SciBlogs

Archive July 2010

Put your trust in a computer… Marcus Wilson Jul 30

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I’ve mentioned before the way that it is tempting to put your faith in the output of a computer program, particularly if it involves impressive graphics and displays words that you don’t understand.

But this phenomenon doesn’t apply just to computers. I’ve been seeing it in my students’ lab work too – where an instrument for measuring something already seems to be connected, how many of my students actually bother to check that the instrument is measuring what they need it to measure? If it gives an output, especially a digital one in a nice font, it has to be right. Hasn’t it?

Engine oil and lubrication Marcus Wilson Jul 29

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We have a couple of cars that are beginning to age, and with that do things like breakdown occasionally and go through oil.  That an engine can survive 200 000 km quite happily is to a large extent down to the lubrication.  Just a few quick calculations can give the scale of the problem the oil has to solve.

How many revolutions of the engine does 200 000 km represent? Doing some very rough calculations, if I run the car at 60 km / h at 2500 rpm (60 km/h is probably a reasonable average and makes the maths nice and easy), that means it does one kilometre a minute, so 2500 revolutions per kilometre.  Multiply that by 200 thousand kilometres gives us 500 million revolutions of the engine in its life so far. Half a billion. That’s a lot of up-and-down in the pistons and opening and shutting of valves, amongst other things.  (Though to put it in perspective, compare it to the number of times someone’s heart will beat during an average lifetime – about 2 billion).

How much damage is done to the cylinder lining, cam shaft, valve seals, etc etc with each revolution? Well, let’s assume for starters that each revolution takes off one plane of atoms from our moving engine surfaces. In other words one atom’s thickness is removed from the inside surface of each cylinder each time the piston passes up and down. In metals, atomic spacings are a few Angstroms, and angstrom being ten to the power of minus ten metres. Let’s say 3 Angstroms for the sake of argument, though I’d have to look it up.  Doesn’t sound much.  But what’s 3 Angstroms times half a billion revolutions?  About 15 centimetres. 

Now, clearly my cam shaft hasn’t reduced in radius by 15 centimetres since the car was new. That means that my initial guess of one atom thickness per revolution is a large overestimate of the damage done. Possibly we are talking of more like one atom thickness removed  per hundred revolutions. I think that’s pretty impressive really, and it means some pretty decent lubrication.

And 200 000 km isn’t really that  large a distance for an engine to do, anyway.

 

Newton’s third law Marcus Wilson Jul 27

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In yesterday’s tutorial I had an enlightening discussion with the students about Newton’s third law. Enlightening for me just as much as I hope it was for them. You’ll find the law in textbooks phrased something like "for every action there is an equal and opposite reaction".

Sounds simple, and to someone who is well trained in physics, it is. But to others, as I found out, it is intensely confusing. We did some examples (prepared by a well-known science publishing company) designed to elucidate the law. This draws out things like:

1. The action and reaction are on different bodies

2. The size of the force on body 1 due to body 2 is equal to the size of that on body 2 due to body 1 (An ‘equal reaction’)

3. The directions of the two forces (remember, forces are vectors) are exactly opposite.

4. The action and reaction are the same ‘kind’ of force. Eg if body 1 exerts a frictional force on body 2, body 2 exerts a frictional force on body 1.

 Having gone through all these, the students all appeared to be understanding it. Except they didn’t. I know they didn’t because I then gave them a practical example – explain how one can walk on a floor but not very easily on ice – and suddenly points 1-4 all went out of the window. Instant forgetting. I was hearing things like – you put a force backwards on the ice with your foot but the reaction is now perpendicular and your foot slides sideways.  Perpendicular? What happened to ‘equal and opposite‘?   Or ‘you put a force on the ice but because its slippery it can’t put a reaction back on you?’. Hmmm. What about the forces being ‘equal and opposite’?

What does this tell me? Well, first, that Newton’s third law isn’t the easy-peasy physics that I might think it is. That one simple sentence hides a series of quite tricky concepts. Secondly, teaching it in one tutorial is wishful thinking. (Actually, this was just one question out of four that I was supposed to cover here – WAY too much for a session). And third, student discussion is really, really valuable. Both to them, and to me, As a group, they were beginning to home in on the correct understanding after much discussion (good for them),  and their comments were telling me that they still didn’t get it (good for me, because it now guides my teaching, which, of course, ends up being good for them).

