SciBlogs

Archive July 2011

Sorry for the lack of entries… Marcus Wilson Jul 28

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My lack of activity on-line can best be explained by my timetable:

Monday: Teach (almost) all day

Tuesday: Teach (almost) all day

Wednesday: Teach all day

Thursday: Teach (almost) all day

Friday Morning:  work on portfolios for postgraduate certificate of tertiary teaching:

Friday Afternoon: prepare teaching for Monday-Thursday next week.

 

Thermal expansion Marcus Wilson Jul 24

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In the last couple of weeks I’ve been fascinated by the amount of thermal expansion demonstrated by the chimney flue in our new house. Like many New Zealand houses, there is a log-burner located in the living area. The flue basically consists of a vertical column of large (steel?) cylinders slotted into each other, going from the burner straight upwards through the ceiling and into the chimney on the roof. We have a two-storey house now, so the flue is pretty tall, and you can see it all the way up. 

When cold, the lower section (cylinder) is wobbly. You can slide it up and down a bit – up until it is snug into the cylinder above it, or down until it is snug into the burner below it. By a ‘bit’, I reckon nearly a centimetre. I can only assume that the whole stack of cylinders is well secured at the top (the second and subsequent ones are all screwed into each other), so the thing doesn’t all drop with gravity.

However, with the fire going, this lower section starts to fit snuggly at both top and bottom ends – and when its really hot, there is no play at all on the cylinder. (N.B. I haven’t actually tried holding the thing and moving it when it’s hot – in case you are worried – but I can see it is fitting very snuggly). I can watch and see it happen. It gets longer. Quite fascinating.

Really, there’s nothing terribly surprising about it, but you don’t usually ‘see’ thermal expansion happen before your eyes. (The effects perhaps are more clearly seen – e.g. buckling of rails, cracking of concrete, etc.) Stainless steel has a thermal expansion coefficient of about 2 times 10 to the power minus five per degree C.   That means a length of 1 metre will expand by 0.02 millimetres for every degree raised.  Raising the temperature by 50 degrees C  will lengthen it by a millimetre. I’m not sure exactly what temperature the exhaust gas gets to – but Wikipedia says 600 degrees C for the combustion of wood inside a properly designed burner is possible – and the bright orange glow from blackbody radiation says that it’s pretty warm in there. So a one metre length would stretch by about 12 millimetres, which I reckon is reasonably consistent with what I see. (The flue itself will be rather lower temperature, though).

 Overall, it’s a nice little demonstration of some physics. And it keeps the house warm.

Teaching contradictions Marcus Wilson Jul 22

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Yesterday I was at a meeting of the university’s ‘Teaching Advocates’.  About eighteen months ago, advocates for good teaching were appointed in every faculty, and I was asked to take on the role for the Faculty of Science and Engineering. Basically, it means that I try to promote good teaching practice – such as by running lunchtime discussion sessions on some aspect of teaching (which reminds me to organize another one) and helping colleagues write learning outcomes for their papers, etc.

One of the topics we discussed was the university’s Teaching Awards. The 2011 round of these is coming up shortly.  It’s fairly clear that different parts of the university have rather different attitudes towards these. Actually, I’ve found with teaching in general that different faculties have very different approaches to things. That’s why I think that we need a teaching advocate in every faculty – to put a local ‘spin’ on things. For example, ‘reflective practice’ seems to be second nature in the Faculty of Education – it’s just something you do – but here in Science and Engineering it is almost taboo. I mean, it’s well-hammered into students that ‘how they feel about something’ isn’t a part of science – and blowing your own trumpet (i.e. identifying things you’ve done well) is akin to pomposity and should be avoided. Moreover, the kind of person who becomes a scientist or engineer (particularly the kind who becomes a physicist) is the sort of person who would at all costs avoid drawing attention to themselves – writing about themselves, especially in a positive light (i.e. putting themselves up to be shot-down) is a complete no-no. Far better to keep a low profile – if you can do ‘invisible’, so much the better.

