Explosive Science - with Chris Bishop

[Music] thank you one of the things I've noticed when I give demonstration lectures is that the most popular demonstrations seem to be the ones that involve explosions so I thought we'd have a lecture entirely about explosions and explosives and the science behind them now just before we begin I do want to say that you should not try to replicate any of these experiments at home and also please don't try to make explosives at home as you'll see in this lecture when explosives detonate they do so extremely quickly much faster than your human sensors can respond to in other words explosives are very unforgiving so please don't make explosives at home okay so we'll begin the lecture then of course by creating an explosion what we're going to do is to recreate on a smaller scale the the largest explosion that ever happened on the surface of the earth in recorded history and it happened in 1883 and it happened on the volcanic island of Krakatoa and there was an explosion which was equivalent to roughly 200 million tonnes of TNT the shock wave travel around the earth seven times the bang was heard three thousand miles away now there's a theory about how the explosion occurs and it goes like this the volcano had been erupting and throwing out larvae and it left behind a gigantic underground chamber and seawater rushed in and filled the chamber now the walls of the chamber were very hot the seawater turned to steam the pressure built up and eventually there is an explosion so we'll try and create that effect on a slightly smaller scale here in the lecture theatre so this is a a glass tube that contains one milliliter of water if we just bring the camera up on the screen you can have a look at this it's been sealed at both ends it contains just one milliliter of water what I'm going to do now is to heat it up and raise the temperature with this Bunsen burner and as the temperature rises the water starts to evaporate and the pressure begins to build up and at some point it will explode it will be quite loud so I'm going to wear ear defenders and you may wish to cover your ears for this one it's a little bit hard to know exactly when it's going to go so this is one milliliter of water when Krakatoa exploded it's thought that around a cubic kilometer of water was turned into steam so the water is turning to steam the pressure is building up and eventually it reached the point where the strength of the container is not sufficient to withstand that pressure and that's when we get the explosion so we call this a physical explosion explosion caused by a build-up and then a sudden release of pressure [Applause] okay so that's an example of a physical explosion the build-up and then the sudden release of pressure now if we want to make an explosion for some practical application this isn't very convenient you have to have the flame we have to wait for the pressure to build up we're not sure exactly when it's going to go off and so on we need a different way of creating explosions and we're going to do that using chemistry and so we're going to have chemicals that explode we call those explosives now for around a thousand years there was only one practical explosive which was known and that's gunpowder and in the trade we refer to gunpowder as black powder they mean the same thing now gunpowder it's just a simple mixture of three ingredients and we've got the ingredients here you can bring the camera up this is the first one this is charcoal familiar from barbecues and so on it's finely powdered this acts as a fuel we have a sulphur one of the elements this lovely yellow powder this also acts as a fuel and it helps to promote the burning and the very important ingredient is this one this is potassium nitrate also known as saltpeter we can think of this as a very concentrated source of oxygen so when this gets hot it gives off oxygen and the charcoal and the sulfur can burn in that released oxygen so the fuels the charcoal and sulfur don't burn with the oxygen of the air they burn with oxygen release from this very concentrated form of oxygen so let's have a look at some gunpowder in this dish I have one and a half grams of a commercial gunpowder and I'm going to ignite this electrically there's a little device in there called an electric match it looks just like a an ordinary match but I can set it off by passing electrical signal to it and I'll do that using this box this is called an exploder well this is not called a detonator we'll find out what the detonator is later on in the lecture that's something quite different so I'll charge this up and then we'll set off one and a half grams of gunpowder just in the open on the bench as you can see where the lovely smoke ring it burns very quickly but you notice we didn't get a bang we didn't get an explosion gunpowder is what we call a low explosive when we set it off in the open it burns very rapidly but it doesn't give us a bang it doesn't explode you get an explosion we need to do something else we need to confine the Gunpowder and so that's what we're doing over here in this cardboard tube we have again one and a half grams of gunpowder the same quantity of the same type of gunpowder again there's an electrical match inside the tube but this time the tube is sealed it's bound up with tape and so the Gunpowder is very tightly confined now what happens when we can find the Gunpowder is that as it burns it produces energy in the form of heat and he can't escape as it can on the bench instead it's trapped and so the temperature rises and when the temperature rises the rate of chemical reaction the rate of burning increases and that increases the rate of heat production and so on we get this feedback leading to very rapid combustion we call that an explosion so again we'll set this off electrically and we'll use a different explode at this time we're going to use this machine this is dates from the early part of the 20th century so DuPont blasting machine so genuine antique blasting machine and remarkably this still works so we'll use this to set off the explosion and at this point I wonder if anybody would