I give the same tutorial to a different group on Friday, and I think I’ll abandon the other three questions and focus on this one.

Students – apologies if I’ve misrepresented your comments. I’m typing from memory. I did find our tutorial very, very enlightening, so thank you for teaching me how to teach. (Even if you didn’t realize that was what you were doing).

Mindless Monday Marcus Wilson Jul 26

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Here’s something that is only tenuously related to physics, but will help you cope with your Monday.

Yahoo’s top ten worst cars ever. Those of you who want some physics can evaluate whether the final statement on the description of the Mini Moke (that you were in danger of falling out on roundabouts) is over-exaggeration.

http://uk.cars.yahoo.com/23072010/36/top-ten-worst-cars-0.html

Fluctuations in Earth’s magnetic field Marcus Wilson Jul 23

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In uncertain times, its good to know there are some things that never change – such as day follows night, my compass needle always points in the same direction, and England will always underperform at the World Cup. Well, scrub the middle one, actually. In our lab, at any rate, there are some shocking variations in the magnetic field.

I know that because I’ve been doing some more playing with our Earth’s field NMR / MRI equipment, and finding that my resonance frequency is harder to pin down than a politician in an expenses scandal.  The system uses the earth’s magnetic field to split the nuclear energy levels of the hydrogen nucleus (proton) in a sample of water; the amount of splitting depends on the strength of the field, and the resonant (Larmor) frequency is proportional to this. When the magnetic field increases, so does the resonant frequency. 

In the lab it’s quite common for the frequency to shift by 0.1% (1 part in a thousand) in an hour. If you leave it a couple of weeks between measurements, it may have shifted a whole lot more – in excess of 1%. Some of the drift in resonance will be because the earth’s field changes,  for example, there is a diurnal cycle in the strength of the field due to the solar wind interacting with the earth’s field. But I suspect a lot of it is because of the nature of the lab – in a reinforced concrete building (i.e. one with lots of steel in it) surrounded by heaps of other lab equipment. There’s a great deal of potential for the earth’s magnetic field to be changed by the lab environment.

It’s no big hassle, usually, because usually the resonance hasn’t drifted too far from where it was when I last looked, but it can take a bit of time to re-optimize the equipment before we get the students to use it.

And remember, that a 1% shift is trivial when compared to the kind of movements the field makes on a longer timescale – e.g. the drift in position of the magnetic poles, and the reversals of the field at times during the earth’s history.  A good overview is found here.

 

Aaaarrrhh first year Marcus Wilson Jul 21

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It’s no secret that I don’t like teaching first year classes.  I find third year undergraduates far easier to teach. I think the main reason for this is that with the third years I don’t have such a large gap between my knowledge of the subject and theirs. That means that I don’t need to think so much about whether I am using words they are not familiar with, or whether my explanation draws on contexts and phenomena that the class hasn’t seen before. I know others take the opposite view – third year classes are harder because the material is more advanced – but to me that’s not a problem. What is a problem is communicating, and it is easier for me to do so with students who are closer to my ways of thinking.  Plus third years tend to speak a lot more and let you know when they don’t follow something, so it is less easy to lose a whole class without knowing it.

On Monday I did a first year tutorial in which I ended up in a horrible tangle  trying to explain something that to me is really simple. To be fair on myself, I think the question that I had to explain (which came from a website) was badly put together, but I should have done rather better than I did. First year teaching takes real practice (I think it does, anyway) . I’m very envious of people like Alison Campbell who excel in teaching large groups of first years.

As part of my PGCert in Tertiary Teaching, I experimented last semester with a method of finding out whether my class (a second year one in this case) is with me or not. (See for example Turpen and Finkelstein, Physical Review Special Topics, Physics Education Research 5, 020101 (2009) ) It’s a well-used method in physics teaching, though I gave it a bit of adeptation for my class. Essentially its formative assessment – ask the class multiple choice questions at the beginning of the lecture relating to last lecture’s material and have the class discuss it in pairs - not to test them for the sake of allocating marks, but for me to know where their understanding is at.  It worked well, I think – there were questions that the class struggled with that I thought they’d have grasped easily. That has got to be good overall for the students, because it allows me to go and unpick their reasoning and correct misconceptions. In a subject like physics, where so often one concept is built on another, the teacher (me) needs to know whether the students have that foundation or not – if not, there really is no point going on.