That’s a bit of an aside.  Back to the teaching awards.  About this time last year, I told my students that they should consider nominating any teachers they felt were worthy of it, and gave them the web-link in order for them to do it. I didn’t say ‘vote-for-me’, I didn’t name anyone, I just thought that the best thing to do was to make sure that my students knew that they could vote, and to encourage them to vote (being reluctant physicists/engineers and all that).    Anyway, that’s what I did. However, as we discussed yesterday, there are many here who believe that we (lecturers) shouldn’t mention anything at all about the awards to students (or even other staff), lest it be mistaken for lobbying.   That I might be lobbying for votes never crossed my mind – maybe it should have done – but lobbying certainly wasn’t my intention anyway.   I should add that votes by students is only one step towards a teaching award – if you have the minimum number of votes (which, of course, biases in favour of people who teach large classes in easy subjects, but that’s another discussion) you then have to prepare a portfolio, which is what the real judging is on.

The problem if one is not allowed to do anything that could be regarded as lobbying (i.e. talk to students) is that most students will probably be  unaware that the awards exist. The awards are mentioned on official university emails and newsletters, etc., but how many people actually read them? Teaching awards are a really important way of recognizing and promoting good teaching (i.e. show that we actually value it) and so we should give them all the publicity we can. Isn’t that the point of an award?  That means talking about them, which is what I’m doing with this blog entry. That’s my view. I know some people would differ on that. Shoot me down.

 

 

 

 

The inverse femtobarn Marcus Wilson Jul 19

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Tomorrow I’m going to Te Aroha to give a talk to their continuing education group on the Large Hadron Collider. This is something I foolishly agreed to do many months ago (maybe it was even last year) before I realized how much lecturing I had this semester. Still, I’ve given the talk a few times before, so it’s not a major effort (it just means I’ll be late for a lab class later in the afternoon.) Anyway, to prepare for tomorrow, I thought I’d look up the latest goings-on at CERN on their twitter page, and found this amusing press release.

 http://press.web.cern.ch/press/PressReleases/Releases2011/PR06.11E.html

In June, the LHC’s two big experiments ATLAS and CMS broke through a milestone in data collection – hitting 1 inverse femtobarn of data. Now, that caused me  to stop and think for a bit. What is an inverse femtobarn’s worth of data?  The press release goes on to say that it is bascially a lot – equivalent to seventy million million collisions.

Breaking down this unit goes something like this.  A barn is a measure of the cross-sectional area of a collision in particle physics. Basically, if you imagine your two colliding particles as balls, ask yourself what is the cross-sectional area of each ball? Obviously, the bigger the cross-sectional area, the more likely they are to hit if you fire them at each other. In particle physics, we can’t really push the ‘balls’ analogy too far, but still it should give you an idea of what we mean by collision cross-section. A barn equates to ten to the minus 28 metres squared. Pretty small in terms of the everyday world, but actually pretty large in particle physics terms. The terminology is attributed to some of the wartime researchers into atomic weapons, who, talking about the ‘size’ of the uranium atom, remarked that it was ‘as big as a barn’. In a manner that only physicists can, they went on to define the units ‘outhouse’ and ‘shed’ for various fractions of a barn, but only the unit ‘barn’ has stuck. 

The ‘femto’ bit is simply a standard prefix, meaning 10 to the power minus 15. So a femtobarn is an area of 10 to the power minus 43 metres squared. A touch on the tiny side.

Now, to discuss collisions, the particle physicist uses something called luminosity, measured in particles  per unit time per unit area. The ‘barn’ comes in here as the unit area, so we can measure luminosity in particles per second per barn, or even particles per second per femtobarn. If we multiply this by the time that we have run the collider for, we get a measure of the integrated (total) luminosity in particles per femtobarn.  If we were to multiply this by the cross-sectional area of the collision (so many femtobarns), we’d get the total number of collisions.

Confused?  I think I am too. Basically, I think (but I am open to being corrected) the inverse femtobarn is a measure of how well the collider has done, allowing for the fact that some collisions are easier to achieve than others. One inverse femtobarn is pretty good going, corresponding to a shed load of data (or maybe a barn load of data) if you pardon the pun.

Hot air rises Marcus Wilson Jul 14

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Well, we have now moved into our new house. We moved last Friday, mostly dodging the heavy showers that have been marauding around the country for the last week. We are slowly unpacking – the place is looking a lot tidier now than it did at the weekend, but it will take a while to find a home for every object we own. Mizuna the cat has taken a little bit of convincing about the move – but he has now emerged from under a bed and seems more at home and is very happy that our ex-neighbour’s cat is no longer is around to bother him.