like to volunteer to come and use this I think you were the first would you like to come on down it's a handful volunteer please try to come and stand there just not yet just turn around just face the audience what's your name Isaac do you know how to use this have you seen this on cartoons what do you think you do okay what you do is you're gonna take hold of the handle with both hands and not yet but when I tell you I'm gonna raise it up okay and then when I tell you I'm going to push it down nice and firmly okay now this is going to be quite loud so I'm gonna give you a pair of ear defenders you pop those on and again you might want to just cover your ears for this one okay so when you're ready off you go okay so that's the effects of confinement on a low explosive like gunpowder it accelerates the rate of burning we get that that nice explosion now gunpowder is just a mixture of fuels sulfur and charcoal and this source of oxygen potassium nitrate we call that an oxidizer and people searched for alternatives to gunpowder other kinds of explosives and so they started to experiment with different choices of fuels and different oxidizers and invented a whole range of different explosives and these are still used today things like fireworks and pyrotechnics I'm going to mention just one of them and it's this one it's called flash powder and again it's a simple mixture between a fuel in this case a very finely powdered metal and an oxidizer not potassium nitrate but a different oxidizer but again a very concentrated source of oxygen so I have one and a half grams of flash powder and it's called flash powder because it produces a very bright flash it's used in the theater for example when you know in the genie appears you want a bright flash in a puff of smoke so that's where you might use flash powder so let's set off one and a half grams of flash powder and again we'll do this in the open on the bench we'll bring the lights down for this but it will be very bright so I suggest you don't look directly at this instead you look off to one side so this is one and a half grams of flash powder now burning in the open so again it's very bright and I suggest you look to one side so here we go [Applause] so again we had a very rapid combustion but we didn't have a bang we didn't have an explosion but we know what to do to get a bang out of our flash powder we need to confine it so that's what we're going to do now so this apparatus consists really just of a heavy cylinder of steel with a hole board down the middle and in the bottom of the hole we have again one and a half grams of exactly the same type of flash powder but now the confinement produced by the metal walls should accelerate the rate of burning and again we should get an explosion again we've got the electrical match and we'll use our oldie blasting machine to set this off so there's a chance now for another volunteer to have a go who'd like to have a go yes on the end there in the yellow so the hanbô volunteer let's come and stand there what's your name Daniel all right don't forget to turn around you saw what to do didn't you so put those on and wait for me to get mine on this will be I think pretty lads are going to recommend that you cover your ears okay off you go Daniel [Music] [Applause] okay so once again then the confinement of this low explosive leads to an accelerated rate of burning and we get an explosion but people were constantly searching for ways to improve explosives to make them burn even faster what could we do to make our explosives burn even faster well the exposes we looked at so far are mixtures they're mixtures here a fuel and a source of oxygen and it seems obvious that the better we can mix them together the faster they will burn because they burn by the fuel coming into contact with the oxidizer so we can test that idea right by using this this is called Lycopodium it's a very fine powder and it's flammable so I'm gonna put a little on a mat and we'll just see if we can set fire to this and there is it's just about you just see a little flame it's just caught fire so this is flammable what's happening is that there's a chemical reaction taking place we call it burning between the fuel or the Lycopodium powder and the oxygen of the air but it's a very slow chemical reaction because it only happens where the fuel comes into contact with the oxygen it only happens at the surface of that little pile so I'll blow that out and what I can do now is try something different we can take some all like a podium powder and this time we'll mix it very thoroughly with the air first and then we'll set fire to it so I'm going to put some more like a podium powder into this tube and I'll use the tube to blow the powder into the air and that will mix the powder in the air very thoroughly there'll be much more surface area of contact between the powder and the oxygen of the air and that should improve the rate of combustion so I'll use this Bunsen burner to set fire to the powder we'll bring the light stand for this and when the lights are down I'll then blow the powder through the flame and we'll see if that increases the rate of combustion [Applause] okay so clearly then mixing the Lycopodium more thoroughly with the air produced a faster rate of combustion but using the oxygen from the air isn't isn't very good because the air has a very low density each cubic meter of air doesn't contain very much oxygen so you can do better if we use a more concentrated form of oxygen so I'm going to use now liquid oxygen - 183 degrees centigrade so this is just oxygen that's been cooled down until it turns into this beautiful blue liquid in the dish I have some ordinary cotton wool so I'm going to soak the cotton wool in the oxygen so the cotton wool will function as the fuel but now we have a much more concentrated source of oxygen and again we'll bring the light stand for this so this is cotton wool burning in liquid oxygen [Applause] so that certainly burnt pretty quickly but a little explosion there but this again it's not a very practical way to make an explosive because liquid oxygen if we leave it sitting around will warm