SciBlogs

Archive November 2011

What’s the buzz? Marcus Wilson Nov 29

No Comments

Nothing to do with physics this bit:  My wife was at home yesterday afternoon, and she phoned me at work.  "Err…we’ve got a bit of a problem…."   My immediate thought was maybe the plumber had severed the main water pipe or something disastrous like that, but no, rather different.   "…I kind of heard this buzzing coming from the chimney and then pinging sounds in the fireplace…."  Yes, our house (more exactly, our fireplace) became the residence-of-choice for a swarm of bees yesterday.  Needless to say, we called in the pest control people who have now convinced the bees that this isn’t a good place for them.

And here’s a joke my brother told me yesterday.   "A Greek, a Portuguese and an Italian walk into a bar. Who offers to buy the drinks?".

Answer: The German.

Finally, slightly more seriously, I got some teaching appraisals for B-semester back yesterday. That’s what the students use to ‘score’ the papers they’ve done and the teaching they’ve had. In one paper I had a perfect score – for every question every student had put down ‘always’ – for both questions about the paper they were doing and for my teaching on it. Wow! Don’t get many of those. I should caveat this slightly by saying there weren’t many students in this paper.

However, in another paper I teach on the scores for the paper (though not for my teaching) gave the impression that there were some rather annoyed students in the group. The problem this creates for me is that while I know they are obviously annoyed at something, the scores on the appraisal forms don’t tell me what it is they are annoyed about. (Which means I can’t do much to fix the problem for next year.) Sure – I’ve had some feedback while the course was going, but nothing to suggest they were entering really-narked-with-something territory. Hopefully, when I get to see the hard copies of the forms they’ll be some constructive comment as to what the problems are – otherwise I’m going to have to chase down some students and try to tease it out of them.

 

 

Hot water Marcus Wilson Nov 28

No Comments

This weekend I decided it was about time that I twiddled all the various taps and valves associated with our water system – just to make sure none of them get seized in the ‘on’ position – wouldn’t be good if can’t get the water turned off when you need to most. Perhaps checking was a good move – because what I ended up with was hot water exiting through the pressure release valve at the top of the tank. The valve wouldn’t reseal, and the only way to stop the tank of water draining non-stop was to turn off the supply to it.  Time to call a plumber in.

This is the valve that sits on top of the tank that is the emergency pressure release. If, say, the thermostat in the tank fails, and the element just piles more and more heat into the tank, one would end up with very hot water under pressure. 

In normal conditions (e.g. a pot of water on the stove), as you shove heat into it the temperature increases, until the water hits boiling point. At this point any further energy you put in doesn’t raise the temperature of the water, instead it is used to turn the water into steam. If you keep the pot on the stove, eventually all the water will boil away. However, if you don’t allow the vapour to escape (pressure cooker), the pressure will build up inside and this acts to raise the boiling point of the water. Up comes the temperature – beyond 100 degrees, and up goes the pressure as well. At some point, if you keep putting more energy into the system, the pressure is going to be too great for the structure to cope with, and something’s going to fail.

The pressure cooker has a valve on the lid that will blow if it gets too hot and pressurized inside, releasing the pressure in a controlled manner, before the pressure cooker explodes. Likewise, the hot water cylinder has a similar device on it. You really don’t want it to get stuck in the closed position (or for the drain pipe to be blocked) hence the warning on the cylinder to release it every six months to check it works. Unfortunately, in my case, the valve is now stuck ‘open’ – which ain’t much use. So my tank is very safe, but it’s also not doing its job, since hot water promptly disappears down the drain. At least that’s better than having an exploding tank.

You can have a look at a water phase diagram here. You can see the boundary line between liquid (water) and gas (steam) phases (the boiling point) and how this increases as pressure increases. If you increase temperature and pressure enough you get to the ‘critical point’ – this is where there is no longer any distinction between water and steam – the two have become the same phase. This is really quite hot indeed!

Rena: Where’s the physics? Marcus Wilson Nov 24

No Comments

On Tuesday night I attended a very informative and lively discussion at Cafe Scientifique on the Rena disaster. (For readers not in NZ, the Rena is the container ship that has been stuck on Astrolabe reef off the coast of Tauranga for the last few weeks, shedding oil and containers into the sea – see for example, this article.) I say ‘attended’; in fact, I was MC for it – but that didn’t make it any less interesting for me. We’d pulled together a panel of five people from the university to talk about various aspects of the situation – two oceanographers, two chemists, and a biologist.