It’s a two-storey house, so slightly unusual for New Zealand, but one part of it is just one-storey with a very high ceiling.  The layout makes for an excellent demonstration of the fact that hot air rises. (N.B. It’s hot air that rises, not heat, as is often mistakenly said.)  With the log fire up and going on the ground floor, the area on the first floor above it gets really nice and toasty, whereas the ground floor itself is not so warm. The warm air has risen, has been trapped by the ceiling, and  so the higher up you are the warmer you are going to be, pretty much.

So, here’s a conundrum. If hot air rises, why is it colder at the top of a mountain than the bottom? It’s certainly warmer at the top of our house than the bottom. What’s the difference?

It’s because there’s another very effect going on.  As you go upwards, atmospheric pressure reduces, and so air expands. Expanding air does work (e.g. compressed air or steam expanding to drive a piston to do useful work) and so must lose heat energy. And that means it cools down. A quick bit of thermodynamics allows you to estimate how quickly temperature must drop with height. Ignoring the effect of moisture, it’s about 1 degree C for every 100 metres climed. This is known as the lapse rate. Consequently you can expect the top of Mt Te Aroha (about 950 m altitude from memory) to be around 9 or 10 degrees colder than Te Aroha township, to give a local example.

That’s if the air is dry.  It gets more complicated if there’s moisture involved, because of latent heat. When water vapour condenses to form water droplets (e.g. a cloud) it gives off heat. That prevents the temperature from dropping as quickly with height. So, in damp conditions (i.e. this week)  the lapse rate can be rather lower than this.  Not that I have any intention of climbing Mt Te Aroha in this weather to demonstrate it.

Teaching new things Marcus Wilson Jul 12

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As I put together my ‘Career Portfolio’ for my Postgraduate Certificate of Tertiary Teaching, I’m struck by the amount of teaching I have done in the last seven years.  By ‘amount’, I’m thinking of not just the number of lectures, tutorials, practicals and other things I do a week, but the number of different papers I have taught while I have been at the University of Waikato.

For those not familiar with the way NZ universities work, I should remark that if you are an undergraduate, you make up your degree by selecting ‘papers’ (sometimes called ‘courses’) – each paper carries a number of points – get enough points at each level (which is broadly but not exactly related to year of study) in the right subject areas then you get your degree. More or less. The rules are complicated but that’s the gist of it. 

 A quick count up tells me that in the last seven years, which  is how long I’ve been teaching at the University of Waikato, I have taught seventeen different papers. By that, I mean I have taught parts of seventeen different papers at some time during those seven years. For most of those seventeen, I haven’t taught all the paper, and for some of those seventeen, I have taught it for several years, but there are seventeen papers in all to which I have contributed teaching. They are distributed across three subject areas – physics, electronic engineering and mechanical engineering, and have been at first, second and third year undergraduate level, plus Master’s level. That sounds a lot to me – on average I have dealt with more than two ‘new’ (i.e. new to me) papers a year. And it doesn’t include extra things like final-year project supervision, directed-study work for individual students, etc. That’s pretty time-consuming, as it amounts to a lot of preparation work.

Often I’ve only taught a paper just once, to cover for someone on study leave or otherwise isn’t able to teach their normal papers in a particular semester. That’s probably the hardest thing at all – you get given, often at very short notice, someone else’s bunch of lecture notes, for an area that is not really your expertise, and have to teach students on it. That can go wrong – sometimes spectacularly – I have experienced this a couple of times in doing teaching for mechanical engineering. Earlier this year I taught half a third-year mechanical engineering paper and was nearly all the way through it before it was pointed out to me that I was using terminology incorrectly. That came from a confusion between physics-speak and engineering-speak – they are subtlely different dialects. I was speaking ‘physics’ – but the students were listening in ‘engineering’, and that was leading to confusion as some technical words are used in different ways.  Teaching well in this situation is really difficult.

Incidently, there’s been a fair bit of research looking at differences between ‘physics’ and ‘engineering’ when it comes to teaching and learning - and that’s relevant when you’re teaching students from both groups in the same class – e.g. Gire, E., Jone, B. & Price, E. (2009) .Characterizing the epistemological development of physics majors. Physical Review Special Topics – Physics Education Research 5, 010103.