up to room temperature and it'll evaporate so it would just sort of disappear after an hour - we need something else something a little bit more permanent and the the breakthrough came when a Swiss chemist Christian shurn bine was doing some chemistry in his kitchen of all places one day and he had a little accident he'd been working with a mixture of nitric and sulfuric acids which is very corrosive mixture and he spilled some on the kitchen table and he reached for a piece of cloth to wipe it up and it was a piece of cotton cloth so he wiped up the acids with the cotton cloth he hung the cloth up to dry over the stove and when it dried out it exploded and he discovered some new chemistry he discovered something called nitro cellulose also known as gun cotton what was happening there was really rather special the effect of the nitric and sulfuric acids was to introduce oxygen and also nitrogen as well into the actual molecules of the fuel itself so the fuel is is cellulose these long-long molecules and natural material and the effect of the acids was introduced oxygen run it into the molecule itself so this is the best possible mixture we have between the fuel and the oxygen having them built into the same molecule so let me show you some nitrous cellulose thanks Chris this is some that I made earlier this is just ordinary ordinary cotton wool I could buy from the chemist and it's been treated with nitric and sulfuric acids it's been washed and dried and it still looks and feels pretty much like cotton wool right up until the point where I set fire to it now this is actually going to be fairly bright so again you might just look slightly to one side so this is nitrous cellulose [Applause] now nitrocellulose when it burns produces relatively little smoke we had quite a large pile of nitrocellulose so we got hardly any smoke compared to the tiny amount of gunpowder for instance the Gunpowder produces a lot of smoke because the products the output the end result of the chemical reaction includes materials like potassium carbonate which are solids at room temperature so if those tiny particles of solid which make up the smoke the nitrous cellulose the molecules rearrange themselves into things like water vapor carbon dioxide nitrogen gas things which are invisible gases present in the air anyway and so the nitrocellulose more or less seems to disappear so no cellulose is used a lot in pyrotechnics and partly because it produces very little smoke it's a very convenient material to use and so what I'm going to do now is put on little pyrotechnic show for you and I hope you'll enjoy this so we'll bring the lights down so this is these are nitrocellulose pyrotechnics [Applause] okay so in all the explosions and the pyrotechnics we've seen there is a sudden release of energy when the explosion occurs so where does that energy come from well we can gain some insight into this this source of energy with a little demonstration and for this I'm going to need one very brave volunteer so who's feeling extremely brave a lot of people volunteering their friends I'd like to come on down and do this one you seem very key you're very brave let's hang on to it if you like to come and stand just there and pop those on what's your name Lucas right that's what I want you to do is hold at your right hand and arm's length that's it I don't know form your hand into a little cup oh just one hand it's fine to cut like that that's right this is just soap solution just sort of washing up liquid really and I would could have put a little puddle of this in your hand you just say you're brave didn't you okay there we go it's going to create a little explosion in your hand now this is this is a electrical power supply and we're passing electricity through a bottle containing water is actually sodium carbonate solution and the effect of the electricity is to break up the water now water just got a bit closer there we go water of course you all know the formula for water h2o water is a compound of hydrogen and oxygen and the effect of the electricity is to break up the molecules to produce hydrogen gas and oxygen gas so this little pile of bubbles in your hand contains a two-to-one mixture of hydrogen and oxygen a little bit more there how brave you feeling quite brave bit nervous I'm not nervous right we'll switch that off now we're going to pop some ear defenders on you for this thanks Chris thanks very much I'm gonna pop some ear defenders on and we'll set fire to this mixture of hydrogen and oxygen you ready hold your arm up straight here we go well done [Applause] okay so that little explosion clearly released some energy but where did the energy come from well hydrogen gas consists of molecules and each molecule contains a pair of hydrogen atoms like this now oxygen is similar oxygen consists of oxygen molecules and each molecule again has a pair of oxygen atoms now our volunteers hand had a had some bubbles inside the bubbles we had a mixture of two parts hydrogen and one part oxygen but it just sat there it didn't do anything until we applied a flame when we apply a flame we're adding a bit of energy we call that activation energy it's a very important concept the activation energy is breaking apart the molecules it's breaking the bonds that hold the molecules together we have to put energy in in order to break those bonds so the bonds the hydrogen oxygen get broken and then new bonds can form the oxygen and the hydrogen can come together to form water so as they come together to form new bonds energy is released and a lot more energy is released by the formation of these new bonds that it cost us to break the original forms and so with two molecules of hydrogen and one of oxygen we can make two molecules of water so this is water h2 so this idea of activation energy is very important one I can illustrate it with this model of a ball on a hill so the ball is that quite a high point on the hill so it has gravitational potential energy compared with the bottom of the hill our energy would be released if the ball could roll from the top to the bottom of the hill but it can't because it's stuck in this