Like probably most people, the family Christmas tree got unceremoniously dumped in the back garden a few days ago. I went out today and let off some aggression lopping off all the branches then chopped the trunk up into a few OK sized logs. Does spruce burn green? Will we get a nice smell of Scandi campfires if I chuck those logs on the wood burner tomorrow night? Or will it smokily smoulder, or crackle and spit resin everywhere?

And from past "things I don't understand" threads, I still don't really get how my induction hob works - if anyone who hasn't tried explaining that to me before and is really bored so wants to give it go.  

And of course if anyone else has some facile conundrums they want to share - please feel free.

Post edited at 22:25

 Welsh Kate 07 Jan 2021
In reply to TobyA:

re Q.1. I have no idea, I'm a Roman historian, not a physicist. But I'm an advocate of experiential learning and would therefore suggest popping it in the microwave and seeing what happens

 mondite 07 Jan 2021
In reply to TobyA:

Since its wood I would assume microwaves would slowly dry it out from the inside regardless of the coating. So probably not an explosive failure but probably have a shorter lifespan.

If you want to test the theory though I would suggest not being near the microwave just in case though.

Post edited at 22:36
In reply to TobyA:

The Japanese are going to launch some satellites built from wood and there are microwaves up there so I say go for it!!!

Disclaimer: Or don't!

Post edited at 22:41
 flatlandrich76 07 Jan 2021
In reply to TobyA:

> 2)  Does spruce burn green?

No, not really.

Will we get a nice smell of Scandi campfires if I chuck those logs on the wood burner tomorrow night?

Probably not, and if you do it also means smoke or gases are coming back in to the room, which isn't good.

Or will it smokily smoulder, or crackle and spit resin everywhere?

Yes, yes and very likely.

 Mr Lopez 07 Jan 2021
In reply to TobyA:

With regards the cup, it's because they are made with some pretty nasty chemicals which will end up in your drink if you stick it in the microwave. The same chemicals still end up in your drink if you put hot drinks inside...

ETA: Some links https://www.chemistryworld.com/news/reusable-bamboo-mugs-leach-dangerous-amounts-of-formaldehyde-and-melamine/4010950.article

https://www.dw.com/en/keep-your-hands-off-bamboo-coffee-cups-german-consumer-group-warns/a-49713624

https://www.google.com/search?q=bamboo+reusable+mug+warnings

Post edited at 22:47
In reply to TobyA:

I would say the bamboo in the microwave would be some sort of fire hazard. 

As for induction hobs, magic? I don't understand them but I know it's a pain to try buy one that isn't these days or even just a hob with knobs rather than touch screen buttons.

 flatlandrich76 07 Jan 2021
In reply to TobyA:

1) Just a guess but I suspect bamboo could absorb water during use or cleaning (or even have sap trapped in the tissues from when it was growing.)  If you then microwave it that water could turn to steam, expand and damage the mug. 

 wintertree 07 Jan 2021
In reply to TobyA:

Spruce needles burn in a rapid flash - high surface area, low thermal mass and high resin content.  An evergreen forest fire must be terrifying.  Throwing a few fresh branches on a bonfire is like hitting the reheat on an Olympus engine.  Wouldn’t want the residue fouling up my chimney.

Post edited at 22:51
 gravy 07 Jan 2021
In reply to Mrs Only a Hill:

There are three main reasons why things aren't microwaveable:

(1) is they aren't transparent to microwaves and absorb them getting very hot - I've a selection of near identical bowls some of which heat and some of which are fine - it's scalding roulette microwaving them.

(2) they have some metal in and the electric fields can cause sparks and plasma.

(3) they aren't up to the rapid heating / heat for some reason (melt, explode, catch fire, crack etc).

(4) no one has tested them and wants the liability

I'll hazard a guess for (1)+(3) and maybe (4). 

Chances are the cup itself heats and falls apart as the glue melts making a dangerous hot mess as the hot drink spills out but also since the material is combustible then maybe it is a fire hazard.

 McHeath 07 Jan 2021
In reply to TobyA:

Q1: A girlfriend gave me one of these; I chucked it when she chucked me. I'm a musician and never seriously considered the physics of compressed bamboo fibre, but maybe I should have left it in her microwave at 700W?  

