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Posts Tagged experiment

The bubble raft and lattice defects Marcus Wilson May 16

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 After doing the washing up a few days ago, I returned to the sink to find a raft of bubbles had formed on the surface of the water. All the bubbles were roughly equal size, and they had aligned themselves into a close-packed lattice, as the photo shows. (Sorry about the quality of the photo – the bubbles were small and the light was poor – and I didn’t have a tripod – but I did my best. )

bubble_raft.jpg

 

Look closely, however, and you’ll see some defects on this lattice. If you follow a row of bubbles, you’ll see in places that a row ends abruptly or another one is inserted. This is a ‘dislocation’. Also, you’ll see single point defects (a bubble missing) and grain boundaries – where parts of the raft with different directions of the bubble rows meet.

All these defects are found in real materials – though of course in a real crystal there is a third dimension which complicates things a bit more. For example, there is a ‘screw dislocation‘ that can occur in a 3d lattice which has no analogue in 2d. 

Making a bubble raft is a good way of teaching about crystal structure and crystal defects – the tricky bit is getting all the bubbles about the same size. I’m not sure how it happened in this case, but it was worth grabbing the camera and having a closer look. 

 

 

 

The sleep machine Marcus Wilson May 06

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I came across this paper while doing a bit of reading about the applications of transcranial magnetic stimulation (TMS). A TMS machine applies pulses of magnetic field to the brain. The rapidly-varying magnetic field induces an electric field (Faraday’s law) and this in turn influences neural activity (but just how and where is an open question).  A team of leading sleep researchers (Marcello Massimin, Guilio Tononi and Reto Huber)  has probed the state of the brain using TMS for various states of consciousness. What they found was very interesting. When in deep sleep, a pulse from the TMS machine generates a single slow wave of activity, which has the same form as the naturally occuring slow waves that are a hallmark of deep sleep.

Now, these slow waves are important - they have been linked with memory consolidation. The more slow waves you have when sleeping, the better you are committing things into memory. And artificially-generated slow waves do the job too. 

In other words, the TMS can be viewed as generating artificially enhanced sleep and therefore artificially improved memory. 

But wait, there’s more (as they say). When someone is awake, a TMS pulse doesn’t do anything. But for someone teetering on the edge of sleep, but still not quite in it, a pulse from the machine can be enough to send them over the edge. So here we have a way of pushing someone into sleep more quickly than they’d otherwise get there. A cure for insomnia? Perhaps not – I mean, TMS machines are hardly unobtrusive, but interesting nonetheless. The remaining question is, do we get one for us or the baby?

 

The amazing vacuum microwave Marcus Wilson Apr 03

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 Happy Easter everyone. Sorry for lack of blog activity – lots of marking has been building up that I’ve needed to get through. 

Yesterday we experienced the vacuum-packing ability of a clip-container in a microwave. In this case, it was being used to cook some vegetables for Benjamin’s dinner. The veges were placed in the microwave, the lid put on, and then zapped for a few seconds. The problem was then taking the lid off, since it had sealed tightly shut. 

I’ve had a comment on my blog about this before, from someone who’s experienced it. I think what’s happening is that, as the contents heat up the air inside expands. It is able to push it’s way out through the seal. The mass of air on the inside is then rather less than what it was to start with. Once the heating has stopped, however, the temperature reduces and the air contracts. However, this time the seal doesn’t let air back in – instead the lid is sealed and the air inside reduces pressure. Consequently we are left with lower pressure on the inside than the outside.

Just how big a pressure difference do we have? Suppose the air inside is heated to 100 C, as opposed to the 20 C that it is on the outside. At constant pressure, volume scales as absolute temperature, so we have a volume increase of about (100 + 273) / (20 + 273) =  1.27 times. That is, about 30% of the air is pushed out in the heating process. This air doesn’t get back in during the cooling. Therefore, once cool, the container has 30% less pressure inside (pressure being proportional to volume at constant temperature).

What does this equate to in everyday terms? Air pressure is about 100 kPa, meaning a force of 100 thousand newtons over an area of 1 metre squared. 30% of this would be 30 000 newtons over a metre squared. Since a kilogram weighs about 10 Newtons, that’s about the equivalent of 3000 kg spread over a metre squared. 

Now, the little container wasn’t a metre squared in area. It’s about 10 cm times 6 cm (approximately) , which is 60 cm2 or 0.006 or a metre squared. Multiply that by 3000 kg per metre squared, gives us 18 kilograms. That is to say, the force due to the air pressure is equivalent to sticking about 18 kg of mass on top. Little wonder it was tough opening. 

This calculation has a few assumptions in it, not least that the air had cooled back to room temperature (it hadn’t). The reality I think is that it would be rather less force. I managed in the end to get a flat knife under the seal and let some air in – that got the lid off. 