little well this little valley if I have a little bit of energy to this nothing much happens but if I had enough energy to get it over the hump we call that the activation energy then the rest of the energy can escape I get the activation energy back again plus all this extra energy so when we set fire to the hydrogen oxygen we are we are adding the activation energy and then we get the activation as you're back again plus a lot of extra energy so that's the idea of activation energy now I'm going to illustrate activation energy with a different kind of experiment and for this we're going to bring the lights down because we're going to use not heat as the source of activation energy but rather light so we're going to do an experiment now which involves a glass tube a heavy glass tube filled with a mixture of hydrogen gas and chlorine gas now again the two gases are sitting there mixed together nothing very much is happening but if we apply some activation energy we can break apart the the bonds of the molecules and they can react to form hydrogen chloride and at the same time release in energy so we're going to use light to provide the source of activation energy now light consists of particles called photons and the energy of each photon depends upon its color in other words it depends upon its wavelength or its frequency so long wavelength light such as this red light the photons have a relatively low energy so what's happening now is the red light from the projector is passing through this tube which contains hydrogen and chlorine but no reaction is taking place because the photons don't have sufficient energy at this point I'm going to put some ear defenders on and then we'll explore some different wavelengths of light so let's go to a higher frequency or shorter wavelength of light and try some yellow blight higher energy but still not enough to get the ball over the hill not enough to reach the activation energy there's some green light higher energy still but not still quite enough energy to get over the hill but let's try the shortest wavelength of all some blue lights with the photons have the highest energy [Applause] so that's an example of activation energy we add a little bit of energy to get the reaction going and then it releases a lot more energy now different reactions will have different levels of activation energy sometimes the that he'll at the top will be quite high that's a high activation energy we need to put a lot of energy in to get the reaction going sometimes the activation energy will be very small I'm going to show you at the reaction now where the activation energy is so low that the reaction happens spontaneously what I'm going to do is to add this gray powder it's called magnesium silicide to some hydrochloric acid now the reaction that takes place produces a gas called silane now you've all heard of methane gas I'm sure methane is natural gas it's the gas that you use for cooking and and so on and a molecule of methane has a carbon atom and four hydrogen atoms while silane is very similar except that it has a silicon atom and four hydrogen atoms and like methane it will burn in the air but the activation energy is so low that this will ignite as soon as it's formed so we'll bring a light stand for this allowed the magnesium silicide to the hydrochloric acid and as soon as the silane gas is formed it will ignite in the air [Applause] okay so there's a system with a very low activation energy what I'm going to show you now is an explosive that has a very low activation energy not so low that it goes off spontaneously but almost this is an explosive called nitrogen triiodide and the molecules consist of nitrogen bonded to iodine but the bonds are very easily disturbed and when that happens an explosion takes place now this will be quite a loud explosion so again I'm going to wear your defenders and you may wish to cover your ears the the nitrogen triiodide is this black solid sitting here on this filter paper all I'm going to do is to give it a gentle tap with this stick and that should be sufficient to set it off so here we go [Applause] now I'm sure being all being observant scientist you'll have noticed that cloud of purple vapor that's produced that's AIA D so the molecule consists of nitrogen combined with iodine it breaks apart to release the iodine and nitrogen now nitrogen plays a very important role in explosives and I'll explain why that is with the help of this model this is a model of a molecule of nitrogen and it consists of two atoms of nitrogen joined together by this very strong triple bond now this is one of the strongest chemical bonds in the whole of chemistry it means that nitrogen is a very unreactive material so nitrogen as you know is about four-fifths of the air it's very hard to get the air the United in the air to react with anything that's because the bond is very strong so to get the nation to react we would have to pull the atoms apart and break this very strong bond that would require a great deal of energy now energy of course is conserved so if we took two atoms of nitrogen and brought them together so that this bond is formed that same amount of energy must be released so in nitrogen gas is formed a lot of energy is released and that was what was happening with that nitrogen triiodide a lot of that energy was coming from the formation of this nitrogen nitrogen bond so and I should have plays an important role in explosives there's a very similar explosive a little bit less sensitive than nitrogen triiodide which plays a very important role because it saves many lives every year it's used in car airbags now a car airbag as you know is a safety device where a bag is inflated moments before well as a collision is occurring your car collides with something the airbag is set off and the airbag will protect you hopefully from injury now I imagine that most of you have never actually seen I hope most of you have never seen an airbag going off the real so what we'll do today is actually set one off for you so we've got not a real car but nevertheless a real steering wheel with a genuine airbag in side and we have a back tree