SciBlogs

Archive May 2011

Schlieren imaging Marcus Wilson May 31

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I’ve been sent this link to a movie of a shock wave from a trombone.  You’ve got to feel sorry for the poor clarinetist who is sitting in front.

This sort of thing can be done neatly with the method of schlieren imagaing. (See some more examples here) This is a ‘simple’ way of picking up changes in refractive index of something (usually air or water) that is transparent, in a way that a normal photo would not. It’s done by putting a knife-edge at the back focal plane of the lens, which cuts out some light rays in such a way that the end effect is to give contrast to the places where refractive index changes rapidly. These places are often shock waves, where the density of air is large. A proper mathematical description gets a bit horrendous (e.g.here, which involves the wonderful mathematical beast the Hilbert Transform), so I’ll leave the discussion there.  But if you don’t mind taking your camera apart and reconstructing it, in principle at least it’s not a difficult thing to do.

P.S. I’ve been rushing through my tax return trying to get it done before departing overseas at the end of this week  (…the blog may take a back seat until the end of June…) and discover that the IRD now refer to ‘natural persons’.  What other sort of person is there?

Wet house, dry house? Marcus Wilson May 27

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We’re looking for a new house at the moment.  We’ve decided to be a lot more environmentally friendly and shift out of Cambridge and move to Hamilton to cut down the pesky commute in the mornings and evenings. We haven’t made much progress, though, with finding a house – what we’re looking for is in short supply in the areas of Hamilton that we want to live in.  What there is seems all to be for auction.  The search is made all the more pressing because we’ve now sold our Cambridge house. We had it on the market just five days. 

Anyway, getting to the physics part, we got a house inspection done of something that we might bid on at auction in a couple of weeks time. That’s the trouble with auction, you’ve got to get your research done before you bid, and that costs you money for a house that you might be outbid on. Doesn’t seem fair to me.  The house inspection people must love it though. One of the bits of technology that is becoming fairly common for house inspections is thermal imaging.

 I know a bit about infra-red physics and thermal imaging, having used it in a previous life, but in a rather different context to the housing people. Mostly we think of thermal imaging as detecting hot things  (hot things give off more infra-red radiation than cold things – other things being equal) - you’ve probably seen imagery of fleeing criminals at night on those police TV shows – or this sort of thing.  However, that’s not the only source of contrast on an infra-red image.

The amount of radiation emitted by an object depends not only on its temperature but on its emissivity. A blackbody is 100% emissive and completely unreflective, but you can also get objects that are very reflective and not very emissive.  N.B. Don’t think that just because something’s white that it is non-emissive in infra-red – it could be very reflective to visible light but very un-reflective (i.e. emissive) to infra-red. Emissivity depends on wavelength.  Emissivity and absorption are related too – a good emitter of infra-red is also a good absorber of infra-red. 

Water is a pretty good emitter and absorber of IR. Although I"ve never carried out a systematic experiment on it, I would image it’s a lot more emissive than dry gib board and wallpaper, as used in houses.   Therefore a damp piece of house is likely to look different to an infra-red imager than a dry piece. And we can get a contrast between the two, even though the two areas may well be at the same temperature. In the visible band, of course, we can’t see water content, since it doesn’t change the reflectivity of the object.

IR will also help detect coldspots, of course, e.g. patches of ceiling that aren’t insulated properly. The systems aren’t cheap, but give you some confidence that the house you might bid on will be warm and dry, as turned out to be the case with this house.  And that’s always a good thing in Waikato.

 

 

 

The science logbook Marcus Wilson May 24

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In order to help my attempt at doing a ‘reflective journal’ as part of my Postgraduate Diploma of Tertiary Teaching, I’ve had a read of an article by Simon Borg:  "The research journal: a tool for promoting and understanding researcher development",  Language Teaching Research 5(2), 156-177 (2001).  I was given it as an example of how journal-keeping can be useful to research. 

This particular example is focused on education – maybe that’s why I found it quite hard going.  Borg talks about the way that he has used his journal as a source of data in order to guide his research.  My initial thinking was that this was all very wishy-washy (I’m a scientist, after all, reflecting is what an electromagnetic wave does when it encounters a change in impedance); how does this apply to science? Then I had a sudden flash of understanding.  What Borg was referring to as a reflective journal, I would refer to as a logbook.

