SciBlogs

Have you checked your tyres recently? Marcus Wilson May 23

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A couple of days ago I checked the tyre pressures on our new car for the first time since buying it (several weeks ago). Obviously we want about the right pressure in the tyre to get it to do its job properly and to help get the best fuel economy out of the car. However, getting the pressure right isn’t an easy thing.

For a start, I’m sure every garage’s air machine reads differently. Certainly the ones with an analogue scale can be all over the place in their readings. Even two readings of the same tyre on the same (analogue) machine can be way different (I’m thinking about the one at our local garage).  That might partly be because I’m not using it correctly – but I think I’m following the instructions.

But also the temperature of the tyre is going to make a reasonable difference to the pressure. Air is going to follow the ideal gas law – roughly anyway, which says that for a given mass of a given gas, pV/T is constant, where p is the pressure, V is the volume, and T the (absolute) temperature. For constant volume (OK, the tyre isn’t exactly constant volume) pressure would increase linearly with temperature. On a cold morning, before you’ve gone very far in the car, the air inside could be sitting at 0 Celcius, or 273 kelvin absolute temperature. On a hot day after an hour on the road, it might be more like 50 Celcius, or 323 kelvin. That’s a difference in temperature of 18%, and will correspond to a similar change in pressure.

However, there are other factors too. A well inflated tyre – on a hot day for example – will stretch the tyre and make it more likely to lose air through the tyre wall. So in summer, tyres will lose more air than in winter.

The petrol-head websites (e.g. this one) say, unsurprisingly, that pressure should be checked with a cold tyre – i.e. early morning with little driving done to get to the air hose. If you have the right pressure then, you won’t be under-inflated at other times. Plus, it’s as close to a practicable standard as you’re likely to get with a functioning car – no-one’s going to take them to a physics lab and leave them overnight in controlled conditions for an accurate measurement every month!

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A refreshing approach to science Marcus Wilson May 17

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A few weeks ago when I was visiting Dunedin I was in conversation with a new PhD student and her supervisor. The PhD student was saying how she felt everyone should help each other in their research – share all their data, share all their methods and know-how, to make the world a better place (or words to that effect.) Her supervisor, rather tongue-in-cheek, replied with the comment something like "You poor misguided thing – you’ll learn. We all started off thinking like that but in a few years you’ll be just the like the rest of us."

OK, so I haven’t remembered the words, but that was the gist of the exchange. The fact is, despite good intentions and probably a belief that we should share our skills and know-how more widely, there is a strong element of keeping-it-in-house when it comes to research. We all want to land those big funding grants, attract more PhD students, and so on, and to do that we need to be the best people in the country or world to do our particular job – so why tell others your trade secrets?

Well, the PhD student isn’t completely misguided in her wish. It does happen. On Tuesday, for example, I visited BrainResource – a company with their main office in Sydney. One of their core businesses is BRAINnet:  collecting together a huge database ofelectroencephalograms – from healthy people, from people with various conditions (epilepsy, depression etc) – in various situations – e.g. resting, sleeping, carrying out particular tasks – for the purposes of facilitating research. This data is made available (in certain forms) to others. The idea is that the database helps bring in research grants – for both the company and for other institutions. Overall, the winner will be the governments and health services that fund these projects, as they’ll get a better outcome, as well as the BrainResource employees who get employed to do a fascinating job.

It’s an interesting model of doing business, but it appears to be working.

 

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Why going downhill is hard work Marcus Wilson May 14

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On Saturday I took the train out to Katoomba, in the Blue Mountains. The train journey certainly gave me a feel for just how vast Sydney is. An hour out of central station, and one is still in the Sydney suburbs. But suddenly the end of the city comes abruptly – the train is suddenly amongst the trees and starts climbing. Another hour gets it to Katoomba, a small town perched on top of precipitous sandstone cliffs.

I quickly got the impression that the way many (most?) tourists see the Blue Mountains is on a coach tour. The coach pulls up in the vast carpark at Echo Point, the tourists pile out with their cameras, take lots of photos of ‘The Three Sisters’, grab an overpriced icecream, then pile back into the coach to go to their next viewpoint, wherever that is. Certainly Echo Point has a feel of an international airport about it.