at the front of the car is a little switch in a moment I'm going to drive thee or push the car into the wall it will close the switch and set off the airbag this is actually quite loud obviously if you're involved in the collision the last thing you're worried about is a is a loud bang but if I can find my ear defenders I'm gonna pop these on and then we'll have a look at a car airbag so here we go [Applause] okay so that's a car airbag that contains an explosive called sodium azide it's a compound of sodium and nitrogen when it decomposes or when it explodes it produces sodium metal and nitrogen gas and the formation of those nitrogen bonds that produces a lot of the energy of that explosion now people searching for new explosives realize that ashen binds discovery of nitrocellulose or guncotton was an extremely important one because he had worked out how to introduce oxygen and nitrogen directly into molecules in order to turn it into explosives and his first discovery was was nitrocellulose or guncotton the people took the same idea and began to search for other explosives in which nitric and sulfuric acids were used to introduce the oxygen and the nitrogen directly into the molecule so people searched lots of different possible compounds that the real discovery was made by an italian chemist Ascanio Sobrero in 1847 when he tried nitrating that is treating with nitric acid not cotton but another substance called glycerin no glycerine is products of byproducts of the manufacture of soap and if we nitrate glycerine we get a substance called nitro glycerine so this is some nitro glycerine this is a small quantity which I made this morning here in the chemistry department and you can see it's a clear liquid it looks a bit like water slightly viscous you'll see in a moment at the Haynes rather differently from water so I'm going to do first of all is to place a few drops of the nitro glycerine onto a heatproof mat and we'll set fire to it and we'll see what happens the rest of it away and let's see what happens if we just heat this with a blowtorch and try and set fire to it so hope you saw there it just burns it burns with a lovely green flame there's no explosion just a very small green flame it behaved a bit like the nitrocellulose it burned very rapidly the nitroglycerine could do something very different and I can illustrate that in the following way I'm gonna take a filter paper and place it on this steel anvil I'm now going to get the same quantity of nitroglycerine and place them on the filter paper and this time I'm going to hit the nitroglycerine with a hammer and the effect should be rather different from before so this is likely to be very loud so I'm going to put ear defenders on I'm also going to put on a face shield and I'm going to wear it love this is definitely one you shouldn't try at home okay so this is just five drops of nitroglycerine so what happened there was very different in the first case it just burned but here's something very different happened we call this detonation and definition is a very specific physical effect and we'll have a look at that again in a moment now nitroglycerine has quite a reputation for being a dangerous material and that's because in the early days there are a lot of accidents when it went off by mistake when it's being made or when it was being transported and although nitroglycerine is an extremely powerful explosive it wasn't a very practical explosive it was just too dangerous to use and this was very disappointing and so people tried to find a solution and the person who really solved the problem was a Swedish chemist called Alfred Nobel and he tried combining the nitroglycerine with other materials in order to make it less sensitive and one day he hit on this material so this is a clay-like material it's called peasel ger and he's a very very absorbent so it can absorb many times its own weight of nitroglycerin and when it does so it remains a very powerful explosive but it's much less sensitive and you call this material dynamite and Dynamite's usually made-up into sticks and the sticks can be pushed into ball holes in rock if we're doing querying or mining or tunneling it's very safe explosive because it's much less sensitive than pure nitroglycerin and so it's safe to store and transport so the Bell had solved one of the great problems with nitroglycerin he'd worked out how to prevent it going off accidentally but there's another problem with nitroglycerin is that it doesn't always go off when you want it to so kind of unreliable explosive and when it's made into dynamite because it's very insensitive it's very hard to get it to go off setting dynamite off is actually very difficult so that led from the Bell other great invention this is called a detonator and this is a modern electric detonator it just consists of a small metal tube and inside is a very small quantity of an explosive that's quite sensitive call it a primary explosive so the exclusive in here is quite easy to set off this one is actually a modern detonator which was set off electrically and the bells original detonator would have been set off by a slow-burning fuse now this was a brilliant invention because the detonator although it contains a sensitive explosive it only contains a tiny quantity of that explosive solo detonators are quite dangerous things there's a limit to how much damage a single detonator can do so the detonator is kept well apart from the the main explosive the dynamite right up until the moment that we want to use the explosive and then the detonator is placed in contact with the explosive the effects of the detonator when it goes off is to create a small explosion that provides a very sharp shot it's rather like the hammer blow and that hammer blow is enough to set off the main explosive so the dynamite is safe because it's very insensitive the detonator is relatively safe because it doesn't contain very much explosive and those two inventions of no bail really opened up the world of high explosives and as a result he became extremely wealthy in fact he became one of the richest people in the world at the time and when he