In reply to flatlandrich76:

Very useful - thanks. If I leave them stacked up next to the wood burner until next Christmas (I'm quite happy with the aesthetics of this) will they burn alright then?

 wintertree 07 Jan 2021
In reply to TobyA:

Induction hob - to really get a good feel you need to go most of the way through an introductory undergraduate electromagnetism course - even then a lot of English language courses and textbooks just present induction as a fact, where as it can be arrived at with a lot more understanding and intuition via determining maxwell’s displacement current from a perspective of conservation and information theory, and then considering the symmetry between E & B fields.  I think doing 1st year physics at ETH  would be my recommendation to really understand it.  If I ever have the time I’m doing a picture book for this level of EMag called “the dyslexic’s guide to electromagnetism”.   I want to do my children’s stories first though.

Noddy answer - it’s a transformer just like in old fashioned power supplies.  If you short the output side of a transformer, it gets very hot and melty.  The pan is the output coil, and it is a short. It is big enough that it can radiate and convect all the heat to the atmosphere before it would melt.  Sort off.  It’s the same as induction charging, if your induction charger was powerful enough to melt your phone. 

Post edited at 23:04
1
In reply to Mr Lopez:

Very thorough, thanks, although I note you've now ruined my Christmas present for me!

I do remember maybe a decade ago when "bamboo" thermal underwear was all the rage, someone here on UKC basically explaining how really its just another type of synthetic fibre and no more environmentally friendly than any other.

I presume when I heat my milk in my Greggs reusable cup or the various other ones I accumulated down the years, there are other chemicals leaching out of them and into my body as well though...? Modern life is rubbish is so many ways, although we still manage to make it on average into our 80s these days unlike our ancestors a couple of centuries ago! So there's that...

 flatlandrich76 07 Jan 2021
In reply to TobyA:

After 12 months indoors they'll be fine to burn (probably smell quite nice for the next few weeks too!) Don't stack it to close or it might start to smolder though. Spruce is a very resinous wood so I wouldn't want to burn lots of it but a few small logs will be fine. 

 marsbar 07 Jan 2021
In reply to TobyA:

I am happy to risk hot milky drinks in the bamboo cups, but I keep a glass jug for microwaving.  

I think the amount of nasty chemicals that leech into the drink would be too much if microwaved.  I also wouldn’t use it for black coffee, as I assume black coffee is more acidic than milky coffee. 

(Having read the links that Mr Lopez shared a while back this was my conclusion.  The German study used an acidic liquid, and the study wasn’t entirely unconnected with the people selling plastic cups). 

Post edited at 23:09
In reply to wintertree:

> Induction hob - to really get a good feel you need to go most of the way through an introductory undergraduate electromagnetism course...

Yeah - last time we did this, I didn't really get much further than some one telling me just to think of it as dancing electrons. So now I have this image of sub-atomic particles doing the Can-Can, which doesn't really help much. I still am vaguely nervous picking up a saucepan off the hob that I'm going to get a shock!

 Mr Lopez 07 Jan 2021
In reply to TobyA:

Sorry about that You can still use it for cold drinks, a stash of smarties, or to keep your pens and pencils in though.

I believe the risk is associated with the way the 'plastic' bonds/doesn't bond with the bamboo fibres, and that normal melanine cups are safe(ish), in that the amounts of stuff they release is not supposed to be a concern. But you never know, just in case i never put melamine cups in the microwave. Kidney stones sound like no fun at all!

In reply to TobyA:

> it is made of "bamboo" to make it even more environmentally friendly.

Unless it is obviously actual bamboo, with obvious fibres and pores, which doesnt sound like a sensible idea for a liquid container, I would imagine it is cellulose acetate, based on bamboo cellulose. So, a plastic (the first plastic, in fact), only not a petrochemical plastic.

I don't think CA likes being microwaved; it will absorb microwave energy, having OH bonds that absorb microwave energy.

[edit] as above, the alternative is that it's a phenolic of some sort (melamine), but I dont see where the bamboo comes into that...

Post edited at 23:34
 Snyggapa 07 Jan 2021
In reply to TobyA:

If you cut a christmas tree it will leak resin like you have never known before onto everything and anything nearby. Everything you touch for 12 months will be sticky. It burns furiously and quickly , ten times so if you still have the needles on.

Burn it outside* , in a large open space, as a science experiment. Then wonder why you had it indoors for a month near the fire.. Or watch a youtube video of the same and take it somewhere to be recycled.

*everyone needs to burn a christmas tree once, but no more as once you have tried it once you end up terrified of them. 

 waitout 07 Jan 2021
In reply to TobyA:

Both questions are totally within the realms of trying out in safety. You're a smart chap, why bother with the thoughts of us forum junkies when you can run the experiments and report back with answers?

