Micron thick steel wool in a microwave and how mirrors work

[Music] this is super fine steel wool and the individual strands are just a few microns thick so that's a few thousandths of a millimeter and it looks like fur you know but I mean even fur is tens of microns whereas this is even thinner than that and it looks amazing but when it really shines literally is when you put it in a microwave they're not beautiful this is actually about a sick of normal speed it's slightly slow-mo here it is a full speed I really just wanted to show you that because it's so pretty but thinking about it it also gives me an opportunity to clarify something that I talked about in a previous video and it gives me the chance to answer something that has been niggling at me for a while I think I have the answer to so the clarification for I mean let's call it what it is it's a correction so in a previous video the one about CDs in a microwave I talked about how metals and electromagnetic waves interact with each other electromagnetic radiation is just an oscillation of electric and magnetic fields and an electric field is just a region of space in which charged particles feel a force an example of charged particle is an electron like the free moving electrons inside a metal so this is what I said happened so you've got this oscillating electric field that goes into the metal and so the charged particles the electrons inside the metal feel a force and they move around that's not really quite right and it's more nuanced and more interesting so imagine you've got a chunk of metal and you put it in an electric field inside the metal you've got these electrons they feel a force because they're inside an electric field and so because they feel a force and because they're free to move they move and so imagine this electron moves over a little bit but all the electrons around it are feeling that force as well so they all kind of bunch up and because they're bunching up well you know this you've got you've got a concentration of negative charge like charges repel each other so these negatively charged electrons are repelling each other and so they feel an opposite force pushing back so you've got the force from the external electric field pushing electrons away and they've got the force pushing back that repulsive force of the clump together a negative charge and naturally you reach an equilibrium where the force is balanced where the electrons aren't moving anymore they're not feeling a force and if they're not feeling the force that must be because there is no net electric field the external electric field is canceled out by the electric field caused by the bunching up of negative charge and the fact is this all happens at the surface of the metal this bunching of electrons or spreading out of electrons cancelling out the external electric field so that within the metal itself there is no electric field it's a surface effect and this helps to explain another thing that's been niggling at me for a while and that is why are you advised not to put metal in a microwave when the inside of a microwave is made of metal surely it would be okay to put something in the microwave that the microwave itself is made of that's my logic anyway and actually the answer is it is okay to put metal in a microwave it's not metal per se it's the shape of the metal that is potentially the problem so first of all why is it okay in principle to put metal in a microwave well imagine you've got your block of metal here and you've got an incoming electromagnetic wave so you've got this oscillating electric field and the electrons at the surface of the metal respond to that by bunching together spreading out bunching together spreading out at the same frequency as the incoming wave and they do that in such a way as to cancel out the incoming wave because they're completely free to move or at least in in an idealized metal they are so you've got this bunching up and spreading out bunching up and spreading out generating an oscillating electric field that exactly cancels out an incoming wave so what does that look like an oscillating electric field that exactly cancels out an incoming wave well that's an outgoing wave and that's reflection that's what reflection is that's why metal surfaces are shiny like when you look in a mirror well first thing is you're looking through a sheet of glass but then behind that is a silver a thin sheet of metal that reflects electromagnetic radiation in this case in the visible spectrum so that you can see your face in the mirror and that's what the inner surface of a microwave is doing it's reflecting the microwaves so they bounce around inside the box and then eventually hit your food and get absorbed by your food so you can safely put a spoon in the microwave and the microwave radiation will just bounce off the surface of the spoon so what happens when it goes wrong like clearly the steel wool isn't reflecting all the microwave radiation some of it must be getting absorbed because it's heating up it's heating up to the point of glowing and eventually burning and that's because of the shape of the metal in this case it's very thin and you might know this from circuits so thinner wires have a higher resistance than thick wires and when you have resistance in a wire its dissipated as heat and light the other reason we're advised not to put metal in a microwave is to avoid Sparks so and you see this with the CD in the microwave the reason that happens is if you've got any sharp edges or points like when the CD initially cracks into those individual kind of tracks that you can see you've got these sharp edges and sharp corners if you push electrons up into those edges and corners then you get this intense buildup of charge you you get a high potential there in that corner strong electric field if you like and that that strong electric field is enough to break down the insulation of the air so air is a good insulator but with a strong enough electric field you can basically tear the atoms apart or at least rip electrons away so that you've got freely moving ions and electrons charged particles they can carry electric current the air is no longer an insulator and you get this this spark essentially like mini lightning in the microwave so those are the reasons not to put metal in a microwave but if you can avoid those then you can put metal in a microwave that's why some microwaves come with a metal grill that you can put in there no sharp edge is no thin bits just for comparison here's normal wire wool in a microwave what's nice is after you turn off the microwave you still see these little glowing spots moving around inside and that's really just the fire triangle doing its thing like for a fire you need heat fuel and oxygen and so the steel wall is like a really thin source of fuel and that's surrounded by oxygen but you've only got heat one end so it's kind of slowly moving along you have this glowing sort of blob at the end it's basically how a fuse wire works or an incense stick you don't need a microwave to see that effect actually you can ignite it with a 9-volt battery you someone suggested that I put polyethylene oxide in the microwave that's the vaso elastic liquid I used in the self siphoning liquid video so I gave it a go and it's somewhat interesting so I'm gonna pop that at the end of this video without comment really but before that I just want to talk about reflections again we talked about fractions earlier I want talk about in relation to the sponsor of this video so this video is sponsored by brilliant org if you don't know what that is yet it's a website full of puzzles and problems you work through these puzzles and problems and by the end you you've learned something it's like it's like online learning but through puzzles and problems and I've said this before I'm a strong believer the best way to learn something is by doing not just watching my videos and that's what you get with brilliant dot-org and I really like the way they've curated the the puzzles and like the the progression of them is really good it's not too hard it's not too easy and you feel Reece Mart by then because you are smarter you've learnt something and I'm really grateful for them continuing to sponsor my videos I really think their hearts in the right place that their philosophy of learning and stuff like that all my interactions with them have been really great so yeah if you want to think like a scientist think like a mathematician go to brilliant dot org forward slash Steve mold the links also in the description there if you use that link it helps me out because they know that the sponsorship is working and as a bonus the first 76 people to use that link get 20% off annual premium membership should they choose to upgrade Oh what was I gonna do with reflections so you know that's the other thing about brilliant is like there's little search box and if you're thinking about particular thing I think about reflections did they type it in and it'll bring up all these courses about whatever it is just doing that today I found this lovely puzzle about reflections like you've got this incident ray it bounces around it comes out and is parallel if it's bounced around five times it's parallel what West the angle be and the way you work out is you get this long equation they all cancels back down again you get the answer it's really good fun and there's just loads of stuff like that okay anyway polyethylene oxide in a microwave here we go you so there you go some things in a microwave I hope you enjoyed that it can give don't get hit subscribe and I'll see you next time [Music]

Channel: Steve Mould

Views: 203,367

Rating: 4.9544706 out of 5

Keywords: Explained, understand, metal, reflection, electromagnetic, radiation, electron, electric filed

Id: oiWZYdr9Zvo

Length: 13min 50sec (830 seconds)

Published: Thu Mar 29 2018