However, escape the crowds, and it becomes rather more pleasant. The way I did that was to head to the bottom of the cliffs. There’s three ways of doing this. 1. Take the ‘railway’ (I assume this is a funicular – I didn’t get a look at it) or the cable car down. 2. Walk. 3. Jump.  I chose option two. That involved heading down the ‘Giant Staircase’ – about nine hundred steps in total down 250 metres or so of nearly sheer sandstone cliff.

One of the surprising experiences going down this number of steps is just how hard going it is. Near the bottom, my legs were burning – rather like having done a REV class in the gym. What’s happening is that I’m exercising muscles that don’t usually get a good workout, and it hurts. In physics terms, I need to provide a force against gravity to slow me down as I descent. The force of gravity alone would cause an acceleration downwards;  in order  to descend at a constant speed I need a force that balances this (Newton’s first law), and that comes from under-used muscle groups within my legs. After several minutes of this, it hurts.

Incidentally, in energy terms this force is not supplying energy to you, unlike the reverse case where it takes energy to climb the steps back up. That’s because the force is in the opposite direction to your movement – therefore it doesn’t do work on you (transfer energy to you). In fact, it is doing the opposite – your legs are absorbing the potential energy lost as you head downwards. No wonder they are burning at the bottom.

Once at the bottom, it was a lovely walk through the bush (though I did get a little worried about what zero- or eight-legged nasties might be lurking in the undergrowth). I went back up a different route – past many lilttle waterfalls. I lost count at 1200 steps. And at 1000 m altitude, or thereabouts at the top, it’s quite an effort for a near-sea-level-dweller like myself.

 

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Electromagnetic fields and the brain Marcus Wilson May 10

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Yesterday afternoon I visited Westmead Hospital and talked to a couple of psychiatrists on the use of electromagnetic fields in treating certain conditions. Treatments like the controversial but effective electroconvulsive therapy for depression, and transcranial magnetic stimulation for Parkinson’s and stroke, are becoming well used. However, there is little understanding of why they work. It’s a case of zapping a brain with an electric or magnetic field  improves a problem a patient is having, but no-one knows why.

The gap in knowledge here are at least three-fold. First, how does the electric field interact with neurons and what does it get them to do? There’s experimental data on how they respond, but the mechanisms of this are unclear. And then, when they do respond in the way they do, how does that lead to a change in neural connections? And finally, what is it about the neural connectivity that causes improvements in symptons of depression or Parkinson’s?

These questions are a mix of many different disciplines: molecular biology, neuroscience, mathematics, and, from my perspective, physics, and probably a good deal more.  Making progress on this question is going to require all of these skills. That’s why a cross-discipline collaborative approach is so important to making advances in science. Being on study leave gives me a good opportunity to explore some of these possibilities. Yesterday’s visit was certainly very enlightening in this regard.

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Virga Marcus Wilson May 08

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I learned a new word today: virga. It was used in a short article in one of the Sydney newspapers, discussing yesterday’s weather. Virga is simply precipitation that evaporates before it hits the ground, and we had some here yesterday. So it was raining, but we didn’t get wet.

Virga can rapidly cool the air around it, causing downdrafts in the atmosphere which then heat as the pressure increases nearer the ground. Personally, I didn’t notice anything particularly bizarre, but then again I was indoors for most of the day.

There’s a nice video of virga on youtube.

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Different place – same old problems Marcus Wilson May 07

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I arrived in Sydney yesterday. I’m based at the University of Sydney for the next two weeks while on study leave. My first morning here was pretty-well taken up by walking round getting administrative things done like getting a key to my office and getting a computer account and internet connection for my laptop.

It was very reminiscent of a day a few years ago when a new PhD student in our research group turned up at Waikato and needed to get keys, out-of-hours-access card, library card, computer account, etc etc. I spent half the day taking her around the campus. Now at Waikato, this problem should now be much diminished by our student centre in the library building. A lot of the administrative things new students need to do are now located in one place. That’s logic for you. Does it work? I’ll have to track down a few new students when I get back and interrogate them.

Now, at Sydney this morning, in order to get a key for my office in the Physics building I had to walk close to a kilometre, through the campus, across a main road, to the security office, which for some reason is located right on the periphery of the campus. Apparently, all keys are issued centrally – departments don’t issue keys for their own buildings. Then I had to walk back again. An easy experience? Hardly. At least getting the computer access could be done through the physics department. One positive thing from the trip, however, was that I’ve now located the swimming pool, which is large and indoor and cheap and available to short-term visitors like me and open early morning. Yay.