Proper data expected of course. Remember your protective eye wear.

In reply to TobyA:

my random questions are all “Hollywood” related and are basically just one question :

Why do talented and successful actors who have enough in the bank to be able to pick and choose their projects, like Liam Neeson, Kate Beckinsale, and Gerard Butler to name three, keep signing up for utter shit? I accept that another big pay cheque is more useful to them than “maintaining a legacy” and they aren’t risking Oscar loss like Eddie Murphy did with Norbit (which torpedoed his chances of winning for Dreamgirls) but O just think “do they look at their own billboards and wistfully remember when they were in proper films?” 

 Eric9Points 08 Jan 2021
In reply to TobyA:

The bamboo is likely to start smouldering if left long enough in the microwave. I once tried to sterilise a wooden spoon that way...

You have never been in a bothy in winter and tried to burn green conifer? Leave it somewhere dry until March.

In reply to Eric9Points:

> The bamboo is likely to start smouldering if left long enough in the microwave. I once tried to sterilise a wooden spoon that way...

I'm coming to the conclusion that, like with thermal undies, the whole "bamboo" thing is just marketing - it's a non petrochemical plastic that's all. So I suspect Mr Lopez's links above explain why - they don't really look like wood in anyway. 

> You have never been in a bothy in winter and tried to burn green conifer? Leave it somewhere dry until March.

I don't think so - I remember carrying coal from the petrol station in to the Ben Alder bothy for example! In Finnish national parks there is always loads of wood ready at camp grounds so people don't damage the living trees. The park authorities provide log splitting and sawing equipment so you can still do the last bits of prep and feel manly though.

I guess a lot of this will be birch, but lots must be spruce or pine of some type. I don't remember it being particularly exciting - but maybe its been lying drying for a year or two by then...

 nufkin 08 Jan 2021
In reply to Blue Straggler:

>  Why do talented and successful actors...keep signing up for utter shit?

A few possibilities, in no particular order:
- For the love of the craft
- To keep their name floating around the top of the top of the casting pool
- Obligations, contractual or personal
- Desperate narcissism
- Poor judgement

(With this last one, I imagine it's sometimes hard to tell guff from gold just from a script or synopsis. And actors have no control over poor direction or post-production mangling)

In reply to nufkin:

Good answers. Maybe I should have put more "disclaimers" into the question, to automatically eliminate "contractual obligation" (see: The Negotiator starring Samuel L Jackson and Kevin Spacey) but that would have made a long winded post, and the question was a little rhetorical. I know some (e.g. Dennis Quaid) simply love working, they like the process, they like being on set. Not about narcissism or obligation. Nicolas Cage is similar, I think he's said "if the script isn't totally shit and I'm available, then why not?"
The contractual thing is interesting though, there is a fair bit of "the studio will fund your edgy dark uncommercial pet project IF you sign up for this crap that will definitely make money" 


As for your final point, the best thing quote I ever read about this came from Matthew Modine, regarding Cutthroat Island:

"I said 'yes' to a script that Michael Douglas had said 'yes' to, and we filmed the script that Michael Douglas had said 'no' to" :D 

 mcdougal 08 Jan 2021
In reply to Snyggapa:

> *everyone needs to burn a christmas tree once, but no more as once you have tried it once you end up terrified of them. 

True, I've never seen a fire like it that didn't involve iron filings and petrol! It gets me that people used to decorate Christmas trees with lit candles; it's a wonder that anyone survived! 

 kestrelspl 08 Jan 2021
In reply to TobyA:

I'll bite on the induction hob one (PhD in particle physics for credentials).

The first important thing to know is that moving charge creates a magnetic field. The second important thing to know is that if the magnetic field on a conductor changes, then that exerts a force on the charges in that wire which can cause a current in the conductor. These are consequences of Maxwell's laws of electromagnetic fields.

The way an induction hob works is by passing a current through a big thick coil in the hob. That generates a magnetic field around that coil. The current used is an alternating current, which means that the magnetic field changes over time allowing it to cause currents to flow in conductors near it.

That's why when you put a pan near the hob it gets a current 'induced' in it. That current then heats the pan by the same effect that makes old fashioned light bulb elements get hot or electric fan heaters work.