That’s another reason why I find third years easier to teach – by the time they reach third year, they have grasped those underlying concepts (if not, they’d be failing bigtime in second year). That means less preparation on my part is required. Maybe I’m just lazy.

Infra-red and heat Marcus Wilson Jul 19

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Here’s a bit of physics that’s coming up in my lectures – what’s the connection between heat and infra-red?  You’ve probably seen imagery from ‘thermal imagers’ or infra-red (IR) cameras, usually on police shows, taken from a helicopter as it follows a suspect fleeing down some alley-way at night.  You’ll see that ‘hot’ things (like the bonnet of a car or a person) will be emitting strongly, whereas colder things emit less. Click here for a more cute example.

Infra-red is part of the electromagnetic spectrum, and is the region with wavelengths a bit longer than visible light (about 0.7 micrometres to about 14 micrometres). Our eyes aren’t sensitive to it, but it is not so difficult to build equipment that is. We ‘see’ because our eyes pick up light that has been reflected from an object (outdoors in the daytime, it will be sunlight that has been reflected). An infra-red camera will ‘see’ because it picks up infra-red light that has been emitted by an object AS WELL AS light that has been reflected (e.g. from the sun).  The sun emits a truck load of infra-red light, just as it does visible light. 

Why do everyday objects emit IR but not visible?  It’s because of their temperature. A blackbody (an object that doesn’t reflect light) will emit energy across a broad spectrum, and just where this spectrum is centred depends on its temperature. Hotter objects have a higher fraction of their emissions at shorter wavelengths than cooler objects. The sun (about 6000 Kelvin) emits a lot of energy in the visible spectrum (400 – 700 nanometre wavelengths), but an object much cooler (such as me) will emit virtually zero energy in the visible spectrum – instead it will be all in the infra-red region (and longer wavelengths still). An object somewhere between the two in temperature (e.g. a log on a fire) will  emit a bit of visible light, but this will be at long-wavelengths, so will look red or perhaps orange – it will never get hot enough to start looking green or blue.  

So an infra-red camera used at night will tend to see warm or hot things. It’s also perfectly possible to use it during the daytime; it will still ‘see’ warm objects, but interpreting the image will be complicated by the fact that the sun also irradiates the scene with infra-red. So in that sense, IR is not quite synonomous with heat – a cold object that happens to be quite reflective and have the sun on it could look quite bright to an infra-red camera.

Also, bear in mind that there are other forms by which heat can move. With infra-red (and visible) we are talking about radiation – remember that heat can pass by convection (e.g. circulation of warm air) and conduction (flow through a solid such as the bottom of your frying pan). And, while we’re at it, remember that visible (and ultraviolet) light carries heat energy that can heat an object as well – bathe yourself in u.v. and you’ll start burning nicely. So we need to be a bit careful using the words infra-red and heat synonomously.

 

Look after your graduates Marcus Wilson Jul 15

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I heard a snippet of Stephen Joyce on the radio this morning saying that the government may link funding of tertiary education to graduate employment – i.e. the amount of money given to The University of Waikato to support its teaching would be linked to the success of its graduates in securing employment. 

On the face of it, I think that’s perfectly reasonable. Of course, I have heard no details, and that is where the issues are likely to be. A government spends a huge fraction of its income on education, and it is utterly sensible (I would say with my tax-payer’s hat on ‘essential’ ) that it carefully considers what value it is getting from its expenditure. But I can think of a few issues here.

What would be meant by graduate employment? Would the proverbial job at McDonalds count as graduate employment? Does the job have to be closely aligned with the actual degree course undertaken? If so, how close? Who defines that? University and polytechnic courses can teach skills that aren’t tied to the actual academic material – our science graduates should be able to do things like write a coherent written report and give presentations – and those skills are useful in a great range of jobs, not just sciency ones.

All that remains to be defined. But one positive spin-off I can see for the universities is that it will force them to keep good track of their graduates. My two old universities (Cambridge and Bristol) send me frequent magazines and emails (in the case of Cambridge) letting me know what’s happening – and one reason is that happy graduates that still feel part of their former university have a habit of giving them money. And some graduates  will go on to earn rather large salaries (I wish), and become a useful source of income for their institution.  In fact, Cambridge is exceptionally skilled at asking for donations – I get at least one phone call a year from someone pursuing this point (in a very nice way) – even now that I am in New Zealand (though just once the caller didn’t register the time-zone difference … he didn’t get a happy graduate on the end of the phone at midnight).

