SciBlogs

Archive September 2010

For example… Marcus Wilson Sep 29

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A couple of days ago I overheard a student asking another staff member if they could see an example of some work done by the previous year’s students, to help them with a current assignment. 

I think most lecturers get asked that fairly frequently, and I’m not quite sure what the best response is. On the face of it, giving an example of good work to a student should help them produce good work themselves – e.g. by understanding what it is about that example that makes it good.  Certainly, when I’ve done things like write a bid for the NZ Marsden Fund (for which, I should declare, I’ve never got beyond the first round – except as a low percentage associate investigator) I’ve tracked down as many successful bids as I can and tried to see what about them leaps out as attractive. (Obviously I’ve failed to spot the required things so far). And when writing an article for a journal I haven’t submitted to before, I have a look at that journal and see what kind of thing they seem to publish. That’s just common sense.

But there’s a drawback to providing example assignments to students.  One is obviously that, in a few students, it tempts plagiarism, which is a serious matter.  But it also discourages novel approaches. All you end up with is a set of assignments that look the same as the previous year’s (or, worse still,  years’). It also suggests that a student is unable to work out what you want to see in the assignment from the instructions you’ve given them – i.e. your instructions are not clear enough.  Confused students are not a good sign.

I’ve begun to look more carefully at the ‘learning outcomes’ for the papers I teach here at Waikato.  The learning outcomes are how we would spot a student who has successfully taken the paper (not a list of topics they were exposed to in lectures) and these should drive the assessments. As we all know, it is the assessments that actually drive what a student will learn. Get the assessments right, with clear, transparent instructions, and learning should happen.  Does giving an example of a completed assignment from a previous year really assist in doing this? – or is it a distraction for the student as he or she works through his or her own assignment?  I’m still not sure.

Dimensional analysis Marcus Wilson Sep 28

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In my experimental physics class, I’ve been doing a bit of work with the students on dimensions and dimensional analysis. Most people who’ve done some physics have some intuition about it, but dimensional analysis puts it on a formal, and often useful footing.

Here’s a brief potted summary for those who don’t want to try to follow Wikipedia’s mathematically-intensive explanation.

Consider the size of your office. (Or living room, or kitchen….). Size is a vague term. What do I mean? I might mean the distance from one wall to the other wall.  This you would measure in metres (if you are metric).  Or, I might mean the area of the room. For a rectangular room, that’s the width times the length. Both those quantities are measured in metres, and so area has a unit of metres squared.  Or I could mean the volume.  Multiply the floor area by the height – and we get a value that is in metres cubed.  Note that the length, the area, and the volume are different kinds of thing. We say the length has a dimension of length (!), the area has a dimension of length squared, the volume a dimension of length cubed.

These dimensions hold true for any length or area or volume.  For example, remember the formula for the volume of a sphere:  four-thirds pi times radius cubed ?  Note that we have radius cubed here.  From the dimensions, we know it has to be radius cubed, (not squared, or to the power of 5/2 or whatever)  because a volume has a dimension of length cubed.  Somewhere in the formula there has to be a length times a length times a length.

But it’s not just lengths we have in physics.  We have time as well. For example, the velocity is how fast you are covering distance  -  i.e. a distance in a given time (e.g. 80 km an hour).  We say it has dimensions of distance per time.  Acceleration would be change in velocity in a given time, and have dimensions of distance per time squared.

And there’s more. We have masses.  A force (equal to mass times acceleration by Newton’s second law) has dimensions of mass times distance per time squared. We can also add in for good measure a dimension of electric current, and a dimension of temperature.

Now, if you are trying to work out how something depends on a lot of variables (e.g. how the drag force on a particular car depends on the density and viscosity of the air and the width and velocity of the car) the need to get dimensions correct means that the form that the equation can have is very restricted.  If I say that force is equal to some combination of density, viscosity, width, and velocity, the dimensions of this combination MUST be equal to the dimensions of force (mass times distance per time squared). This severly restricts the possible equations, and is very, very useful.

This kind of reasoning is taken to its extreme in the study of fluid flow. Physicists ably supported by engineers construct a plethora of dimensionless numbers (numbers whose dimensions cancel out – an example is the aspect ratio of a room – the length divided by the width) from quantities such as density, viscosity, velocity, temperature, acceleration due to gravity, size, heat input …  Instead of discussing the behaviour of the fluid in terms of density, velocity, etc, they discuss it in terms of these dimensionless numbers, which results in much much shorter and more manageable equations.

So, have a go at thinking in dimensions, and, when you next come across a physics equation, check for yourself that the dimensions of the left- and right-hand sides are the same. (If they’re not, something has gone wrong).

