Exploding Wires

Neal our technician suddenly got inspiration from his old physics teacher he's devised a machine which will allow him to put a high current through a piece of thin wire and as he turns the knob the current goes up and up and up until the wire fails no I'm ready when you are for this one so we started with copper what happens is that as you put the current through the copper he gets hotter and hotter and hotter and in this case copper melts before it catches fire so suddenly you see the wire sagging down and down and down and eventually it breaks and as it breaks there is a flash as the copper vaporizes and you may remember that copper gives a nice green color in the vapor so you see a very quick flash of green light so Neil had a great ambition to try and burn aluminium so he got piece of aluminium wire and heated it up but like copper aluminium melts before it catches fire in air so suddenly you'd start sagging sagging sagging and after a little bit its breaks Neil was a bit disappointed nothing burned so we tried magnesium didn't have magnesium wire so we tried magnesium ribbon that's relatively flat material and as you heat that because the heated metal expands the magnesium started arching upwards and then suddenly it drooped there as probably many of you know that once you set fire to magnesium it keeps burning so the whole lot burnt away with clouds of white sort of smoke which essentially is finely divided magnesium oxide [Music] magnesium is a metal that once you light it it generates so much heat that the burning becomes self-sustaining it's probably because the metal doesn't conduct away the heat very much from the area that's burning whereas aluminium and copper if you set it on fire and remove the source of heat it quickly goes out probably the metal just cools down too fast [Music] Neal suddenly remembered he had some special wire he took Brady to his special store he calls it the dark hole doesn't want me going there and might start taking things from it in the dark hole was the remains of a superconducting magnet a very powerful magnet that was used in an instrument some years ago the magnet consists of a coil and Neil cut off a piece of this alloy the wire that's used for these magnets has to be insulated otherwise it would short-circuit and it's insulated with varnish so as it heats up there's a bit of smoking as the varnish vaporizes slightly disappointingly it behaves much the same as the other metals SAG's flashes and breaks next Neil produced a piece of tungsten wire tungsten wire is what's used in old-fashioned lightbulbs it was really interesting as you heat it up it gets hotter and hotter tungsten has the highest melting point of any of the metals that we used and it starts to burn doesn't melt at all and just gets thinner and thinner and thinner until it fails he ended up with quite a terrifying looking spike a really narrow tungsten so professor this did fail quite quickly but light bulbs don't fail quickly ideally what's the difference between this and a light bulb the difference from the light bulb is that in a light bulb the tungsten is not as hot as at the end of Neil's experiment and of course the bulb itself is filled with argon gas rather than with air Neil and I were both really quite fascinated by this very narrow spike that is made and I think it is because one part of the wire must have started getting thinner than the rest and therefore hotter and therefore preferentially the hottest part will have burnt fastest Brady really wanted to do gold Neil was a bit reticent gold is expensive so he put a glass dish underneath now I must say that gold was probably the most disappointing of them all because it heated up it melted it broke it didn't give a really very convincing flash Gold doesn't have a beautiful green color like copper but Brady was satisfied Anil didn't lose any gold I think the most exciting metal and this was a real surprise was molybdenum when we heated up molybdenum even when I was watching the camera from the distance there seemed to be some sort of pulsating on the picture and when we looked at the metal afterwards it was extraordinary because instead of having a smooth wire there were little blobs of molybdenum metal all the way along the wire and these blobs were really quite evenly spaced now it turns out that this phenomenon has been seen in exploding wires using very high speed cameras I think it's unusual i and none of my colleagues had seen like anything like this before now you probably or some of you who specialist in this area probably have some really good explanation of why this happened I think what happened is that the molybdenum ëtil began to melt on the surface of the wire so you imagine the wire covered with a thin layer of liquid and because liquid has surface tension this is why liquid forms drops I think that the liquid separated into little drops because the surface tension pulled it together and so you get what would normally be little drops of liquid but they solidified so we've somehow got them fossilized along this wire all my colleagues that I've shown these bits of wire to have got really excited it's really surprising that you can get such an even spacing I did a bit of research on this phenomenon and I came across a word I'd never heard before called angeloids angeloids is the word that mathematicians use to describe this sort of object I would be very interested to hear from any of you who has a really good explanation of what happened Neil thinks that the spacing might be related to the frequency of the mains so he's keen to try and use direct current rather than alternating current and see whether we get the same effect I think there'll be no difference I had the idea that we should turn the apparatus through 90 degrees so the wire is vertical and it would be much more difficult for it to sag and what we found was indeed it didn't sag so much at least until it break and we got a much better shot of the flesh of green light as the cup of vaporized there was still slight bending but it was much less than before just as we were packing up I persuaded Neel to make a coil of copper we put this coil vertically my idea was that the coil would be floppier than the straight piece of wire so it might sink more but then it collapsed quite spectacularly once it's broken [Music] but I think chemically what was interesting to me was that if you looked at the bit that was broken you could see the black copper to oxide on the surface of the copper showing that there was at least some reaction with oxygen but very much less than saying the burning magnesium I was really grateful to Neil because he devised a demonstration which almost turned into an experiment which made me think I never thought that piece of molybdenum wire could be in the least bit interesting and Here I am the day after Neil did the experiment still thinking about it and anything that makes scientists think is really good and anything that makes us think differently is brilliant [Music] you [Music] you

Channel: Periodic Videos

Views: 349,061

Rating: 4.9566517 out of 5

Keywords: periodic, videos, chemistry, wires, electricity, current, gold, copper, molydbenum

Id: kexteGM2V2s

Length: 10min 58sec (658 seconds)

Published: Tue May 21 2019

Reddit Comments

[u/MeatBallSandWedge]

Great video Dr. Haran.

Ok, here is my hypothesys for the molybdenum unduloids: the beads are not molybdenum metal, they are molybdenum trioxide which is what results from heating molybdenum in air. Molybdenum trioxide has a much lower melting point (802C) than molybdenum metal (2623C). This molten molybdenum trioxide forms when the wire is heated and then beads up on the wire. Those beads freeze solid as soon as the wire breaks and the current stops causing the wire to cool down.

To clarify: the beads result from surface tension acting on the molten molybdenum trioxide that forms on the hot, oxidizing molybdenum wire.

[u/CandyCoatedFarts]

Every time I see you guy/gals do stuff like this I get the urge to try it myself

[u/mubd1234]

At my high school many years ago, a friend of mine swiped a few materials from the science lab and bridged the live/neutral conductors in an outdoor power point. The breaker must've blown because after a quick experiment with some magnesium wire, the power point didn't work again until someone reset it a few days later.

[u/Gusfoo]

Neat. Can you carry this subject forward to https://en.wikipedia.org/wiki/Exploding-bridgewire_detonator#Use_in_nuclear_weapons ?

[u/4FPSwarrior]

I agree with Neil frequency might be couse for this unduloid shape wire. Profesors expanation also seems valid but since tungsten has same structure(bcc) and similar properties I would expect this phenomena to occur as well.

[u/Astaro]

I'm surprised they didn't hook up some graphite, just for fun.

The professor's theory about the metals behaving differently due to heat loss could probably be tested by supporting the wire on different insulators.

[u/Celarion]

I'd like to repeat the molybdenum experiment with some magnetic field viewing film in front of it, to see the evolution of magnetic fields around the wire as it melts and burns.