died in his will he gave his vast fortune to the creation of the Nobel Prizes which are today the most prestigious prizes in science and there's even one of the elements named after him it's called an abelian now dynamite was sort of the beginning really there were lots of other explosives invented I'll mention one of them if you take a solvent called toluene and treat that with nitric and sulfuric acids you get tri nitro toluene and the abbreviation for that is TNT again an explosive that many people have heard of so don't confuse TNT with dynamite they're quite different dynamite consists of nitroglycerine absorbed into diesel girl or some other material whereas TNT is a completely different chemical compound and people continue to research new kinds of explosives this is a very new one this is the latest British explosive this has been available for a couple of months now this is a plastic explosive is called pe7 and a plastic explosive and explosive hits her squidgy and moldable so this is like sort of plasticine or playdough this is a very powerful explosive but I should just say that none of these explosives are live this is actually a training block of material it's identical to real p7 in every respect except that the explosive component has been replaced by an inert material there's also been colored blue so that you can tell that it's inert the real explosive is white so when we looked at the combustion of gunpowder we saw that gunpowder in the open just Birds it burns very rapidly this is a burning process there's no explosion when we hit the nitroglycerine with a hammer we get something very different we get this loud explosion we call that a detonation the the burning of gunpowder and other explosives of that kind we give her a fancy word we call it deflagration just means burning so low explosives burn whereas nitroglycerin is something very different we call it a high explosive and high explosives detonate I want to explain the difference between a low explosive and a high explosion explain what's meant by detonation so first of all going to demonstrate this idea of deflagration or burning so this is a metal track and it contains a powder the powder is actually finely powdered nitrocellulose it's the kind of powder that's used to reload the the bullets for guns and what i'm going to do is to set fire to this trail of powder at one end and you can see what's happening the trail of powder is burning along what's going on here is that a piece of the a pile of the parody is burning and releasing energy in the form of heat that heat energy is raising the temperature of the next piece of powder providing the activation energy so that it can burn it then releases heat and in turn ignites the piece of powder to the next the next door to it so what's happening then is the flame is progressing by the transfer of heat so that's what we call deflagration that's a low explosive detonation is something completely different and to explain this I'm going to need about six volunteers something like that I'd like to come on out any guys in the front sure we have yes gone you guys come out how about from you to there eight of you come on then there's not a handful of volunteers if you'd like to form a row oh gosh we could do to come to this end maybe if you come to this if you come and stand just here just really it's a roughly height order okay that'll do right what I want you to do is to face this wall what's your name Tom all right you just stand upright like that now what's your name Bridget okay if you just stand back a little bit face Tom's back put your hands on his shoulder shoulders have gripped them very tightly if you stand back a little bit have your arms very rigid not too rigid that's good so if I move you he'll move okay so keep your arms locked and rigid okay you do the same thing what's your name Harry okay put your arms on her shoulder stand back a bit keep your arms nice and lot that you've moved together okay same thing what's your name EQ okay you are Joe mark mark okay put your hand on his shoulders arms nice and rigid so you lock together that's good right okay can you each put your arms on your shoulders what's your name Kim and you are a beat all right arms on her shoulders good now what I'm gonna try to do keep your arms nice and rigid and locked I'm gonna give you a shove and we'll see what happens okay I'm gonna do that one more time so I give you a push good the push sort of works its way down now what I want to imagine if each of our volunteers is like a molecule in a block of explosive and their arms are like the bonds between the molecules if one of the molecules gets a push or knock with a hammer then the wave travels through the block now if there's a block of regular material that would travel at a particular speed we call that the speed of sound this compression wave traveling through a material we call a sound wave it's how you're hearing my voice right now through compression waves traveling through the air and those compression waves those sound waves can travel through solid materials as well okay let's have a big hand for volunteers I want you to imagine that this little dot is a source of sound waves so those circles spreading out of sound waves the light circles are regions of high pressure and in between we have regions of low pressure so that sound wave spreading out from an object now let's imagine that the object starts to move well you could see what happens the sound waves ahead of the object pile up and get closer together and the ones behind it are sort of trailed out and they get further apart now we call this the Doppler effect it's something that's very familiar if you hear a car go past it makes a a particular noise it goes yeah okay the note the pitch drops as it goes past you and you can see why ahead of the object the sound waves are bunched up the wavelength is shorter the frequency or the pitch is higher and likewise at the back the waves are spread out so the frequency or the pitch is lower now what would happen if the object moved at the speed of sound well here's the object now moving at the same speed of sound waves