If you are doing science research, it is almost essential to keep a logbook. This is where you write down what you are doing, why, sketch out your methods, record your data (OK, so most of this goes on the computer now), analyze it, write down your thoughts on what is happening, identify what you need to change, discuss what you might try next, etc.

This, I believe, is basically what Borg was getting at. Phrased in the ‘logbook’ way, rather than ‘reflective journal’, it sounds a whole lot less intimidating and perfectly achieveable to a hardened scientist like me.  Logbooks are essential in science – they allow you to come back to something you did yesterday, last week or last year, and see what was happening.  They are a record of your work – but not just of your work – also the motivation for it and the thinking around it. If you don’t write those things down, they don’t get recorded, and the logbook is the place to put them.  They are a vital research tool, just as Borg has found in the education context. (And, in some cases, a vital piece of evidence in patent disputes that you actually thought of an idea independently and before someone else)

As an example, I remember a few years ago back in the UK when I had a real job that I was needing to do a particular experiment.  Now, I was reasonably certain that an ex-colleague (who had left the company a year or so previously) had done this experiment already. I went searching in the filing cabinets for his logbook, found it, and within ten minutes or so had located his discussion on this experiment. Such was his good record keeping and writing, I was able to discern exactly what he had done and what he had found out, just from reading the log-book. And that meant I didn’t have to spend a week repeating the experiment.  The logbook pays for itself in the end!

 

 

The physics of slugs Marcus Wilson May 19

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At Tuesday night’s Cafe Scientifique, we had a very entertaining discussion, led by Mike Wilson from AgResearch, on that most cute and cuddly animal, the slug. Let’s face it, in terms of looks and popularity, the slug doesn’t have a lot going for it, but it’s certainly a very interesting creature.

I learnt a fair bit about its biology, but the most interesting bits for me were the physics. For example, how slugs move is a good example. They create waves that travel down their body – the bits of the slug that contact the ground then push the slug along. If you look at the photo of the underneath of the slug-on-glass, on the handout here, you’ll be able to see the wave pattern on the bottom of the slug.

Then there’s slug navigation. Slugs have homes, apparently, which they will return to after their foraging runs around dawn and dusk.  How do they navigate their way back?  I wondered whether it was scent, but, according to Mike, it might well be by magnetic field. Wow.  I knew some birds did that, but the slug? Clever little creatures.

But the physics isn’t all in the slug’s favour.  There are a few physics based weapons of mass slug destruction out there for the gardener’s use. The slug electric fence sounds an exciting one. Just two strips of wire, separated by an insulator, and wired to a battery. The sluggy crawls over them, and completes the circuit between the two, and gets a shock. That’s usually sufficient to knock them off the wire. However, Mike commented that some slugs can get stuck across the two wires and get fried. Not very nice for the poor molluscs. Some slug fences are just single-wire based – you can use the ground as the other wire – so if the slug is in contact with both the wire and the ground it will form part of a circuit and get a zap. It was suggested that we don’t even have to wire-up the fence – the slugs will get a shock off just a piece of copper wire due to electrochemical effects – in a similar way to the way you can get a small shock if you start chewing tin foil and have amalgam fillings.

But, in my experience, by far the best and most bizarre slug zapping machine is simply the yoghurt-pot filled with beer. Slugs love it, and drown happy.

 

Physicists on film Marcus Wilson May 17

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Last night we went to the movies. We had free passes that needed using by the end of the week, so we turned up at Chartwell early evening without knowing what was showing. Not wishing to see a film about adultery, we decided against ‘Water for Elephants’ (or whatever it’s called) and picked Thor. 

If you haven’t seen this film, I’ve got just one word of advice. Don’t. We paid nothing for it, and it was  worth nearly every cent. It takes a ridiculous scenario, embellished with wooden acting, mixed with overcooked special effects, and held together with a plot so transparent you could stick it in a frame and call it a window. I can only think that Antony Hopkins was paid a huge amount for his appearance in a film that clearly didn’t deserve his presence.  

And then there is the gripe with the physicist characters. Three of the characters are physicists. They are portrayed as sad losers, with no friends, a single-minded focus on astrophysics, driving a van packed with electronics with lots of flashing LEDs and ‘beep’ and ‘ping’ sounds. Seldom do we see on film physicist characters who bear much relation to real physicists. True, it’s a (bad) work of fiction, and their relativity obsession was rather essential to what passed for the plot, but how much does that portrayal influence people’s thinking about what physicists are like.  I wonder. All I can say is real physicists aren’t like that.