Windy Hamilton Marcus Wilson Sep 24

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With our low-pressure-area-the-size-of-Australia still very much in residence, I’m beginning to forget what a still, sunny day is like.  I have to say that in my experience (six years) Hamilton is a pretty wind-free place. It’s certainly a lot quieter than where I lived near Portsmouth in the UK, which would be frequently blasted by south-west winds. That was one of the places where the trees tended to grow on a lean, denoting the direction of the prevailing wind. You could use them as a compass. So this week’s wind (which took down part of our fence at home) doesn’t feel all that unusual to me, though it probably does to long-term Hamiltonians.

So why is it so windy?  When I first learned about high and low pressure areas, I had in mind that the wind should blow from high pressure to low pressure. That seemed perfectly logical, and couldn’t understand why the weathermen talked about winds blowing anticlockwise around a low-pressure system (northern hemisphere). Why not towards the centre of the system, to even out the pressure?

The reasoning is actually quite subtle and involves balancing the effects of the pressure gradient with that of the Coriolis force.  Imagine you are at the north pole, and throw a ball southwards. OK, you don’t have much of a choice of direction, so let’s say you throw it southwards down the Greenwich Meridian. As the ball travels, the earth rotates underneath it. The rotation of the earth is from west to east, so, the path the ball appears to take to an observer on the earth is a curved one, bending towards the right. The faster the ball goes, the greater the force that appears to be acting. That’s the coriolis effect – in a rotating frame, there is an effective sideways force on something that is moving.  In the southern hemisphere, the sideways force is in the opposite direction.

Go back to the (southern-hemisphere) low pressure system and imagine a lump of air moving from high pressure to low pressure. As it moves it will experience a sideways coriolis force, which will bend it towards the left (anti-clockwise). When it has bent through 90 degrees, the coriolis force now is in the opposite direction (towards the high pressure) to that due to the pressure gradient.  At the right velocity, the two forces balance, and the air continues to move at a constant velocity (Newton’s first law) which will be along the isobars, perpendicular to the gradient.  In the southern hemisphere, this will be clockwise about a low pressure area, and in the northern hemisphere is will be anticlockwise.

Where the isobars are close together, as we have had for the last week, we have a strong pressure gradient  and we would expect a strong wind in order to give enough coriolis force to balance the pressure force. 

That’s quite a tricky explanation to do in words – I hope I’ve got it right.  I think after writing this there was good reason to be a little confused back at school as to what was happening.

Quantum finance Marcus Wilson Sep 22

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I would say I generally think captialism is a good idea, but after my experience on Tuesday night I am beginning to wonder. Does any of this sound familar?

Ten years ago I bought a policy (call it policy 1) with financial organisation A. It was a ten-year one – paying out just a few days ago (Little did anyone predict a global financial crisis at the time…but that is superfluous to the story).

About six months ago organisation A got gobbled-up by organisation B. They wrote to me to tell me the details and that, as far as I was concerned, nothing had changed.

A few months later, they wrote to me again, asking what I wanted to do with the funds from the now-maturing policy. I wrote back to say that I wanted them transferred to another policy (call it policy 2) I happened to have with organisation B.

Then I get a letter from B advising me that part of organisation B has been flogged off to organisation C. This includes the bit that looked after policy 2, but not the bit that looked after the now-maturing policy 1, which remains with B.

Everything has gone quiet about the maturing policy, so I phoned B. The number I had didn’t work, but I got a referral to a new number, which I phoned, which turned out to be C. Organisation C was very kind, and gave me the number of B’s new offices.

I phoned B and they said yes my policy 1 had matured and I would have to talk with C, because that’s who now owned policy 2, where policy 1′s proceeds (that remaining after the credit crunch) should have been put. C, however, said that since they didn’t take over the policy 1, it was B’s business not their’s. At this point I gave up for the evening, after no doubt helping lift NZ Telecom’s share price a bit by several international phonecalls to the UK.

So, to summarise, my finances seem to be locked in some quantum state, that can be described as neither policy 1 or 2, but some combination of the two, located in some superposition of organisations B and C.  This entangled financial state desperately needs some collapsing into an eigenfunction – namely my bank account, though getting it to do that seems like it will be a bit of a mission.

Clever lenses Marcus Wilson Sep 21

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I’ve just bought a new pair of glasses. The prescription is a little different from my old pair, meaning that although everything is slightly sharper the world seems to curve downwards a bit towards my right. That’s just my brain getting used to the new way in which the world is projected onto my retina – give it a few days and it will figure out what the image is supposed to be and things will look level again.

We’ve got a talk coming up at cafe scientifique on the human visual system. The particular question John Perrone will talk about is how the brain can make sense of an image that is continually jumping around on the retina – you move your head, and the image moves on the back of the eye – but your brain still makes sense of it all and you don’t see a blurry, shaky world out there.

