Wednesday, January 11, 2006
for the geeks
Note : This is quite a geeky entry, and quite informative I hope. I have tried to keep my explainations and scientific terms as basic as possible with the hope that it can be understood by everybody - even those without a science background. However, if you still do not understand some stuffs and you are really interested, drop me a comment or an email and I will try to humour you to my best efforts. Labels: reviews
Some time back, I managed to land my hands on a fair bit of dry ice. If you had been paying attention in Science Class in school, you would know that dry ice is actually frozen carbon dioxide which is about -78.5 degree celcius. There are loads of uses for dry ice and if you are free to indulge in some reading material, click here.
Of course, since I had no practical purpose with it, I started by doing what I used to do as a kid - dumping a fair bit of it in the toilet bowl and watch it 'smoke'. The 'smoke' is actually water vapour mixed with carbon dioxide which is released as the dry ice 'melts' from solid to gas.
After a while though, that's not much fun. You do get to hear the gurgling sound of air bubbles being discharged through the water, but that's about it. You could of course pee in it and watch your piss dissapear through the 'smoke' but all of that is pretty pointless.
And so, I proceeded along with an age-old experiment I came up with when I was a kid. I've actualy blogged about it before. It basically involved filling a water bottle with some water, dumping in the dry ice, capping it shut, and watching it explode. Now, bearing in mind that I did that as a kid and had not much inclination towards safety, the approach this time around was way different. Besides, this time around, I knew the theories behind the explosion of a so-called 'pressure vessel'.
When dealing with pressure vessels, or in this case, the water bottle, the two main stresses in the vessel are the hoop stress and longitudinal stress. The magnitude of these stresses and the build of the vessel determine whether or not the vessel fails under the intense pressure.
*pictures sourced from http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/pressure_vessel.cfm. Click here for more reading info.
So basically, longitudinal stress is the stress acting in the longitudinal direction of a cylindrical vessel. In the case of the water bottle, this is the stress acting to pop open the cap and the bottom of the bottle. Hoop stress, however is the stress acting against the walls of the cylinder trying to split open the cylinder. In a cylindrical vessel, the hoop stress is twice that of the longitudinal stress. This is explains why, when grilling a sausage, the sausage splits along the length of the sausage. This is because the larger hoop stress due to the steam from inside the sausage breaks up the skin, and hence splitting it along its length.
And now, to the experiment itself. Please note that playing with pressure is EXTREMELY DANGEROUS and should only be done by someone who understands and respects the danger involved. Yes, I did it when I was a kid, and looking back, it probably wasn't the smartest thing I did. Proper safety precautions have to be taken. While researching some info for this article, I bumped into this piece of news about kids playing with dry ice, and oblivious to its effects, blew up a plastic bottle which effectively left a mother blind in one eye.
I took A LOT of precautions here, and yet I cannot stress how dangerous it is unless you have access to a proper lab with a safety barrier which you can stand behind. I wore safety glasses and worked behind a shatterproof Plexiglass sheet. I worked in the middle of the driveway at home, with NO spectators. Using a glass bottle or a metal container is a strict NO-NO, not something I would do even if I had a proper lab to work with. I even chose a small and light 300ml plastic bottle - which is considerably less dangerous I feel - than the average Coke bottle.
I filled up the bottle somewhere about 2/3 full. There is a logic behind this number. We know that water is incompressible. This means that the amount of free space you leave would directly influence your explosion. Imagine a balloon filled up with water until it bursts. When it bursts, it just burst. No loud bang, no bits of rubber flying everywhere, nothing. But if you blow a balloon till it bursts, you get a loud bang and if you are unlucky a random piece of balloon will smack you in the face or lips.
Therefore, the more water you put in the bottle will give you a smaller explosion, which in hindsight MIGHT be safer. However, that would also mean that there is less space in which the air can compress and hence would explode faster (maybe even in your hands after you cap the bottle). Using less water would mean that more air would compress, which produces a bigger bang (dangerous) but would give you more time to get away from the bottle. The danger here, especially if you use a large bottle is that you leave too much space for air and you underestimate the amount of dry ice required. This means that it is possible for all the dry ice to have turned into gas, but the pressure isn't enough to explode the bottle. In this case, you would have a great trouble disposing the bottle. Even approaching the bottle which is under high pressure would be dangerous the say the least. What would you do then? To be honest, I don't even know. That is why, I preferred to make certain of a successful and safer experiment albeit with a smaller bang.