One immediate question that sprang to my mind is ‘Where’s the physicist?’ I think  that often when people start putting ‘environment’ and ‘science’ together, the immediate thought is biology, and then maybe a bit of chemistry/geology/oceanography/ecology etc, with physics left off the end.  In fact, there was  physics talked about, in terms of what the oil does to the surface of the ocean. The oil changes the surface tension which in turn changes the pattern of the waves, and therefore the ways in which sunlight is reflected and infra-red light is emitted, and this kind of change can, if its substantial enough, be monitored from space. This means you have a way of remotely monitoring where the oil goes. At this point I could have jumped in with a few comments, having experience from my previous job of what the sea ‘looks like’ in infra-red. However, being MC, and having questions come from the floor thick and fast,  I felt it might be better to allow others to talk.

My point here is that there is physics in ‘environmental’ stuff – it’s not all about the biology.

There was also a fair bit of discussion around the way ‘science’ is done – for example, the way that controlled experiments need to be done before jumping to conclusions. As an example, Chris Hendy, a chemist, described measurements of the arsenic content of the oil on the beaches (which appears on the face of it to be high!) However,  the ‘oil sample’ he had was a mix of oil and sand, so, to get a meaningful measurement, he had to also measure the arsenic content of the sand alone. And, lo and behold, it’s the sand that has the arsenic in, not the oil. (Where’s it come from? – probably it’s of geothermal origin)

So, overall, science got a good hearing, as it should at Cafe Scientifique.  Unfortunately, as one of our panel commented, it looks like Rena isn’t going to lead to a ‘funding bonanza’ for the scientist.

Finally, I should have said a couple of weeks ago that PhysicsStop is now three years old. Wow. For the record, this is the 465th entry.

Elections and antimatter Marcus Wilson Nov 21

No Comments

Reading opinion polls in the papers tends to cause despair and hilarity in roughly equal measure. Despair, not because my current favourite party might be nose-diving in the polls, rather because of the journalists scant regard for statistics, and hilarity for the way a story is built up where no story exists.

A really, really rough understanding of statistics is all that’s required here. Really crudely put, the more people you interview, the more accurate your poll is going to be. Of course, that assumes you are actually carrying out the poll well in the first place, not just interviewing whoever you come across on the street or whoever answers the telephone, because such crude selection methods will inevitably bias your results -  opinion pollsters know better than this.

Every poll that’s published has buried away at the bottom the number of people interviewed and the ‘margin of error’ (call it M  percent).  This latter is something to take note of – basically it means that if the poll says the major party’s share of he vote is X percent, their true popularity is most likely to lie in the range of X – M to X + M.  Given that many polls are based on around 750 people, giving a margin of error of 3%, that’s a huge range.  So if a party polled at say 40% one week, and polled at 38% the following week, their sudden drop in popularity isn’t a story at all. It may well be that they have dropped, but you can’t tell on the basis of these two polls alone. Also possible is that they’ve stayed exactly where they were in popularity.

For minor parties, struggling on 5% or lower, the margin of error needs recalculating. Compared to the fraction of the vote they are getting, the margin of error can be really high indeed,.

Of course, what you can do to improve matters is combine the results of lots of polls. Basically, what you are doing here is pushing your sample size upwards, and reducing your margin of error. A helpful geek or two have been doing this for you – have a look at this Wikipedia page which shows how the different parties have polled since the last election, and, more importantly, a measure of the margin of error in the calculations. Note the size of it for the minor parties, compared with the fraction of the vote they are getting

So where does the anti-matter come in? Physicists are currently looking at some results from the LHCb experiment at the Large Hadron Collider, which look like they are indicating charge-parity (CP) violation in a particular decay process. What that means is that there is an asymmetry between the way matter and its corresponding anti-matter behaves. Swap matter for anti-matter, reverse the directions in space, and you don’t get the same results as one might naively expect. Now CP violation isn’t new, but this would be a new manifestation of it.

The question then, is do the statistics show CP violation? Is the measured asymmetry bigger than the margin of error (what we’d call the standard uncertainty) in the results? Yes it is, and very much so, in fact about 3.5 times. So, by the standards of even the most cautious political reporter, we have a story here. However, for physicists, three and a half times still isn’t quite big enough. There’s still a small chance (roughly 1 in a 1000) that this result is just statistical fluctuation. One in a thousand doesn’t sound big, but when you have an interesting result like this one it pays to be cautious. The ‘standard’ for these kind of experiments is considered 5 margins of error, before people will start jumping up and down to say that they have discovered something. 

How can we go from 3.5 times to 5 times?  Simple – more data – just like for the opinion polls.