Structural failure: Jam yesterday Marcus Wilson Jan 23

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We’ve had a bumper crop of plums from our two plum trees. Way more than we can eat our way through in the short plum season. It appears that we aren’t the only ones – the last couple of weeks have seen bucket loads of free plums turn up in the tea-room here. (And yet they are several dollars a kilo at the supermarket.) So, yesterday, Karen had a go making plum sauce, to add to the plum jam and frozen plums we already possess. As far as I can work out, the difference between plum sauce and plum jam is merely that plum sauce is more saucey. The mechanism appears to be the same – everything gets cooked up in a large pot and the hot sauce gets added to sterilized preserving jars. The lids go on and should seal shut as the temperature inside the jar drops and the small amount of air inside loses pressure.

A lid that’s popped downwards is a good sign that there is a decent seal between the lid and the jar. If there weren’t, then air could get in and equalize the pressure between inside and outside, and up would pop the lid. Conversely, if there were, for some reason, some multiplying nasties in the contents, producing carbon-dioxide, the pressure would build up inside and the lid will buldge upwards. That’s a sure sign that what’s inside isn’t edible. The trick is to spot it before it explodes at the back of the pantry and splatters jam and glass everywhere.

That’s why there’s a warning on shop-bought jam – if the ‘button’ isn’t down, don’t eat the contents.

However, all this only works if the jar is up to scratch. I came home from work yesterday to discover that parts of the kitchen had been painted in plum sauce. This wasn’t the work of the baby – it was down to a jar that had failed. When the sauce went in, the lid went on, and, sure enough, the lid popped downwards. But it wasn’t the only thing that ‘popped.’  The jar did as well, leaving it with no bottom.

There are a couple of reasons I can think of why this might have happened. First, it could simply be down to rapid thermal expansion of the glass. When the hot sauce goes in, the inside of the glass jar gets hot much more quickly than the outside, and so there is stress built up as a result of different amounts of thermal expansion on the inside and outside. This is what happens pouring boiling water into a cold glass.

Or, perhaps the bottom of the jar acted as a popping lid. If there were some air trapped at the bottom, it would reduce its pressure as it cooled, and create a force on the bottom of the jar. If that force were great enough, it could break the glass. An implosion, rather than an explosion.

Now, neither should have happened because the jar concerned was a proper preserving jar, designed for the purpose of having hot stuff poured into it and being sealed. But, for some reason, it did. I was tempted to bring in the jar to work today and get one of our materials engineers to examine it to determine the mechanism of failure. It would have been interesting, but I thought they had better things to do with their time, so we may never know exactly what happened here. But the good news is that there are still several intact jars of sauce, so we will be well supplied for the coming months.

 

 

 

Happy Christmas Marcus Wilson Dec 17

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Blogging has ground to a halt in recent days, as I try to get other things done, such as research proposals, reviewing a PhD thesis, supervising a summer student, and attending numerous parties. I’m off on holidays very soon, but will be back early January.

I’m currently grappling with an ever increasing parameter space in one of our computer models. These parameters are for things associated with various neural processes – such as how quickly does release of a particular neurotransmitter occur, and under what conditions? There are a lot of things we need to know, but in some cases they haven’t been measured very well.

The solution is easy. Consult some friendly experimental neuroscientists. Which we did last week. But that didn’t help, because they didn’t know either. In fact, it made it worse, because they drew our attention to another raft of things that we should be considering, but didn’t have much data on either. Indeed, they looked to us to help reduce their parameter space through our modelling.

I get the impression that there is very little known at the cross-over between physics and neurophysiology. A good reason to study it, then.

Undiscovery in physics Marcus Wilson Nov 28

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With the recent undiscovery of Sandy Island I’ve begun wondering what other things might be ripe for undiscovery. Wasps, for example. Wouldn’t it be great if we realized that there wasn’t actually any evidence for the existence of wasps after all. Their discovery had been just a mistake made by an entomologist back in the depths of history. We can all tell our children not to worry about them – they don’t exist. Our chickens would love to see the neighbour’s cat undiscovered (as would we – at least from our garden).  I’m sure a variety of places might feature strongly too. Hamilton is bound to be on the list of some people; but, I can assure you, the last time I looked it was still there.

I don’t think in physics there has been a great deal of undiscovery in the last few centuries. I struggle to think of any real undiscoveries.  Sure, there have been changes to our thinking. For example, relativity superseded Newtonian physics, but it would be wrong to say that Einstein undiscovered Newton’s Laws of motion. The latter are still a cornerstone of physics – but their applicability has been reduced to the realm where things aren’t travelling close to the speed of light. That would be more like discovering the coastline of Sandy Island is a bit different to what the maps have it. One might say that the Michelson-Morley experiment undiscovered the aether, but in reality the aether had never been discovered – it was just a well-accepted hypothesis. Likewise Joule’s experiments with heat put pay to the idea that heat was a fluid, but since no-one had claimed (supported by real evidence)  to have observed this fluid, it wasn’t really an undiscovery either.