The effects were nothing short of astounding. The results didn't match that of the experiment I did as a kid, but that was one done with a 1.5L bottle and a huge air space. I have positioned the bottle in a horizontal position (which thinking about it, wasn't very clever). The bottle flew like a rocket about 10 metres and hit the brick fence seperating my house and my neighbour's.
The bottom part tore apart and flew in the opposite direction of the bottle (ie. bottle flew 10metres to the right but the bottom tore apart and flew 1 metre to the left).
I tried to piece them together, but as you can see, a large sizeble chunk is missing. I searched the whole compound but couldn't find any distinct blue plastic bits.
Perhaps not surprisingly, the whole chunk came off quite cleanly, that is to say, an almost circular section. This is because the pressure is uniform inside the bottle and hence it would tear apart the bottle 'uniformly'.
The damage done to the cap was nothing short of devastating. The cap is probably the hardest part of the whole bottle and yet it got busted that easily. Unfortunately, as you can see from the picture on the right, in addition to the 'cracks', the cap sustained a 'bruising' which was probably a result of it's impact against the wall. Therefore, it is impossible to tell whether the cap broke due to the pressure from within the bottle, or from impact. Nevertheless, it still gives you an indication of the magnitude of the forces we are dealing with. If you were to take your average Coke bottle and banged it against the wall with a quick swing of your arm, I still doubt that you would be able to crack the cap as such.
Ok, if you had been paying attention in the earlier lesson about hoop stress and longitudinal stress and the sausage, you would be wondering - why did the the cap and the base crack? If the theory holds, shouldn't the bottle split along its vertical? Truth is, I don't know. Theoretically, it SHOULD.
However, there are a couple of explainations I can think off. First was the position in which I left the bottle. I left it lying down on the floor on it's side. This then applies a force (which is equivalent of it's weight) on it's side which might have slightly countered the pressure acting from the inside.
The more probable explaination is the way in which it is manufactured. The theory accounts for a cylindrical vessel. The bottle, however, is not a cylinder per-se. It has grooves running around its circumference. Unless we do an in-depth analysis (only possible with the help of certain software), there is no way we can figure out what effect the grooves have on the pressure. Also, since it is common knowledge that the hoop stress is twice that of the longitudinal stress, and the cylinder will fail along the vertical, designers might have countered that problem by making the bottle thicker around the vertical section.
There are loads of ways to improve this experiment - like hanging it from a string to prevent it's deadweight from having any effect on it. There are many variables as well - the amount of dry ice used, the original temperature of the water (might affect brittleness of the plastic) and the volume of water vs air. One day, and I am serious here, one day, my house shall have it's own lab in which I can work in. This would be the first real experiment which I would properly conduct. Till then, one last message...
KIDS, DON'T FUCKING TRY THIS AT HOME!
no injuries, thank God, and everyone was pleased...ok thrilled by the explosion. but really, on hindsight that was a bloody stupid thing to do in public.
maybe try the experiment again with one of those chinese new year cookie jars which are more cylindrical than coke bottles.
Was bored so started reading SOME of your previous entries, like the ones of the straight A students, Miss Malaysia, etc.
I think that sometimes even all these 'grand' unis that supposedly judge you by other things look more at your results than anything else lar. One of my friends doesn't even SPEAK properly, he just SLURS half the time and I don't understand him at all and he's on the waiting list to study Medicine in Cambridge! Not that I'm saying he doesn't deserve it or anything.
And EST is a stupid subject probably invented to help students get more As so that maybe one day Malaysia will be in the Guiness Book of World Records for having students with the most As at a particular level, and they're STILL thinking of introducing MORE useless subjects, though I'm not sure if those subjects are going to be SPM subjects but...ugh.
Anyway, your blog is REALLY a bitching blog more than anything else and sometimes you're a bit too arrogant, but still, very funny. =p
lishun : In actual fact, those jars, while cylindrical, are not made to withstand any sort of pressure. As such it might just blow up in an instant (and in my hands). I still think bottles made for carbonated drinks are the way to go, never mind whether they are cylindrical or not. Maybe next time with some load cells (for measuring pressure), I can get more quatitative results.
bex : Go look at the September archives for something called 'The Great Review'. Hehehe...you might even find that educational, just like the other 100000 women who read it.
That would definitely be something you don't want to try at home. Not your own home anyway.
Anyway RYC: I thought Puteri Gunung Ledang was *okay* also, but I would rather learn to knit than watch it again. And M. Nasir was badass but when he got down in the mud, I was far from turned on. So it defeats the purpose (of the mud), dontcha think?