But given that rather few people are likely to go and see this film, I guess the impact is hardly going to be significant.

Paint Marcus Wilson May 13

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Question: What’s more tedious than watching paint dry? Answer: Waiting for a dry day so you can put the paint on in the first place.

Or, to be closer to the truth, waiting for a dry day so that the guy you’ve hired to do the painting can get on and finish it.  Getting the outside of a house in Waikato painted in May always had ‘stupid idea’ written all over it. It should have been done earlier in the year, but, for various reasons, it slipped to April, and then it was the Easter holiday, and then it started raining, and, about a month later, it’s still not quite finished. Hopefully, when I get home tonight, the scaffolding will be down and it will be all done, but, given the showers that have been marauding through the region today, I have my doubts.

Anyway, since paint is at the forefront of my mind at the moment, it’s worth a comment on just how much science is in it.  Water-based emulsion paint is fascinating – what is most perplexing is that the  paint uses water as a solvent so you can apply it easily, but dry paint keeps water out. There’s lots of physics and chemistry going on with regard to how the components of the paint work together so that it does what it should. Getting paint to dry nicely without cracking, rippling, blistering and so on needs a bit of experience, as I’ve found out in my own efforts at painting, and is basically why I get someone else to do it. (Plus I’m not going up on ladders/scaffolding to do the high bits.) Certainly one thing is to make sure the surface is dry before you start – otherwise water can get trapped underneath which doesn’t do the wood much good and, once the sun gets on it, will cause the paint to blister as the water underneath evaporates and pushes up the dry skin of the paint.

Therein, of course, lies the problem. Where does one get a couple of dry days from in May?

I’m not a paint expert, but a bit of browsing throws up some quite nice articles on what is happening with paint. For example, here’s a nice easy-to-read one from Harvard. The message: Interesting physics is everywhere. 

Have a good weekend.

Student DIY Marcus Wilson May 10

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Here’s a summary of a conversation I had with a student in a lab yesterday.  I can’t remember the exact words, but it went something along these lines:

Student (showing me some work): Is this right?

Me: What do you think?

Student: I don’t know

Me: Well, is there anything about it that makes you suspect that it isn’t right?

Student: No

Me: So do you think you’ve done it correctly? – I mean, what about it suggests that it’s doing what is should be doing?

Student: I don’t know. Is it right?

And round in circles we went.  I think from the student’s point of view, he was just unconfident in his ability. But what I was trying to do was to get the student to take the initiative with assessing his own work.  It’s all very easy for me to say ‘Yeah, that’s right’, but I suspect if I did my student would be learning nothing from it. Someone doing physics in ‘the real world’ will have to make their own judgements on whether they have done a task correctly. Knowing what to expect, and how things should behave, is a key step to doing any kind of laboratory or modelling work. Put another way, if you are selling a product you need to be able to recognize if it’s faulty. Too often we set students activities that lead them to think that they can always look up the answer in a textbook, website, or get model answers from a teacher, etc – i.e. take away the need for them to do any independent thinking and critically assess their own work.  

So, in a lab situation, with an unconfident student, I am reluctant to say ‘yeah, that looks right to me’ without the student first thinking about whether it looks right to him or herself. It’s probably infuriating for the student, who probably just wants me to say ‘yes’ or ‘no’, but I am sure they will learn more if I don’t.

Three heads are better than one Marcus Wilson May 09

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It’s often surprising how different people can bring different approaches to the same problem, but in a way that gets you moving forward. I experienced a good example last week. A PhD student has been tangled in a nasty net of circuit analysis, trying to understand how a particular circuit does what it does. I’ve been helping him out on this, as has an electronics professor here.

After a couple of head-scratching meetings, between which we all were thinking about what was going on, we cracked it.  I think the key thing was that all three of us were talking about it together. We each had grasped different aspects of what was going on, and, putting them together, we got it assembled into a complete picture. I think if we had attempted to solve it individually, we would probably still be at it, maybe for a while yet. This of course is much to the relief of the PhD student concerned, who can now move on.

It’s a fairly trivial example, but one that drives home the fact that progress is quicker when you collaborate, particularly when you talk to people whose strengths are in areas different to yours.  Shaun Hendy, a fellow sciblogger, has talked a lot about innovation networks, NZ as a city of four million, and the like.