 

 

 

 

 

 

 

 

You know you need a new car when… Marcus Wilson Nov 17

2 Comments

Our cars (one of them in particular) are beginning to cause a few too many problems. Last night we were left stranded again, just outside Cambridge, and had to have those friendly AA guys rescue us. This time there was a sudden "Thwunk" sound and then a rapid "fwap-fwap-fwap" from underneath the bonnet. We pulled over to investigate. On opening the bonnet it didn’t take long even for a mechanical-phobe like me to spot the problem. We’d shed a drive belt (well, at least one), which seemed to have got itself mangled and around other belts. It was kind of like black rubber shredded spaghetti.

The garage are working on it as I write – they advise that belts usually don’t fly off of their own accord and quite probably its because one of the units being driven has failed in some way. Will find out shortly. Technically, I guess, we weren’t stranded, in that the engine would still go, but it was hard to ascertain what other systems had and hadn’t got power, so best not to risk it, especially down SH1.

What happens to a drive belt when it breaks suddenly (say) would make a nice problem to discuss in a first year physics class. There’s lots of physics principles buried in it. The belt is under tension, so there’s a force here. Then it’s rotating fast, so there is the angular momentum to think about. Possibly it’s been knocked sideways too, and there could be linear momentum to consider. And of course there is gravity. Which effects dominate the behaviour? It might make for an interesting discussion.

It will be interesting to find out, once the spaghetti has been untied, just how many belts (or parts of belts) we have in the car. That is, did we lose one onto the road (i.e. did gravity win) or are all belts accounted for?

Bending of beams Marcus Wilson Nov 15

No Comments

The ceiling in our new house is held up by seven large, curved, steel beams. There are also steel beams holding up parts of the upper floor. These beams are I-beams - so-called because they resemble the capital letter ‘I’ in shape (except they don’t in a sans-serif format as this blog gets published in.) A better description for a sans serif format would be an ‘H’ beam – imagine the letter ‘H’ rotated by 90 degrees – that’s what the cross-section of the beams look like.

The beam has its distinctive shape for good reason. The idea is to put the a lot of steel into places where the stresses are greatest. If you imagine supporting a beam by its two ends, and then applying a load in the middle, the beam will bend slightly. The top surface will be on the inside of the bend, and become slightly shorter and so be in compression; the bottom surface will be on the outside, and become a bit longer and be in tension (stretching). A line running through the centre of the beam along its length won’t change in length at all. This is called the ‘neutral axis’. The stresses are largest on the inside and outside surfaces of the curve, and this is where most material needs to be. So an I-beam has two flanges – one on the innermost surface of the curve, one on the outermost.  In this way one can get the strength of a much thicker beam of uniform area, without the weight and cost.

The centre bit of the beam is only 3 or 4  mm thick – which makes the thing look quite precarious, but it’s quite sufficient for the task. (Amendment 16 Nov 11 – that’s probably not true – 5 or 6 mm I think is a better estimate. It’s a tricky thing to measure since you can’t put a ruler against it, except at the ends of the beams, which are outside the house a few metres off the ground. I wasn’t going up there last night for the sake of a blog entry.)

A mathematical way of describing this is through the ‘second moment of area‘. Basically, this is a measure of how far away the bulk of the beam’s cross-sectional area is away from the neutral axis. (Formally, its the mean square distance from the neutral axis). A stiff beam needs a large second moment of area.  There’s a whole mathematically intensive theory describing how beams deform when loaded (involving a fourth order differential equation) that architects would do well to have some knowledge about.

 

 

11:11 (and 11 seconds) on 11/11/11 (or 11/11/11 if you are American) Marcus Wilson Nov 11

2 Comments

I’ve scheduled this post  to appear today (the 11th November 2011) at 11:11 am plus eleven seconds (NZ summer time).

Why? Because I can.

Electricity Show Marcus Wilson Nov 10

No Comments

I’m guessing many of you readers saw the ‘Sunday’ programme last Sunday. (Would be silly if were on another day of the week). Carlos Van Camp, the guy with the lightning show, was pretty impressive.

What was refreshing was how the whole piece was presented without mangling the science, which is something that media are adept at doing. Whenever I hear electricity talked about on TV, radio, in the papers, it’s probable that there’ll be statements about currents of 40 kilovolts, charges of 10 megawatts and so forth. Pick some random numbers, mix and match electrical sounding names with electrical sounding units, use them all entirely out of context, spice with health-scare comments, blame it all on the government (or the previous government)  and you’ve got your story.

But not in this case – mostly, I think, because they let the guy himself do the talking.