 

 

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How to cheat aging Marcus Wilson May 04

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It’s ‘Wellness Focus’ week here, and there are all kinds of wonderful activities going on to promote health among the employees of the University of Waikato. I’ve just finished a REV class, which has finished me off for the whole afternoon, I think.  Yesterday, I had a free health check – where my blood pressure, cholesterol etc was measured, and the nurse went through a lifestyle questionnaire with me assessing my risk of heart problems (which turns out to be low). One question on the list is ‘what is your age?’. It obviously affects your risk of a heart attack, though the nurse said it’s one factor that you can’t do anything about.

Well, I thought, that depends on how pedantic you want to be about it. I’ll be travelling to Sydney very soon as part of my study leave, and that is going to slow down my aging by a few nanoseconds. That’s simply a consequence of special relativity. The important factor here is denoted by the Greek letter gamma by physicists, and is the reciprocal of the square root of [1 - (v^2/c^2)].  In this expression, v is your velocity, and c is the velocity of light, or 299,792,458 metres per second.  (The ^2 means ’square’). It tells you how much time slows when you are travelling at this speed. 

What is gamma for a commercial jet aircraft? At 900 km/h, or 250 m/s, it comes to 1.00000000000035 . That means, when I’m on board the plane, for every 1 second of aging I do, people on the ground will age 1.00000000000035 seconds, that is and extra  3.5 times 10 to the power of minus 13 seconds. Over the course of a three hour flight that comes about an extra 4 times 10 to the power of minus 9 seconds, or 4 ns.   Not enough so you’d notice, but it’s measurable with atomic clocks.

One of the problems with teaching special relativity is that its effects nearly all lie outside the realm of everyday experience. They are only apparent when something travels very fast. However, they are extremely important in physics. That’s the motivation for this piece of software I was told about a couple of weeks ago from the Australian National University in Canberra. The software, which you can download for free, lets you fly a spaceship around a city-scape at close to light speed, and observe some of the effects that are occurring, or observe different clocks at speed to see the time dilation effect.  It’s well worth a play, but to get the most out of it you should follow through the student instructions which come with it. The research article that comes with it is worth a read too.

 

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Mysterious power generation Marcus Wilson Apr 30

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One consequence of being a physicist is that you can’t go anywhere without seeing physics calculations that need doing. I’ve just been to our library hunting down books on the medical technique of transcranial magnetic stimulation (TMS), which was an interesting exercise in itself, since one textbook I found also has a chapter on homeopathy. Hmm. So how much do I trust its section on TMS then?

Anyway, our new, glorious library and student centre gets better every time I go in. Not only has a new cafe opened up inside, but there are now screens telling you just how eco-friendly the building is being right now, by displaying data on the building’s power consumption, solar power generation, water consumption and water capture, etc.

So, in the last month (which I assume means April) the building has used 182 792 kWh of power (that certainly makes my electricity bill look tiddly!) but has generated a cool 1 847 531 kWh from its solar panels. Now, I know April has been unusually sunny this year (shame the sunshine couldn’t have come in summer when it was supposed to) but there is something clearly wrong with this figure.

One metre squared of area, under full sun, gets about 1 kW of power on it. That means in about an hour it captures 1 kWh of energy. I don’t know how much of the building is covered in solar panel or other capture device, but I reckon the footprint of the building is about square with a side of 40 or 50 metres, so let’s say about 2000 m2 in roof area. So, if that were covered in solar panel, under full sun it would capture about 2000 kWh in one hour. In April there are 720 hours, so that gives us 1 440 000 kWh of energy.

But I’ve assumed that the panels are illuminated 24 hours a day! That’s clearly rubbish. So halve that, since half the time it’s night. In April, the sun isn’t anywhere near the zenith, so let’s halve that again. We are down to about 400 000 kWh. Then there are cloudy days (albeit not too many this month) which will take it down again, and still a large factor to apply because the power conversion from light to electricity or hot water isn’t 100% efficient. I reckon we might be down to a more reasonable estimate of 100 000 kWh.

So what does the 1 847 531 kWh represent?  It’s either a mistake, or I’m misinterpreting the display.