Underlying modern science (by which I mean Galileo and beyond) is experimental evidence. No change in understanding of science, in any discipline, is going to happen without well collected and well analyzed data. This makes undiscovery of something (by which I mean overturning of some knowledge, theory or principle that has been believed based upon evidence, as opposed to mere hypothesis) unlikely. There have been a few instances of reports of new things that have been made prematurely, with unreliable evidence, such as cold fusion and faster-than-light neutrinos, and these have been embarrassing for the groups concerned and undiscovered very rapidly.  But undiscovery here has happened because they were never properly discovered in the first place.

That said, neither was Sandy Island properly discovered. My spell-checker’s underlining of the word undiscovery may be for good reason.

I’d love to hear readers’ thoughts on this one. Is there any piece of modern science that has been genuinely undiscovered?

 

 

 

 

Validity in experiment Marcus Wilson Nov 20

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Last Friday I was at the Waikato Science Teachers’ one day conference in Cambridge. There was a wide range of different material talked about, which made for an interesting day. One of the questions which was tackled (led by Simon Taylor)  was ‘When is an experiment valid?’ Or, what is ‘validity’ all about when it comes to science. Some thoughts from the audience were ‘When it does what it is supposed to’, ‘When it agrees with theory’, ‘When it is controlled’, ‘When it is repeatable’, ‘When it measures what it is supposed to’.

All of these I think are reasonable responses, depending upon the situation. f you are illustrating a physical principle to a class then you certainly want your experiment to behave – we all know that physics experiments are too often characterized by the fact that they don’t work – especially when they have a big audience.

But an experiment can never ‘not work’. It does what it does. The fact that it didn’t do what you wanted or expected could be for a variety of reasons – bad experimental design, poor control, statistical variation, or maybe because of some ‘new’ phenomenon. After all, where would science be if every experiment agreed with current theory? Major strides forward in science have usually been triggered through experiments that didn’t do what the experimenters were expecting.

I think a good definition of ‘valid’ would be ‘that the experiment measures what you think it is measuring.’ That, from memory, was basically Simon’s point. If we achieve that, then it doesn’t matter whether we find a new phenomenon, validate an existing theory, or just make a mundane measurement of electric current in the lab. We’ve done some good experimental physics.

Sorry – short entry this one – baby wants attention.

 

Experimental physics is easy on paper Marcus Wilson Oct 18

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Currently, down in the depths of C-building, there’s a master’s student trying to carry out the Stern-Gerlach experiment. (and also here). This is one of the classic experiments in quantum mechanics – specifically demonstrating the quantisation of angular momentum.

If you look at the text books, it’s simple enough. Pass a beam of atoms through an inhomogeneous magnetic field (i.e. one that is stronger in one region of space than another) and, hey-presto, the beam gets split into two (or more) beams, depending on the magnetic moment of the atoms. The non-uniformity of the field is essential. If the atom has a magnetic moment that lines up with the field, then it will have a negative potential energy due to the field and will move towards the region of strong field, where its energy is most negative. Conversely, if it has a magnetic moment that is against the field, it will have a positive potential energy and will move towards the region of weak field, where its energy is least positive. So the beam splits. The key result, though, is that the beam doesn’t split into a continuum, which would mean any magnetic moment were possible, it splits into discrete beams, showing that only certain values of magnetic moment are allowed. This is what quantization means – things are split into discrete amounts. What the experiment is doing, is measuring the magnetic moment in a particular direction.

Stern and Gerlach did this experiment in 1922. Having seen our poor student struggle with the apparatus, they must have put in some considerable effort, that’s now been glossed over in most books. There are all kinds of issues that need attending to. Preparing a beam of atoms (in our case sodium – we’d like to use potassium but that’s a little bit too exciting from a safety point of view) is tricky. The sample needs to be heated so that atoms are evaporated. We need a high vacuum, meaning that atoms do not collide with air molecules on their way down the apparatus. We need to make sure that we are detecting our sodium atoms not contaminant atoms that are coming from elsewhere.  And, most frustratingly this afternoon for student, we need to find where the beam is going an align it so that it falls on the detector.