The next cool bit (see what I did there) is "Why doesn't the hob get hot?". The reason for that is that the coil in the hob is big and thick, so just like the hefty power cables you see for bigger bits of equipment it doesn't get hot even if quite a lot of current passes through it. The pan on the other hand is much thinner so the current induced in it makes it heat up a lot. Just like an old fashioned light bulb element gets so hot it glows, but the thicker cable to it which has exactly the same current down it stays at room temperature.

I've skipped over some details like how the voltage, current and geometry of the hob coil can mean that you get very different currents in the hob and the pan, and how magnetic fields in objects aren't the same as magnetic fields in vacuum, but the above should be enough to roughly understand what's going on.

Post edited at 14:08
In reply to TobyA:

> Does spruce burn green? Will we get a nice smell of Scandi campfires if I chuck those logs on the wood burner tomorrow night? Or will it smokily smoulder, or crackle and spit resin everywhere?

It's fine once it's dry (and the leafy bits make excellent kindling as well smelling great).  The bad news that it will be ready in time for next winter's fires.  

Our tree is still up, which is really unusual for us - we are normally ruthless with Christmas decorations once the New Year is in - but somehow we're in no hurry to take away the lights this year.  Still, this weekend I'll be chopping it up to dry in the barn. 

 nniff 08 Jan 2021
In reply to TobyA:

Microwave ovens.  Many things that I do not understand.

Tin foil, forks and metallic elements in glazed pottery will fizz and spark and get otherwise excited. If I heat a liquid in our microwave I am told by the manufacturer that I must put a teaspoon in it so that it heats properly.  Our last microwave fried itself because we heated too much coffee without a spoon (expensive mistake). 

The microwave has a wire rack in it. It also has a big metal baking tray thing.  Both of these must be removed if full power is used. 

A ceramic cup is fine, but if it has a crack in it, someone is going to lose some skin. 

Cheap microwaves have a little roundabout.  Expensive ones do not. 

The cheaper they are, the easier they are to operate.

If you leave a baking potato in too long, the bottom of it will turn into an inedible biscuit.

Heating fish in an office microwave is a hanging offence  What does a microwave do to make fish smell so strongly?

Dark magic indeed. 

Post edited at 16:24
 Eric9Points 08 Jan 2021
In reply to nniff:

Never heard of putting a spoon in the water.

The electromagnetic field that is set up inside the oven alternates at the same frequency as water molecules want to vibrate at so the water soaks up the microwave energy and that's how thing heat up. You're heating the water that the food contains. I guess other molecules are probably excited at or near the same frequency or harmonics thereof but that's a deduction based on some bits of crockery heating up in a microwave and some not.

The other thing about microwaves is that the bit that generates the energy, the klystron, tends to wear out over time, or at least the ones used in the radars I used to design wear out so they will become less effective as they age.

The other thing to note is that microwaves at that frequency are phuqing dangerous. They will cook you just the same as they will heat up a pie. The reason microwaves don't regularly kill people is that almost all the energy is kept inside the oven because it's a metal box that won't let the radiation (radio waves) pass through it. The window in the door has a metal mesh in it too and the wavelength of the microwaves is too big for them to pass through the holes. So if you hear someone worrying about 5g you can point out that there's likely more radiation leaking out of their oven into the kitchen than there is from 5g and it's at a frequency which is really bad for you.

 kestrelspl 08 Jan 2021
In reply to nniff:

The metals thing is often due to the pointyness. Pointy metal things are a lot lot worse than big rounded bits because the electric fields that can build up around points are very very high. This is why lightning conductors are pointy, because it makes it much more likely that the discharge will occur there compared to a similarly heighted round thing.

The edges of the thin films of metal on things like plates with metal foil on them are particularly bad. See also butter packaging and tin foil...

Post edited at 17:56
 cb294 08 Jan 2021
In reply to wintertree:

Every time I drive to Berlin I pass Halbe, 60 km south of Berlin and the site of one of the last battles of WWII in Europe, fought in parallel to the battle of Berlin.

This battle was fought for four days in April 1945 in a continuously burning pine forest, with troops moving between burning patches of forest.

There is also a memorial and a war cemetery in Halbe, but there was not much left to bury.

CB

 cb294 08 Jan 2021
In reply to nniff:

The coolest bit about microwaves is that you can use them in combination with a large bar of chocolate to measure the speed of light!

300g Lindt or Marabou work best, according to many diligent control experiments....*

When we did this with the children we came up with 280.000 km, which I found impressively precise.