I once saw a demonstration of a piece of real-time camera-shake elimination software. A video camera was stuck on the end of a long, flexible pole, and its picture was fed live to a screen. The guy demonstrating it then struck the camera hard, and it bounced quite violently on the end of the pole, but the image stayed almost stationary. I don’t know what processing was going on in the computer (presumably it wasn’t specific to the type of shake at the end of a pole – but I don’t know) but it looked pretty impressive.

A camera and the eye are similar in many respects, but there are a few differences. For a start, the eye has a pretty impressive bit of image processing ‘hardware’ behind it, namely the brain. The iris is pretty well the same – except in the eye it is in front of the lens, whereas for the camera it is behind the lens. Then there’s the focusing method. For a camera, the focus is changed by changing the distance between the lens and the sensor (or ‘film’ for people who remember what that is) – to focus on close objects you increase the distance of the lens from the sensor – while the eye focuses by changing the shape of the lens – to look at nearer things the lens is squashed to a lens with greater power – to look at far away objects it reverts to a lower power lens.

A short-sighted person like me has an lens to retina distance that is too long – which means the image from far-away objects comes into focus before the retina. To correct for this my glasses are made from diverging lenses – that counteract slightly the power of the lenses in my eyes, so that the image slips backwards a bit and lands nicely on my retina in focus. And I can see again.

 

Seal level rises Marcus Wilson Sep 18

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I received an email yesterday alerting me to an upcoming news brief on the implications of seal level rise as a result of climate change.

A rise in seal level is something that I hadn’t thought of before. I’m not sure whether we are talking about specific species of seal, like the New Zealand fur seal, or just seals in general.  And by level, do we mean population, or do we mean height above current sea level – i.e. would a large enough rise in seal level mean that they started taking up residence in Auckland’s suburbs in more significant numbers?

Maybe it was just an uncannily interesting typo.

The academic poster Marcus Wilson Sep 16

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I was having a conversation this morning about the status of the poster at an academic conference. At most conferences there will be one or more ‘poster sessions’. A ‘poster presentation’ is an alternative to an oral presentation – instead of preparing power point slides to send your audience to sleep, you do the same with a poster – maybe A0 size – in lots of colour. During the ‘poster session’ you stand by your poster, glass of wine in one hand (the other is left empty to wave and point enthusiastically at your masterpiece), and try to waylay – Ancient Mariner style – any poor conference attendee passing by and inflict the story of your academic research upon him. (At physics conference it is, usually, ‘him’, rather than ‘her’) 

Just how a conference organizing committee allocates presentations to ‘oral’ and ‘poster’ is one of the mysteries of modern science. For example, at the last three Australian Institute of Physics conferences I have been given a poster presentation slot, whereas my colleague has been given orals (actually, to be precise - he didn’t go to the last one – so it’s too of the last three). Why? I have no idea. So I was pleasantly, I think, shocked to get an email yesterday saying my abstract for AIP2010 had been accepted for an oral presentation. What did I do differently? Who knows. Interestingly, my PhD student, who also put in an abstract, has been allocated an oral presentation too.

There is a view amongst some people that a poster presentation is inferior to an oral presentation – i.e. being invited to give a poster is a bit of a snub at your research. I’m not sure whether there is any truth in this, but I would say that posters range from lousy to excellent in exactly the same way as oral presentations can range from lousy to excellent. 

There are a couple of advantages to a poster, however. First, is that you can spend time with the people who are really interested, and discuss in exactly amount of detail that they want. You don’t have the problem of talking to an audience that has both experts and non-experts in it. And secondly, you get to take it back home with you, and stick it on the wall of your lab, office, corrider, or, if very sad, your living room.  Posters are great for provoking interest in your research amongst undergraduate students, and thus are one method for persuading the best of them that they really want to do a PhD in your area. 

With this in mind I’m busy rearranging some of the physics posters our research group has amassed over the last few years. A bit of strategic placement is in order, methinks…

Measuring the speed of light Marcus Wilson Sep 15

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Earthquakes are not the only thing that can cause a building to move. Simple expansion and contraction as a building heats and cools can move walls around. Not so you’d notice with the naked eye, but certainly noticeable  if you have mirrors attached to the walls to guide a laser beam around a lab.

That’s what we have in one of our teaching labs for measuring the speed of light. We use a time-of-flight approach – basically we modulate the intensity of a laser beam so that it flips on and off a few million times a second – we send the light on a fifty metre path round the lab, and time how long it takes to complete the path.  There’s a little more to it than that, but that’s the guts of it.

But the problem we have in our new lab, that didn’t affect us too badly in the old one, is that the mirrors on the walls keep moving. It’s a real pain; one week the equipment can work beautifully, the next week it is almost useless. To adjust the mirrors involves getting up on a ladder – while at the same time someone else watches an oscilloscope output – that’s not something we’re going to do for the sake of a couple of undergraduates getting a good result.  The movement in the walls is tiny, but it is enough to mean that the returned laser beam no longer hits the detector.