The stereotypical drawing of the apparatus we see in the quantum textbooks overlooks most of what actually has to go on to get this to work. It’s slow going, tedious, and frustrating, but hopefully the student will nail it in the end. This is all too reminiscent of the reasons why I became a theoretical physicist rather than an experimental one, and the old adage…"Biology experiments wriggle, Chemistry experiments smell, and Physics experiments don’t work"

 

 

 

Expensive cat physics Marcus Wilson Sep 25

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With baby Benjamin taking our attention, poor Mizuna the cat has been rather neglected recently. Unfortunately, this has proved an expensive (for us) and painful (for him) mistake. A couple of weeks ago, I noticed one morning that he was clearly in pain, and desperately trying to urinate. CLANG, CLANG, go the alarm bells. Male, neutered cat, urinary problem – there’s no mucking about with this one, but straight to the vet. It was obvious it was another bout of FLUTD - he had one about 18 months ago.

Last time, without a baby to cause distractions, we spotted it early enough and the vet had it under control fairly straightforwardly. This time, however, I picked it up much later, and the poor kitty had actually become blocked (which, for non-cat owners, means his urethra was blocked with crystals meaning he couldn’t urinate). A blocked cat is on a rapid, painful trip to kitty-heaven if he (and it is far more likely to be ‘he’ than ‘she’) doesn’t get immediate treatment.

So, a week and a half and a bill the size of an Australian Institute of Physics Congress fee later, Mizuna is back with us and appears to be doing well. But the lesson has been learned – watch the cat. 

Reading about the condition is quite fascinating. In fact, I’m left in great surprise that a male cat has a working urinary system at all. The urethra, at the base of the penis, has a tiny diameter, and that means it can block really easily. Even without a blockage, it can’t be easy for the moggie to pass urine. The flow of fluid down a very narrow pipe is governed by the Hagen-Poiseuille law - which is that the flow rate is proportional to the diameter to the power four, times the pressure difference.   That’s a steep power law – double the diameter and you get sixteen times the flow (so long as the flow stays laminar in nature) – on the other hand, a halving of diameter gives you one sixteenth of the flow rate for the same pressure. A narrow tube really isn’t much good for passing water down, and that’s the male cat on a good day. 

Interestingly, Poiseuille did his experiments with a view to understanding the flow of blood in the human body. Blood has a rather higher viscosity than water (yes, blood really is ‘thicker’, in this regard, than water) which means that Poiseuille flow remains applicable for larger diameter ‘pipes’ (i.e. arteries). Here, the steep power law is important in terms of heart problems – a narrowing of the arteries, if only by a small amount, is sufficient to reduce the flow of blood quite significantly. On the other hand, a stent doesn’t have to open up the artery by a lot in order to restore a good flow rate. I should add for completeness that there are a whole lot of other issues involved with blood flow; it’s not just Poiseuille’s law, but it does indicate that the width of the artery has a large role to play.

Going back to the cat, the last resort for a blockage is the perineal urethrostomy, which, in crude terms, is cutting off his willy. That is, getting rid of the smallest part of the pipe. Fortunately, Mizuna didn’t have to go down this route – and whether we would have agreed to pay for it is another issue too.

 

The Higgs Boson for not-so-dummies Marcus Wilson Sep 11

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A few months ago I agreed to do a physics talk for the Hamilton Junior Naturalists (Junats). When pushed for a title I decided on the Large Hadron Collider and the Higgs Boson. Hmm. How does one go about explaining the Higgs Boson to an 11 year old? (I’ve got to Friday evening to come up with a decent answer). In fact, how does one explain the Higgs Boson to a physicist (i.e. me)?  Particle Physicists, especially the ultra-theoretical ones, speak a language that is almost incomprehensible to everyone else, including other physicists.Think of it as being a being a kiwi trying to listen to a broad Glasgow accent. If I listen carefully to a particle physicist, I can hear words that I recognize, but just what they are trying to get across I can only get an inkling of.

Here’s an amusing but not terribly helpful video on minutephysics on why the Higgs is just so important.

http://youtu.be/9Uh5mTxRQcg

Did you get that?  (If you did, please explain it to me.) What irritates me about this video is the way that maths is used as an excuse for something being the way that it is. "Toss in the ingredients (in this case the Higgs field), let the math machine ferment, and out comes the answer (in this case mass)" Particle physicists, you have got to do better than that. You can’t say that something is the way that it is because the maths says so. No, you’ve created the maths to describe the situation you have. Sure, there can be unexpected solutions that pop out that in fact represent reality, and that gives you confidence that you are on the right track with your mathematical description, but, fundamentally, you have to be describing something PHYSICAL for it to be at all meaningful.  Maths would exist quite happily in a universe of complete nothingness – physics, on the other hand, wouldn’t.

If you are like me and need a bit more help here, there’s a few more videos to choose from.

http://youtu.be/RIg1Vh7uPyw   (Fermilab)

youtu.be/KPoxewA-URo  (Brian Cox’s extended effort).

Enjoy. I’ll let you know how Friday night goes.

 

 

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