and now the sound waves all pile up at the front of the object they will catch up with each other and so you get this region a very intense pressure and let's just make it move a little bit faster than sound and you can see there's these these sort of bound wave we call it a shock wave that's where the sound waves are all catching up with each other so that shock wave is is something that would be produced for example by an aircraft when it's traveling faster than the speed of sound so a supersonic aircraft would create the shock wave we can hear the effect of that shock wave on the ground in the form of a sonic boom so a shock wave is the mechanism by which a high explosive detonates so the high explosive the the it's not heat being propagated from one region to another like in our trail of powder it's not just heat energy raising the temperature of the next piece of explosive it's rather the passage of a very fast shockwave traveling much much faster than the speed of sound okay thank you for the computer screen so how does a shockwave set off the explosive well I could demonstrate that with with this apparatus this is a plastic a clear plastic tube and it has a piston a plunger which is sealed and I'm going to take some cotton wool this is just ordinary cotton wool it's not gun cotton or anything just ordinary cotton wool take a little piece of this and place it in the end of the tube and then I'm going to seal the tube so what I'm going to do now is to compress the air in this too but I'm going to do so very quickly now if you've ever pumped up your bike tires quickly you know if you're in a hurry and you pumped up your tires really quickly you'd notice that the tires and maybe the pump as well get hot so we compress a gas it gets hot so I'm going to compress this gas very rapidly and if we bring the lights down we'll see what what effect this has so if there's just a little bit of cotton wool right there in the in the end of the tube so here we go so they've got so hot [Applause] that became so hot at the cotton will actually ignited and there's nothing left in there now Justi because I can empty to you when a shock wave passes through a high explosive that shock wave is a region of very intense pressure and that pressure heats up the explosive and causes it to detonate and that detonation releases energy which is what drives the shock wave onwards and that shock wave travels much faster than the speed of sound now what I like to do now is to try to demonstrate for you a detonation shock wave here in the lecture theater and that's quite difficult because if I couldn't if I just set off a block of plastic explosive that would be the end of the lecture I can't do that obviously so I'm going to find a way of demonstrating a detonation and to do that I'm going to use some of this this is called shock tubing it's actually a commercial product it's used on a enormous scale by the explosives industry and it consists of a plastic tube a hollow plastic tube and the inside of the tube is coated with a very fine powder of a high explosive cord hmx and this explosive is about 70 percent more powerful than TNT but there's just a very light dusting of it so I want to do now if we can bring the but the camera on screen is to just step back about what I'm going to do is to blow through the tube and watch to see if you can see some of this H and X peridot coming out of the tube okay so it's just held on by static electricity so I can blow us out of the tube now if I just took a piece of shot tubing and set fire to it it'd be rather like the nitroglycerin that would just burn but if instead we can provide a very sharp shock to the tube we can initiate a detonation that will pass down the tube at very high speed so let's have a look at that and to illustrate shock to you can show you how it works I've got a little demonstration in which I've used shock tubing to spell out the the name of the explosive that it contains so this says hmx and hopefully this will help you to remember the name of the explosive so I'm going to initiate this shop tubing we're obviously with a shock and that's going to be provided by this device which produces a voltage of two and a half pounds and volts and then discharges it to make a little lightning strike if you like so I'm just going to point this at you right safe and fiery so you should just be able to see a little spark okay over here I'll try over there so it's not producing very much energy but it's producing that energy in a very short interval of time in the form of a shock and that can initiate a detonation wave traveling down the shop tube so these letters consist of just a single length of shock tube wound in and out of the board and so will fire the shock tube and hopefully you'll be able to see the detonation of the of the tubing now this this will be lose quite a bright flash so if we bring the lights down you should be able to see the shock tubing firing quite clearly it makes a little bit of a noise you may or may not wish to cover your ears okay so here we go this is shock tubing here we go okay so that was a detonation wave passing through a few meters of shop cheering but it's extremely quick like that that you could see the actual detonation wave pass along the tubing so what I'm going to do now is to set off a longer section of shock tubing and see if we can actually see the detonation wave passing on the tube and for this I'm going to need four volunteers who'd like to come and hold the shop tubing while we set it off some of the hands are going down I noticed have you on the end there and he'll surely have will have you and yes you can come on down and let's have you as well it's the hand for all volunteers please okay you stay there you stay there if you come with me you come with me to this end you maybe there we go if you'd like to come and stand about here now we're going to give you some eye protection and some hearing protection or give you a glove to hold as well and this is the shop tubing so if you if you come with me stand about there what I want you to do what's your name Oh Albert right Albert I want you to hold that end of the