CB

* Cool the chocolate in the fridge, put it onto four toothpicks in the microwave with the carousel removed, the closer to half height the better. Turn power on for a few secs, find melty bits of chocolate by poking the bar with two more toothpicks. Measure the distance. The melty bits correspond to the nodes of the standing wave in the resonator, so multiply by two and then with the frequency of your microwave you can read off the sticker at the back.

 elsewhere 08 Jan 2021
In reply to cb294:

Chocolate is an excellent medium for education.

Pedant alert - melty bits at antinodes or nodes?

Post edited at 19:34
 greg_may_ 08 Jan 2021
In reply to cb294:

Wow. Ok that is a useful experiment I can use with my year 11s! 

 Babika 08 Jan 2021
In reply to Blue Straggler:

> my random questions are all “Hollywood” related and are basically just one question :

> Why do talented and successful actors who have enough in the bank to be able to pick and choose their projects, like Liam Neeson, Kate Beckinsale, and Gerard Butler to name three, keep signing up for utter shit? 

In the same vein I often wonder why do really wealthy people go to such great lengths to avoid tax? 

Its a genuine question.

 cb294 08 Jan 2021
In reply to greg_may_:

It is from a book called "How to fossilize my hamster", even though I saw it elsewhere first.

Tons of other suggestions in there that could be used as exciting school science experiments.

Fossilizing the hamster unfortunately does not work!

CB

 wintertree 08 Jan 2021
In reply to Blue Straggler:

> Why do talented and successful actors [...] keep signing up for utter shit?

Perhaps they hope to replicate the Nicholas Cage or Gary Busey magic.

 Chris Craggs Global Crag Moderator UKC Supporter UKC Supporter 08 Jan 2021
In reply to TobyA:

I watched something years ago, a reporter asked a scientist to explain magnetism to him - the scientist said it wasn’t possible without an understanding of some basic principles. The reporter struggled with that,

Chris

In reply to nniff:

> Heating fish in an office microwave is a hanging offence  What does a microwave do to make fish smell so strongly?

One of the bosses in my office cut the plug off the office microwave with a pair of pliers on the third day of someone heating smoked haddock soup up 🤣

 Timmd 08 Jan 2021
In reply to TobyA:

I just burnt my tree in my terraced house garden which was quite satisfying, I cut it as far along the trunk as loppers would work, then cut the branches off and made 'a heap of tree' on top of some cardboard and card, and it burnt down within minutes. I think I missed it being collected for processing, it's greener for it to burn than to rot apparently, because methane is meant to be emitted when an Xmas tree rots.

Post edited at 22:07
 Hat Dude 08 Jan 2021
In reply to kestrelspl:

Q1 Don't trust a coffee cup made from the same stuff that underpants and socks are made from

 nufkin 09 Jan 2021
In reply to nniff:

>  The microwave has a wire rack in it. It also has a big metal baking tray thing.  Both of these must be removed if full power is used.

My understanding, only recently learnt, is that microwaves don't have different strengths; the power setting just varies the duration of the waves' generation, rather than making them more or less 'microwavey'. I don't know how that relates to metal inside, but it did make me feel like I was somehow more master than slave to the machine subsequently

 Reach>Talent 09 Jan 2021
In reply to TobyA:

From personal experience of heating bamboo it is normally a bad idea:

Bamboo contains lots of little air pockets which can explode violently when heated, burning bamboo sounds a lot like gun fire. I wonder if a bamboo mug (which I assume is a laminated lump of bits of bamboo) would behave similarly. 

I was told you shouldn't burn green wood in a log burner as it cakes your flue in "manky black gunk". I can't say I have ever checked this myself so no idea if it is true.

 Reach>Talent 09 Jan 2021
In reply to nufkin:

> My understanding, only recently learnt, is that microwaves don't have different strengths; the power setting just varies the duration of the waves' generation, rather than making them more or less 'microwavey'. 

Yep, microwaves aren't on all the time. I had someone who really knows their stuff try to explain microwaves to me and they failed miserably, as far as I am concerned radio frequency engineering is basically witchcraft.

 cb294 09 Jan 2021
In reply to elsewhere:

Always assumed nodes, but thinking about it antinodes is probably right!

CB

In reply to TobyA:

Induction hob - The coil in the hob creates a magnetic field which flips back and forth at mains frequency (50hz).  The 'magnetically coericble' atoms in the pan try to align themselves with the field, and because it continually flips back and forth so do the atoms. This makes them get hot.  It's the flipping that's the important part.