Anyway, my point is that I think I’ve done pretty well in having a go at teaching on seventeen different papers. Some of them I’ve done for several years now, and am getting both more confident and more innovative with them, but there are a couple, I have to say, that I would refuse to teach again if I were asked to. Or, at least, teach very very reluctantly. I wonder if my experience here is the similar to other lecturers (particularly those who have spent less than about ten years teaching) .  I’d love to hear your comments.

 

 

The Faraday Suit Marcus Wilson Jul 11

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Annette Taylor has sent me this link.  Rather than turning off a high-voltage power line when you work on it, you just wear one of these special anti-electricity suits.

Of course, there’s nothing hi-tech about them (at least, not conceptually). If you put one of these on, you are in your personal Faraday Cage, and the electric field inside will be zero. Another way of thinking about it is that any electrical current will travel along the suit, not into you, so you are protected. 

It would be fun to try one out, I reckon. 

12 July 2011:  Annette has advised me I’ve got the wrong link there. Try this one: http://www.number8network.co.nz/2011/07/03/471/

Apparent weight? Apparently not Marcus Wilson Jul 08

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Here’s a thorny problem that no doubt every physics teacher has grappled with. In a space-station, orbiting the earth, are you weightless?  

There are at least two ways of answering this:

1. Yes, you are. Let’s face it, you float around inside the space-station, water forms large blobs, some plants don’t know which way up is, pendulums don’t swing, and you need frightening machines to help you go to the toilet.

2. No, you still have weight, because you still feel the gravitational force of the earth. If you didn’t, there would be nothing keeping you in orbit.  You ‘feel’ weightless, because you and the space-station are accelerating at the same rate towards the centre of the earth (centripetal acceleration).

There is confusion here because we are often very loose with what we mean when we say ‘weight’. We often think of weight as being the force gravity exerts on us, and so we run into problems because when we are in the space-station, the earth’s gravitation field is still there, and so we are forced to conclude that we must still have weight. To alleviate this conundrum, often we talk about ‘apparent’ weight. Since the space-station is accelerating towards the centre of the earth at the same rate as its occupants, the occupants feel as if they are weightless – so they have no ‘apparent’ weight.

 In the article  "Apparent Weight: A Concept that Is Confusing and Unnecessary" in "The Physics Teacher", Albert Bartlett argues that us physicists should get our act together when it comes to talking about weight. If we abandon the concept of apparent weight and the equally confusing "acceleration due to gravity", and stick to a decent definition of weight and use "free-fall acceleration", the confusion should be alleviated.  I’m inclined to agree with him, and also stick my hand up as being guitly of  sometimes doing just what he wants stopped.

If you’re physics-educated, have a read and see what you think (the article is downloadable for free).

If we stick with the definition that an object’s weight (a force) is what a spring-balance would read when the object is placed on it, we should have no problems. In this case the weight might result from gravity, but it could also result from being in a rotating frame (e.g. a rotating spacestation in deep space), or a combination of the two (e.g. being at the equator on the earth), or, bringing in general relativity, in an accelerating lift.  Take your bathroom scales into work and go up and down in the lifts, and note that your weight really does change when the lift accelerates and decelerates.

Thanks to Steve Chrystall for sending this article to me.

Bartlett, A. A. "Apparent Weight": a concept that is confusing and unnecessary. (2010). The Physics Teacher, 48(8), 552.

Electricity and heat Marcus Wilson Jul 06

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There’s an advert that’s well-featured on the television at the moment for a plug-in wall-mounted heater.  As part of the advert, the product is described as ‘efficient’.  Now, I’m not at all saying that these heaters aren’t a good purchase, but a bit of physics tells me that this statement about efficiency doesn’t really mean very much – all heaters that work by converting elecricity to heat are going to be 100% efficient.

Energy comes in various forms. There’s electrical energy, movement (kinetic) energy, sound energy, gravitational potential energy (the energy stored in a hydro lake), light and heat, just to name some. Energy can convert between forms, but is always conserved – you can’t create energy out of nothing (at least, not for long periods of time, but I’ll leave quantum electrodynamics aside hre) and you can’t destroy it.   In which case, you might ask the question, "If it’s never destroyed or used up, why do we have to pay for it?"