tube you just hold that for a moment and we'll wind this out across the full width of the lecture theatre okay hold that just hold it in your gloved hand what's your name Adam okay now if you'd like to come with me a little bit you like to come with me hopefully there's enough to go all the way across if you stand about there hold that and if you come with me what's your name Bradley and you're gonna be down this end look pretty good you hold up there what's your name Adam yeah okay what I want you to do hold it near at the end now it's very important I want you to point that away from your your face okay because there's a little spit of flame come out at the end hold it up nice and tight we're going to try and stretch this out so if you step forward a little bit hold it up nice and high so everybody can see that's good if you just step forward a little bit that's good and excellent good okay so what we're going to do now is to start a shock wave a detonation wave at this end of the tubing we'll see if we can see it passing all the way down to the other end of the tube so again we'll bring the lights down for this and here we go three two one [Applause] okay just stay we are for a moment you just stay where you are if you'd like to head back to your seat so the two volunteers at the end can go back to their seats let's just thank the once again okay so you know you may or may not think you saw that shockwave travelling down the tube but I can tell you now that it was travelling at 2100 meters per second that's nearly seven times the speed of sound she would be quite lucky if you did actually see it but I really wanted to show you a detonation wave what did you see a detonation wave passing at seven times the speed of sound so what we've done and I think this may be the first time this has ever been done is we've wound nearly a kilometer because I mean just over 800 meters of shock tubing wound around the walls of the lecture theater so we're going to bring the two ends out now for the middle of the lecture theater thank you Chris all right you come with me you're going to hold this end okay that's it you stand there and you come with me we may overlap you a little bit that's fine keep going you've got you've got a few knots in it shouldn't matter okay so I think this is a first we tried a smaller scale demonstration earlier in the week it was it was very interesting so this is the first time we've tried it with such an enormous length of this shop tubing so this should take about point four of a second just under half a second for the shock wave to travel along this tube eight times around the lecture theatre and then back along this to you if it all works I hope you're happy so you are completely surrounded by an explosive that's seventy percent more powerful than TNT okay so here we go then we'll bring the lights down just hold up nice and high and we're firing in three two one [Applause] so when an explosive detonates of course it produces gas so we've now got 800 meters of tubing full of high-pressure gas and if I'd bring that you hear that gas escaping extraordinary let's have a big hand for two volunteers thank you okay so that pretty much brings us to to the end of the lecture obviously it's a lecture about explosions so of course we have to end with a big explosion before we do though I just want to say a big thank you first of all to the Department of Chemistry for hosting this I'd like to thank our sponsors of course Microsoft Research event horizon and Lemaitre for their very generous support and also like to ask you to join me in thanking somebody who's put a lot of effort into helping me prepare and deliver this lecture and that's Chris Braxton okay so we need a good explosion to finish and we're going to create another physical explosion if you remember at the beginning of the lecture we had that little tube of glass water which we heated up the pressure built up and it burst there was a physical explosion so we're gonna do something similar we're going to scale it up a little bit and instead of using water this time you're going to use liquid nitrogen so this is liquid nitrogen listen this is at minus 196 degrees centigrade so nitrogen remember that's about four-fifths of the air if we cool it down to minus 196 degrees centigrade extremely cold it becomes a liquid so we have some liquid nitrogen here and if we were to add a liquid nitrogen to warm water it would raise the temperature of the liquid nitrogen it would evaporate and turn into a gas and in the process it would expand rather like the water boiling and turning to steam so first of all demonstrate that by adding some liquid nitrogen into some hot water [Applause] okay so let me add the liquid nitrogen to the hot water the liquid nitrogen turns from a liquid into a gas and it expands and that's fine because it can expand and escape into the room but what would happen if we did the same thing that we used a closed container a sealed container well the container that we're going to use is this it's a 1/2 liter fizzy drinks bottle so gonna put some liquid nitrogen into here screw the lid on so it becomes a sealed container and of course when you're taught how to use liquid nitrogen you're told under no circumstances keep your liquid nitrogen in a sealed container you'll find out why in a moment and then going to drop it into here we have a bucket inside the bucket contains some more water so the nitrogen will evaporate and turn into a gas would like to expand but it'll be trapped inside the bottle the pressure will build up and and you can guess the rest so let's get some liquid nitrogen I'm going to fill this about 1/3 full of liquid nitrogen there we go so this is the final demonstration of the lecture I hope you've enjoyed yourselves and we hope you enjoy the final demo here we go [Applause]

Channel: The Royal Institution

Views: 857,814

Rating: 4.8978677 out of 5

Keywords: Chemistry, Explosion, Microsoft, Cambridge, Science, Boom, Fireworks, Element, Dynamite, Rocket, Chris Bishop

Id: uFQdcKJUijQ

Length: 60min 8sec (3608 seconds)

Published: Thu Nov 01 2012

Reddit Comments

[u/TuahaSultan]

Nice