More money from graduates means there is less reliance on government income, which means there is less for us lecturers  to fret about when ministers hint at changes to the funding system. I wait to see what will happen

Peak Oil, peak platinum, peak physics Marcus Wilson Jul 14

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I guess most of us now are familiar with the concept of Peak Oil. At some point (maybe about now) oil production will peak, then it will be in decline as reserves run out. The only way around this issue is to reduce our use of it, which means, for example, reconsider our transport options.

At cafe scientitique last week, Shaun Hendy drew our attention to some other resources that are looking decidedly finite. The specific ones he drew on are platinum and palladium, which are used for high-tech applications. Since demand for high-tech equipment is growing rather faster than our demand for oil, one might speculate we could be hitting peak platinum before peak oil, and perhaps even that peak platinum might hit our civilized western mobile-phone-and-ipod-and-general-gadget-loving cities harder than peak oil. The price of platinum  certainly has gone up sharply in the last few years, and nearly all of it comes from a single source in South Africa (imagine the politics if nearly all our oil came from a single source…) We may have to become more clever in recycling high-tech equipment.

Another ‘peak’ example, though of a slightly different form, I read about in this month’s Physics World magazine. The world is running short of Helium 3.  (That’s the isotope of helium that has two protons and one neutron in its nucleus.) Helium 3 has previously been in abundant supply as a by-product of nuclear weapons development – tritium (an isotope of hydrogen), which is produced in nuclear-weapons reactors, decays into Helium 3.

Why is this an issue? Well, helium 3 has some seriously odd properties when it gets cold (e.g. see Wikipedia) which means that amongst other things it can be used to cool systems to very low temperatures (much less than 1 Kelvin). And low temperatures are important for investigating basic physics, such as quantum effects – for example there is minimum thermal noise to mask what is going on. Some condensed matter physicists are getting a little worried that not enough is being produced. Recently, supplies have been sidetracked into screening the US borders for smuggling of nuclear material (ironically, the very stuff that produced the helium 3 in the first place; helium 3 captures neutrons very easily so can be used as the basis of a detector). 

The most pessimistic condensed matter physicists might even go as far to say that we have reached ‘peak physics’, unless we are able to get a decent supply of Helium 3 back to the physics labs. It’s not an issue I’d thought about until reading the article.  I suspect there may be several other materials that also fall into this ‘peak’ category; you might be able to think of a few.

 

Yuk physics Marcus Wilson Jul 13

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What’s small, brown, damp, disguisting, goes round in circles, and is still alive?

Answer: A cockroach in a washing machine.  Finding one in amongst the sheets as you haul them into the laundry basket isn’t very pleasant, I can tell you. Unfortunately, in the northern North Island, the Gisborne cockroach (an Aussie import) is rather ubiquitous and you just have to get used to having them turn up just where you don’t want them.  Our useless ball of fur that masquerades as a cat isn’t much help – he’s never shown much interest in racing across the floor and intercepting one as it scuttles from under the cooker to under the fridge, though he is quite adept at racing across the floor and intercepting a box of cat biscuits before it reaches his food bowl.

Cockroaches are nigh-on indestructible. The rather soggy specimen I pulled from the washing machine had evidently not only survived the washing, but also a spin. That’s pretty impressive. A rough calculation of the centripetal acceleration in a washing machine spin is fairly simple:  At say about 400 rpm for a top load spin, we have an angular velocity of 2 pi  times 400 rpm / 60 s min-1  which gives us about 40 radians per second. Acceleration is then angular velocity squared times radius of the spinning object (maybe about 30 cm), giving 40 s-1 times 40 s-1 times 0.3 m or about 500 m s-2.  Compare that to the acceleration due to gravity of 10 m s-2. The cockroach is experiencing about 50 G of force. 

Imagine if you weighed 50 times what you do now?  50 times 65 kg is 3.5 tonnes.  If that mass were placed on you I doubt you’d survive very long  (A large car is about 2 tonnes in weight). But the cockroach – that’s another story.  This one did loose a couple of legs in the process, but he was still kicking with the remaining ones at the end of it.  Yuk.

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