This is why you need pans made of magnetically coercible metal i.e. iron or steel. As we all know, iron will hold magnetism well and thus it's atoms respond well to the changing magnetic field. Cast iron pans work amazingly well on an induction hob.  The techy name of the cause of the heating is 'eddy currents' I think.  The coil doesn't get hot because it's made of a metal whose atoms aren't magnetically coercible.

Post edited at 02:04
 kestrelspl 10 Jan 2021
In reply to Toerag:

I don't think that's right. It's the current flowing that makes things get hot, regardless of whether it's from magnetism or something else. The coil in the hob also has a current flowing through it, it's just a thick enough conductor that it's resistance is low enough that not much heating happens.

 wercat 10 Jan 2021
In reply to Toerag:

I was under the impression that the induction hobs were a source of somewhat higher frequencies than mains?   We need an expert (forgive me if you are one!)  to come along!  

I know there is major concern about RF emissions from wireless car charging

Post edited at 12:52
 wercat 10 Jan 2021
In reply to kestrelspl:

eddy current?

 geode 10 Jan 2021
In reply to TobyA:

> I'm coming to the conclusion that, like with thermal undies, the whole "bamboo" thing is just marketing - it's a non petrochemical plastic that's all. So I suspect Mr Lopez's links above explain why - they don't really look like wood in anyway. 

yeah, i was expecting a real bamboo cup not this fakery:

https://www.seasandstraws.com/bamboo-cups.html

 AllanMac 10 Jan 2021
In reply to TobyA:

Burning unseasoned (green) softwood in a woodburner for any length of time will gunk up the flue with acidic creosote deposits, which may later cause a chimney fire, as well as corroding the chimney. Moisture content of logs for burning, whether hardwood or softwood, should be less than 20%. 

Water vapour mixed with tree resin is not good, but the same resin in dry seasoned wood makes it burn faster, and is carried out of the chimney as less harmful smoke, rather than steam condensing on the flue wall.

 mbh 10 Jan 2021
In reply to TobyA:

I cut up my Christmas tree. I took the branches to the recycling centre and will wait until next year before burning the trunk.

I want to know about the water content of the logs I have lazily stacked behind the house. When I say lazily, I mean they have got wet because my shanty construction of old slates from our roof didn't really do the job.

While they might feel wet when I bring them in, is it all just surface stuff that a few days inside near the fire will put to rights (while making my house more damp, I suppose) - they do feel dryer - or do they still lose their intrinsic moisture if left outside, even in the wet. 

An estate I went to once that had 50 or so holiday homes all provided with heat from a biomass district heating scheme fed by their own wood used to leave cut trees stacked outside for a year, then chipped and inside for another year before using them. What was the point of the first year if not to reduce water content?

 Timmd 10 Jan 2021
In reply to mbh: A guilds of master craftsmen furniture and cabinet maker I know once told me that wood outside which gets wet is still 'dry' in the having lost it's moisture content sense, it's just got wet from the rain. I suppose in damp enough conditions it could start to rot, but other than that I was lead to believe it is okay to burn.

In reply to kestrelspl:

https://en.wikipedia.org/wiki/Induction_heating  Look at the coil in the first pic - lots smaller diameter than the object it's heating.  It's not a low resistance of the induction coil that stops it heating up compared to the pan, it's the fact that the pan is made of steel / iron with magnetic properties.

 kestrelspl 11 Jan 2021
In reply to Toerag:

The reason in that picture that the thing it's heating gets a lot hotter is that it's wrapped round the bar 10 times, thereby multiplying the current in the heated thing by 10.  That's what I was glossing over above in my explanation above when I said the geometry of the coil can mean there's a very different current in the coil and the pan. So to be more precise I should have said that the cross-sectional area of the coil multiplied by the number of turns in the coil (and some other geometric factors) is larger than the cross-sectional area of the pan.

You are right though that there's an extra effect I hadn't considered in my last post that means that magnetic materials get heated more effectively than non-magnetic ones. Changing the magnetic field the pan experiences changing causes two things. First is the induction of a current that I described above. Second is that all the atoms in a magnetic pan act like little bar magnets and the change of magnetic field can cause them to flip. That flip also causes some heating. The balance between which of those effects causes the most heating depends strongly on the particular geometry of the coil and the pan and the particular material.

In an induction hob it seems both are important at a similar level, but the reason the coil doesn't get hot is definitely that it has sufficient diameter for the current running through it not to cause much heating.  http://www.imajeenyus.com/electronics/20120509_induction_cooker_with_external_coil/index.shtml 

Post edited at 15:27

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