Unfortunately, with energy, it is easier to convert it in some directions rather than others. Formally, this is encapsulated with the second law of thermodynamics – that entropy (‘disorder’) increases with time. Take friction for example. A rolling car on a flat road will eventually come to rest because of friction in the moving parts of the car, between the tyres and the road, etc. (and due to air resistance). Here, the movement (kinetic) energy of the car has been converted to heat energy.  That process happens naturally on its own. What doesn’t happen naturally is that a car sitting stationary on a flat road starts moving, because heat in its parts turns itself into kinetic energy.  Energy does not flow naturally that way.

Essentially, the second law of thermodynamics tells us that producing heat is really straightforward. Take just about any physical entity containing some form of energy, and it will eventually generate heat. It’s not difficult.  So, our plug in wall-heater, is designed to take energy from the power station (transmitted through the electricity grid) and turn it into heat. This it can do perfectly; every bit of energy that gets to the heater through the electricity supply gets turned into heat.   (Contrast that with a light bulb, where some gets turned into light, and some to heat, or with your TV, where some goes to light, some to sound, and some to heat). So the heater is 100% efficient at converting electrical energy to heat energy.

 Just where that heat goes next is determined by your house design, particularly how much insulation is in the roof – hopefully it hangs around inside your house for a while, before leaving (still as heat).

And just to end, you can get more than 100% ‘efficiency’ though in electrical heating – namely the heat pump. In this case heat energy is transferred from the air outside (yes, cold air contains heat) and is pumped inside. This flow doesn’t happen naturally (second law of thermodynamics), and there is a cost you pay. You need to input more energy to get it to happen – that’s the electricity supply to your heat pump – but you get that energy back as heat as well, so you don’t lose out. 

So a claim of 100% efficiency for an electrical heater doesn’t mean a great deal – don’t take much heed of it when deciding what plug-in electrical heater to buy!

Rockets Marcus Wilson Jul 05

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So, last Thursday and Friday, we had a great demonstration of some rockets, thanks to Steve Chrystall. Some of our visiting school students had made water rockets, and these were launched across our sports field at lunch time.  There were some pretty impressive entries (both in terms of distance travelled and blowing-up on the launch pad). The furthest student rocket travelled a staggering 130 metres, though Steve himself demonstrated one that  from memory reached around 180 metres.

We also had a demonstration of some pyrotechnic rockets – two on Thursday, two on Friday.  Of those, we had two and a half successful firings – the ‘half’ being a rocket travelled a couple of metres into the air before exploding early.

Rockets are extremely simple in concept. They are a demonstration of the conservation of momentum. The idea is that the rocket throws out exhaust backwards at high speed – this exhaust carries momentum – and as a result the rocket gains a momentum in the opposite direction. Overall, the momentum starts at zero (stationary rocket and fuel) and ends at zero (fast rocket and fast fexhaust, in opposite directions). As fuel you can use anything that can be expelled at high velocity. Commercial rockets use exhaust gases that have been created as a result of a chemical reaction (e.g. hydrogen with oxygen). You can make a tiny chemical rocket using vinegar and backing soda in an old 35mm film cannister (it releases carbon dioxide) but in the case of water rockets we can use water with compressed air to achieve the same thing.

The ‘design’ is really straightforward. Take a 1 litre soft-drink bottle, drink the contents, then place a bit of water inside and put a bung in the end, in which a valve (like on a bicycle or car tyre) has been inserted.  Then pump up the air inside to about 80 psi (about 50 kPa) – high enough for a good effect but not too high to blow open the bottle. (You’ll need to hold the bung in place while you do this.) Finally you let the bung fly out. The air/water mix comes out the back carrying momentum, and the rocket accelerates in the opposite direction.

You’ll need to experiment a bit with getting the right amount of air and water in the rocket, and putting a bit of weight on the nose so that it flies straight, but getting that right is part of the fun. Probably the most difficult bit is constructing a launch mechanism that holds the bung in place while you pump up the pressure but then releases the bung at your command.  

See if you can get one travelling over a hundred metres.

 

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