The equipment is also complicated by the fact that the heating for the lab (and this lab is COLD) consists of ducted hot air blown out of the ceiling, straight above where the laser beam goes. Putting turbulent hot air into the beam doesn’t do much for its ability to travel in a straight line and remain a tight beam. Hot air has a different refractive index to cold air, and this means that the beam takes a bit of a shaky path from one mirror to the next, and spreads out a bit enroute; neither of which is particularly ideal. The solution to this one is relatively easy – let my students freeze – but its not a popular one.

Today, the experiment is working just fine, so I’m happy. What it will do tomorrow is anyone’s guess.

 

Why are eggs egg-shaped? Marcus Wilson Sep 14

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Here’s a nice bit of news that’s come out of Christchurch in the last few days. The kiwi eggs being incubated at Willowbank wildlife reserve survived the earthquake, and one of them has now hatched into an adorable baby bird, who now carries the name ‘Richter’.  (N.B. Kiwi aren’t adorable – from what I’ve heard they are vicious things – but very very endangered, so anything that warms people to them has to be good)

At first thought it might be surprising that an egg can survive through an earthquake, but eggs are well shaped for that purpose. The round shape means there are no sharp edges.  Sharp edges can act as stress concentrators. It is at these points that structures are most likely to fail. It’s why you can usually rip a piece of paper in half fairly cleanly. Once you have a small tear in it, the stress will be concentrated at the tip of the tear, and the tear will propagate forward. It’s the same reason why a small crack in your car windscreen can easily turn into a larger crack -  the stress is concentrated at the ends of the cracks.

Round things such as eggs obviously don’t have sharp points to them and therefore it is much harder to get a crack formed in the first place. Also, the shape ensures the shell is strong in compression (squashing) but weaker in tension (stretching). External forces are likely to squash it, whereas internal forces (the chick trying to get out) will stretch it, and it breaks more easily.

Egg shells also have a wonderful structure to them, that makes them very strong for their weight. It’s not really surprising eggs have to be like that, when you consider that they have to be built inside the female bird and go through the stress of being laid and then sat on. So I was not completely surprised to hear that the eggs had survived the quake.

Here’s a nice egg strength experiment to do at home, which I got from this Q+A site. It includes a great electron microscope picture of the lattice structure of an egg shell, which will give you some idea of its strength compared to its weight.

Pity the poor PhD student… Marcus Wilson Sep 10

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I’ve recently been asked to be an examiner for a PhD thesis.  This is the first time I’ve been given this honour. It is a slightly disconcerting thought as the success of the last three (or possibly more) years of a student’s life hangs on what I choose to say about his thesis.

When I think about it, the PhD examination seems a bizarre process. A student puts three years into researching a topic that possibly only a handful people in the world will ever be interested in, and then spends six months putting together a piece of writing (the thesis) that makes the previous three years sound productive, knowing full well that (if he or she is lucky) only three people will actually read it (themselves, their supervisor, and an examiner – and the last two might not even look at it properly).

So, out of duty to the poor student, I have been filling every available free second in the last couple of weeks reading through his thesis. At the end of this, I have to submit a report back to his university (in Australia) in which I make a recommendation about whether he should be awarded a PhD. Different universities have different procedures regarding thesis examination – in my case in Bristol many many years ago I had an oral exam with an external examiner and an internal examiner (the latter not being my supervisor – he had no part in the exam process). In New Zealand, getting an external examiner to come and do an oral exam can be prohibitively expensive (and Waikato insists on an examiner from outside NZ) so in practice this examiner usually submits questions to the candidate via an internal examiner. However, for the thesis I have been asked to examine, I merely need to make comments on it; I don’t get the chance to ask questions back to the student.

When the thesis arrived in the post, I was rather expecting to find with it a document on what I should look for as an examiner – i.e. some guidance on what would be ‘acceptable’ and ‘unacceptable’ theses.  Interestingly, there isn’t any. It really is left up to me to give a decision based on what I think.  Other qualifications seem to have extremely robust examination practices, where exam questions are extensively reviewed beforehand, then marking is subject to moderation, etc. etc., while the success of a PhD thesis hangs on someone’s whim.  Or so it would seem. I can of course compare it to other successful PhD theses I have read completely (let’s be honest here – this means three – my own one, that of a student whose PhD thesis I read during my PhD work, and that of a work colleague back in the UK who asked me to review it for her before she submitted it), but is that a really robust way of doing it?  I only have one of those theses available to me now, anyway.

The other interesting and unexpected thing is that I get paid for this examination. Not mega-bucks, but neither is it a trivial amount. I wonder whether my PhD examiner got paid (and does getting paid influence the chance of the examiner returning a good report?..I wonder…)

For the record, I am attempting to give an honest, unbiased report back on this student, whose future career hangs by a thread in front of me while I hold a pair of scissors…..

 

 

 

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