 

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Experiments with known answers Marcus Wilson Apr 26

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Earlier this week I read through a student’s work placement report. Our engineering students all go out on two work placements over the course of their study with us, and need to provide reports on these. I was slightly amused to read about the student’s views on the novel experience of doing experiments where you don’t know the outcome before you start. What he meant was, for the first time in his training, he needed to undertake an experiment where he couldn’t go and look up the outcome in a textbook.

That I think is something worth thinking about. Nearly all the experiments that students do in class are things that have been done many, many times before. There’s good reasons for that – they are (mostly) reliable, the equipment is available at sensible cost, the experiment can easily be discussed in terms of underlying theory, it’s not dependent on the expertise of a single teacher that will be lost if the teacher leaves, it saves planning new experiments from year-to-year, and so on.

But it also means that students can look-up what the answers should be. That’s got a couple of clear problems: 1. The student loses their ability to critically judge the credibility of their own work and 2. there is the temptation to ‘cook’ the results so you can get a better mark. The first of these is a real skill that a scientist needs to develop – if you can’t look critically at what you’ve done and decide whether you trust the results then your science is on shaky ground. In real science, there is no answer to look up in a book – so how would you know if you’ve made a mistake?

The second is an interesting point, too. I would say if students find that they are getting better marks by making up results to conform to what the textbook says there is something going wrong with the marking process. Assessment should always be done with a view to promote learning. Copying things out of a book is not learning. I’d rather have a student try and fail to produce valid results, but, at the end of the experiment, understand what they were doing wrongly and why, and be able to correct it next time, than just to produce a graph that looks nice. Any experimentalist knows that most of his or her lab time is taken trying to troubleshoot the experiment. So why do we set experimental labs for our students that teach them otherwise?

This isn’t an easy problem to get around, especially when you have a huge class of students. I tackle it with the second and third year classes by marking a student’s work with the student present (in fact, getting them to mark it themselves first, then tell me why they deserve the mark they think they do). That way we can talk through the difficult issues and they can see that I’m not just after graphs that look good.  But in a first year class, where there are 100 or more students to shuttle through in a week, that’s not practical.

The placement student was very happy to have the opportunity to tackle a real experiment. Perhaps the answer lies in bringing in our research more into our teaching – getting the students to do some real research things.

 

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Convection and continuity Marcus Wilson Apr 24

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With the coming of the colder evenings, we’ve had our new heat pump going. It’s quite a powerful beast, as it has a large volume to heat, and it comes with a plethora of different settings – temperature, fan speed, air flow angle and so forth. It’s taken some experimentation to get some decent settings on it.

One thing that’s clear is that the heat pump generates a lot of convection in the house. That, of course, is the point – that’s how the heat transfers from the indoor unit to the room – but in our case the air currents can be pretty strong. There are two ways this happens – ‘Free’ convection and ‘Forced’ convection.

Forced convection just describes the air flow due to the fact that there is a fan inside the unit – it’s blowing the air around. Given that air isn’t created or destroyed, this means that the air has to circulate around the house. Free convection describes the currents caused by warm air being a lower density than cold air. The warm air will be buoyant, and rise, which means (by continuity again) that colder air has to sink.  The two mechanisms are both in place, and they are both very evident. First of all, one can feel the air moving around (and see the light shade swing a little bit as the air blows past), and secondly upstairs becomes nice and toastie while downstairs is rather cooler.

So, the problem then becomes setting the unit so that we get the ‘best’ situation. One obvious problem is that the forced convection results in the airflow into the unit  being rather cooler than  the average room temperature. The pump measures the temperature of the incoming air, and then attempts to take this to the target temperature you set. So if you set the target temperature as 19 Celsius, it means that the air temperature of the majority of the area, especially upstairs, will get to rather higher than 19 C. That we can cope with, simply by setting the target temperature a bit lower.

We’ve also found that the angle of air flow leaving the pump makes a large difference to the distribution of heat around the house. If it’s too flat, it just scoots across the downstairs ceiling, and then heads upwards when the ceiling runs out – we end up with an oven upstairs and a fridge downstairs. If it’s angled too far down, it blasts into the dining room table – it keeps you dinner nice and warm but it feels like you are eating dinner in a Canterbury Nor’wester. Again, with some experimentation, we think we’ve got a reasonable setting.

So that’s all good for now, but there is a nagging doubt that when it gets much colder still the convection currents will change again and we’ll have to ‘re-optimize’.

 

 

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