So I very much encourage students to talk with each other when studying. They are likely to learn far more by interacting with their peers rather than talking directly with me. (Of course they can take this too far and hand in identical assignments, which sometimes happens, but the fact that they want to think about a problem together is good). Plus. when they get out into industry and have to tackle real problems, that’s the sort of thing they’ll have to do.

 

My first go with Votapedia Marcus Wilson May 06

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Yesterday I had my first attempt at using the Votapedia audience response system.  For those that don’t know it, it’s a web-based thing that’s come out of CSIRO in Australia, and in the broadest of broad terms it gives a teacher (me) the facility to do a Who Wants to be a Millionaire? ask-the-audience question. Last year, I gave all my students cards (actually, bits of paper) with A, B, C, and D on them, so when I ask a question they can vote on it.  I use this as part of formative assessment in each lecture – first so that I know whether my students have understood the point I was trying to make, and secondly so that the student knows whether they’ve grasped it or whether they need to do some work on it, and thirdly as a tool for allowing peer discussion.

Ideally, I’d use audience clickers, (as in WWTBAM) but they haven’t yet made an appearance at Waikato, so Votapedia is a cheap imitation.  Here, the user (me) sets up an account on Votapedia (www.urvoting.com) and enters his or her question into the system through the web.  When ready, the question is ‘activated’, and the students get to see the question, possible responses, and phone numbers to ring to vote. The votes are tallyed automatically and displayed nicely on a bar graph.

Sounds easy?  Well, here are my experinces. I’m not the only person to have blogged about this, e.g. see https://davidtjones.wordpress.com/category/ilecture/votapedia/ .

First, registering wasn’t totally straightforward, and I had to call in our IT people to advise, but got there in the end. Once I was a bona-fide user, setting up the questions was a piece-of-cake. However, in the lecture room yesterday morning it all got a bit muddled.  First, the room happens to be in the basement of the world’s most ugly (now that the Tricorn centre in Portsmouth has been demolished) reinforced concrete building. About quarter of the class couldn’t get mobile phone reception. Then, it seemed that responses were very slow to come in.  From someone voting, it was many, many seconds before the response was noted by the system.  People were confused about whether their vote had got through (the call ‘drops’) and a voter hears a number not obtainable tone.

But, perhaps most significantly, only about a third the class actually voted, whereas with the A, B, C, D cards it was close to 100%. Of course, some of them couldn’t get reception, some of them might not have had the cellphone with them (what, students not have their phone glued to their hand?), and some might not have believed me when I said it wouldn’t cost them anything. Perhaps some just couldn’t be bothered – given that I had no real way of knowing whether they were voting or not. To get formative assessment to work properly, I need a decent chunk of the class to vote.

I’m going to give it another go, and then as a class we’ll make a decision on whether to stick at it or go back to the cards. From my point of view, if only a third of the class votes, for whatever reason, it won’t be worth the fuss.

Piezoelectricity Marcus Wilson May 04

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I went to a very informative seminar this morning by Peter Pott, of the Technical University, Darmstadt, Germany. He gave a very nice introduction to the world of piezoelectric devices.

In short, a piezoelectric thing is something that acquires a voltage across it when it is squeezed, stretched, sheared, etc. They can be used as sensors, e.g. in microphones, or, perhaps more usefully, as actuators.  In the case of an actuator, you would apply a voltage to the material, and it would distort accordingly. That means you can use them to move things in response to an electrical stimulus.

Of course, electric motors do the same, but tend to be big and involve lots of parts. A piezoelectric device is dead simple, really.

Possibly the most familiar example is in gas lighters – squash a crystal and it gives enough voltage to throw a spark across a gap, which will ignite the gas. More high tech, but also now well used, is controlling the fuel injection within a diesel engine. This application is pretty taxing for the piezo-device, because it gets so hot, close to the so-called Curie temperature at which the crystals change their structure and lose their piezoelectric state.

Other examples include precise control of mirrors in telescopes (adaptive optics) to ‘cancel’ the distortions imposed by the atmosphere; some very, very cool motor designs, including the famous Burleigh Inchworm, which can position an object extremely precisely; and active control of vibrations.  

Finally, Peter explained that since a Piezoelectric device doesn’t have any electric current flowing through it, it isn’t magnetic (unlike a motor) and so you can use them safely in conjunction with MRI machines. 

Overall it was a nice seminar and I learnt a lot about these devices, particularly about the very many places where they are used. 

 

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