SciBlogs

Archive September 2012

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.

 

What do students make of physics and maths… Marcus Wilson Sep 19

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Last week I did my first interview with students regarding a small research project I’m doing – looking at the ways that students percieve the relationship between physics and maths and how similar they are in their thinking to expert physicists.

It was an interesting interview. I had four students, and their was quite a bit of discussion going on between them. I haven’t done a full analysis of what they said yet – that takes a while for a 35minute interview – the first part is getting an accurate transcript of what was said. When I went through university, transcribing voice recordings wasn’t part of a physics degree, so it’s taking me a while, but I’ll get there eventually. (I don’t have the budget available to farm this job onto someone else.)

But it seems that the group I talked to has a very mature view of the links between physics and maths, by which I mean the views of the students  seemed to be mostly well-aligned with those of practising physicists (I’ve already done this latter group.) An obvious parallel to look at is the work of Gire, Jones and Price, which showed that students’ views on the nature of physics in general were  somewhat below those of practising physicists (not really surprising), but that physics students had a much more mature view than engineering students. Will it be similar in the specific case of the maths and physics links? I’ll have to wait and see. What I find most interesting about the Gire et al paper, is that the beliefs of the physics students about physics basically didn’t change for their first three years – it was only when they hit fourth year / graduate level that it suddenly lept upwards to come close to that of practising physicists.   Reference: E. Gire, B. Jones and E. Price. Characterizing the epistemological development of physics majors. (2009) Physical Review Special Topics – Physics Education Research, 5, 010103.

One obvious outcome from this interview group was the fairly limited idea that the students had of the career paths that their study could take them to. They had chosen their subjects because they liked them, or though they would like them, without much regard to what would happen after university.  An area that we can help with as lecturers, perhaps.

 Finally, I can’t resist putting this snippet of conversation in, particularly since I was mathematician-bashing in my last entry.

Student:  I reckon you can learn the parts, and get how it works [i.e. the maths], but you can’t fully understand it until you see how it works in real life [i.e. the physics]

Me: Do you think a hardened mathematician would disagree with you there?

Student: Yeah, but they’d be missing out.

 

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.

 

 

How many significant figures? Marcus Wilson Sep 07

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The overstating of accuracy is something that physics teachers have to continually correct. Just because one’s calculator gives an answer to ten significant figures doesn’t mean one should quote it to ten significant figures.

I’ve just looked up the location of Nakedbus’s Auckland City bus stop. It was very easy to do – drawing from Google Maps. For those who are interested, one can find the bus stop at 36.8432758738427 degrees south, 174.766430854797 degrees east. That locates it to about 100 nanometres (100 billionths of a metre)  in position. No excuse for waiting in the wrong place, then.

 

Early warnings of a change of state Marcus Wilson Sep 05

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It’s been wonderful watching Benjamin grow in the last nine and a bit weeks. He’s now become fully interactive – he’ll respond to what we do and what’s going on around him. hearing him ‘talk’ is fun – he can come out with an excited string of goos and gahs when he’s happy. 

Naturally, though, there’s still only a relatively small range of behaviours he exhibits.  Sleeping, feeding, squarking, gazing carefully at something, or goo-ing happily is pretty well the extent of it still. These behaviours are mutually exclusive – he’s only ever in one at a time, and transitions between them many times a day. To be one step ahead of him, we need to pick up the signs of a change of state.

In physics, there’s several ways that can be done. For example, at a first-order phase transition (e.g. water turning to ice) there’s several hallmarks that a change of state is imminent. In particular, the natural variations in the system begin to grow – they get longer and larger. One can measure these to determine just how close to transition a system is. Also, the susceptibility to an external stimulus gets greater – poke the system somehow and its response is larger than if it were further from transition (it is more ‘irritable’). We do this kind of thing with our brain modelling – e.g. look at the transition from deep sleep into REM (rapid-eye-movement) sleep – there are indications that can be picked up through electrical recording from the scalp (the electroencephalogram or EEG) that a state change might be imminent.

 I wonder if we see similar things with baby. For example, as he transitions from squarking to sleeping, the noise he makes does seem to fluctuate more – with loud periods and quiet periods, before ceasing altogether. Of course, anecdote isn’t data, and we certainly haven’t done a scientific analysis of this, but I wonder if it is the case. I certainly think we are better at anticipating what he’s about to do – so maybe there are subtle signs that we’re picking up.

Which brings me to the cat.  He too has only a few modes of behaviour – eating, sleeping, washing, gazing out of the window, being an adorable cuddly purring kitty and then being completely loopy (in this last case you don’t want your arm anywhere near him).  Unfortunately, as is the way with most cats, the transition between these last two states can be very quick indeed. The warm, cuddly, purring moggie can turn into a viscous arm-shredding machine in a couple of seconds. Is there warning? Maybe a couple of tail twitches and a short pause in the purring. One needs to be very clued into phase-transition theory to pick these up.

 

Static electricity Marcus Wilson Sep 03

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Static electricity is, above all, fun, but it also can be annoying at times. I was at a conference in Queenstown last week, and was haunted by static everywhere I went. I’m guessing that atmospheric conditions were pretty dry, meaning that electric charge didn’t leak away quickly. When you know that you are going to get a shock when you press the lift call button at the hotel it makes you behave in odd ways. I’m sure someone watching on a security camera must have wondered what I was up to.

A Van der Graaf generator would have worked a treat, but I forgot to pack one.

I had one incident with a lolly (sweet, candy, depending on your origin) wrapper. It stuck to me and wouldn’t budge. I could swipe it from my clothes, but then it stuck to my hand. I could then transfer it from one hand to another, or back to my clothes. But I couldn’t shake it off. It was well and truly attached.

Electric charge sits on the surface of objects. That means the amount an object acquires is broadly proportional to its surface area. That makes small objects particularly susceptible to its effects. The force of gravity on an object is clearly proportional to its mass (density times volume), so the relative strength of the electrostatic force compared to the gravitational force is proportional to the surface to volume ratio.  Small objects have a much greater ratio than large objects. A lolly wrapper therefore glues itself to you, whereas a chair just gives you a shock.

 

 

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