I loved the lightning fighters – here you can see that the electrical discharge comes from the tips of the ‘swords’. That’s because the electrical field is greatest around objects that curve most strongly (i.e. pointy things – especially conductors). Therefore the air will break-down electrically at these points first, and it’s where a spark will originate from. That’s why a lightning conductor is the most likely point on a building to be struck by lightning – they don’t get hit just because they are taller than anything else, but because they have been designed to increase the electrical field around them and encourage a hit to them, not elsewhere on the building. You can also see the way the spark jumps about along different routes between the two sword tips.

I would be a touch concerned however, about the crowd standing so close. I would want very careful control of them. You wouldn’t want to find yourself accidently standing between the two guys.

If you haven’t seen it, you can watch it at the link above.

Thermal expansion again Marcus Wilson Nov 09

No Comments

Our new house (new to us – it’s twelve years old) can make some ferocious noises sometimes. It has some huge steel beams supporting the roof – these being held up as far as I can tell partly by steel supports and partly by concrete walls. I’ve been trying to work out just what is supporting what in the house – for example there appears to be one beam that is held up only at one end. That can’t be right – there must be some kind of structure buried away that I can’t see – but it does look a bit odd.

With all that steel, and warm days and cool nights, there’s a lot of thermal expansion and contraction going on. At about the time you want to start sleeping, the house can start banging as  the steel moves slightly to relieve stresses that are beginning to build up. I’m sure the architect knew what he was doing, but it it’s a bit disconcerting sometimes.

Thermal expansion can be used to good effect however. The classic example is the bimetallic strip used as a thermostat. Here, strips of two dissimilar metals are placed together. The two have different coefficients of expansion – so as the temperature changes there is stress built up within the strip. To relieve this stress,  the combined material bends.  Physicists talk about the strain energy contained within the strip – in parts of the strip that are stretched, for example, the atoms are further apart than they would normally be leading to extra energy – in parts that are squashed the atoms are closer together, again leading to extra energy. Overall, the strip moves to minimize that strain energy, which means changing the amount it bends. This bending can be used to make and break an electrical contact, and, hey presto, you have a thermostat – a switch that is switched on and off by temperature. Wikipedia has a nice video of a spring uncurling when it is heated.

A similar effect is seen in our cat Mizuna. In this case, rather than acting to minimize the strain energy, he acts to maximize his heat energy. On a cold morning, he’s curled up; but as the temperature increases, he slowly unfurls. Plonk him on a hot deck on a summer afternoon and he’ll be stretched out, maximizing his total body area exposed to the sun. We could use him as a thermostat too, though I don’t think he’d take too kindly to being used as part of an electrical circuit.

 

Bugs and more bugs Marcus Wilson Nov 07

2 Comments

I’ve spent a frustrating day trying to drive out the bugs in one of my computer programmes. It’s a piece of computer code that is implementing equations describing how neurons talk to each other. I knew there was a bug because my modelled neurons were clearly not firing at the rate they should be. It wasn’t an easy bug to find – and on the way I ran across another one – but I got there in the end (about four hours work). Having found and fixed that bug, it’s clear that there is at least one more in there, and I’m now trying to track that one down.  Although my neurons now fire at the rate I expect, my neurons don’t communicate with each other properly. This one’s proving really awkward to find. Haven’t got there yet.

Finding bugs really is a slow process sometimes. It needs to be done very systematically. if you know what result you should get out for a given set of conditions, you can test every piece of the computer programme to see if it is doing what it should. Slowly you can home in on what the problem is.

Often, when you find them, bugs can be really small things. Like a ‘+’ sign instead of a ‘*’ sign, or a variable name spelt incorrectly. At least the days of FORTRAN77 are over – when variable names could have a maximum of six characters. That led to many variables having very similar names and opening up a lot of potential avenues for bugs to creep in.

My most difficult bug so far was a FORTRAN90 bug in a HUGE programme (thousands of lines of code) that I was working on in my previous job.  (N.B. I didn’t write the programme – I was tasked with making some modifications to it.) It was clear that one piece of it wasn’t doing what it should, but finding the bug took me a week. In the end I had to work my way through the code line by line, checking calculations with a calculator, until I got it. And when I found it, it was  unbelievably trivial. Just a typing mistake – a single character that shouldn’t have been there, but it altered what the line of code was telling the computer to do, and resulted in a big difference in the result of the end calculation.

A lot of theoretical physics research these days is just what I’ve described. Forget analyzing strange equations describing black holes or neutrinos or quasi-fluids and predicting new phenomena – All too often what you are really doing is looking at computer code that ain’t working and pulling your hair out.

 

Network-wide options by YD - Freelance Wordpress Developer