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July 2013 Archives

We're three weeks into our 'B' semester here. One of the papers I'm teaching (just on the fringes of) is our main first year physics paper. When I say 'on the fringes of', it means I'm supervising one laboratory session a week. It's good to keep in contact with what's going on at first year - all my other teaching is 2nd or 3rd year or graduate level.

One thing that seems to be happening is that, every year, the incoming cohort of students is less confident in their use of equipment in their physics lab classes. Is this because they are doing less and less practical work at school? One thing for sure is that students can find even the most simple piece of equipment a real stumbling block.

The main purpose of the laboratory classes is to give students the chance to experience the physics that is talked about by lecturers for themselves, and thus give them a practical learning opportunity. It's one thing to hear a lecturer talk about the force on a current carrying wire in a magnetic field, which can be pretty abstract  - it's another to measure the force for yourself and verify that indeed the force is proportional to the current.

However, these opportunities are often clouded by unfamiliar and frightening equipment. What often happens is that the student spends the laboratory trying to figure out how to use the equipment, rather than looking at the physics the experiment is designed to illustrate. The chance for meaningful learning is thus rather diminished.

From my observations I can see two ways that this occurs. First, is the obvious "I don't understand the equipment". "Which wire do I connect to which terminal?", "Which mirror do I need to use for part A", and so forth. The focus of the student is on the equipment, not the science. The second way is blind trust in the equipment. A student tries to take a reading of current and connects the multimeter in parallel with the component, and then assumes it must be right. Having a digital display rather than analogue seems to up someone's confidence in the equipment. If it's hi-tec, it has to be right (even when it is used incorrectly).

It's a tricky balance to get right. On the one hand, a student needs to use experimental equipment correctly, but on the other we don't want our lab classes simply to be about learning to use pieces of equipment that most students will never come across again. In fact, at second year electronics, we've now got a module on common pieces of lab equipment, in which the focus is to learn how to use multimeters, signal generators, oscilloscopes etc correctly.

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Hopefully people of Seddon (both Seddon, Marlborough and Seddon, Melbourne) are able to enjoy this piece. What a fantastic example of low quality journalism. The correction is itself pretty lousy, with 'facts' being taken second-hand from other websites, including the grandaddy of them all, Wikipedia.


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When I was young (about six-ish)  I had a variety of ambitions. Some of them I shared with a lot of other boys of my age, such as being a train driver and playing cricket for England. Some were more particular to me, such as becoming a biologist and discovering a new colour. 

Needless to say I failed on all accounts. One I got close to - being a physicist is not so far away from being a biologist.  I've at least watched England play cricket (including an England v India match at Lord's - in the members' guests area - that was rather neat) and stood on the footplate of a steam engine. Discovering a new colour, however, is something I was not likely to achieve from the outset.

I had a vague idea that if I mixed enough paints together I'd hit on a combination that no-one had tried before (maybe purple and green with just a hint of orange) and, hey-presto, they'd mix together to some entirely colour previously unknown to science. The colour would naturally be named after me, and become an instant hit with home decorators. Out would go 'Magnolia', in would come 'Wilurple'. 

I gave up on the ambition long before I found out why it was unlikely to work. The CIE colour chart encapsulates the situation neatly. There are only three different colour receptors ('cones')  in the human eye. By having the 'red', 'green' and 'blue' cones stimulated differently, one sees different colours. The CIE chart puts all possible colours onto a 2d grid. One defines the variable 'x' as being the fraction of the total stimulation that is accounted for by the red cones; the variable 'y' as the fraction of the total that is accounted for by the green cones. (One could define 'z' in a similar way for the blue cones, but it is redundant since x plus y plus z must equal 1.) Then 'x' and 'y' defines a colour. The chart shows it. 

All possible colours are shown on this chart. The outside of the curved space shows the colours of the spectrum - those stimulated by a pure wavelength of light. The others are due to combinations of wavelengths. At x=1/3, y=1/3 (and so z=1/3) there is white. It isn't possible to go outside this chart, and therefore it contains all possible colours. D'oh.

But, there is hope. The response of the green cones of the eye is entirely overlapped by those of the red and the blue. This means it isn't possible to find a wavelength of light that stimulates JUST the green cones. If, somehow, one could stimulate cells artificially, one might be able to trigger green cones to fire without any response from red and blue. And then the person would be seeing a colour they've never experienced before. 


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 Just a couple of hours ago, I was thinking that I really need to do another blog entry for the week, but (a) can't think what to do it on and (b) don't have time to do it because I have a lab class for the afternoon. Well, the events in the lab class have (in)conveniently both of those problems. 

It's the first lab class for our second year Experimental Physics paper. As the name suggest, this paper is based in the laboratory. There are a few lectures, but these are to support the teaching of lab techniques. The first session is always an introductory one. I talk about some general issues with doing experiments, some safety issues, what I expect from the students, and so on, and then we work through an experiment as a group. The idea is that I model what I would like to see in their experimental work - e.g. making several measurements of something, not just one, plotting a graph carefully, dealing with experimental uncertainty (both random and systematic), making good notes in a logbook, working to the right number of significant figures, and so on. I've found that doing this in the first session really helps students when they get set loose on some equipment for themselves the next session. They have some idea what I'm expecting of them.  

 The experiment I usually do with the class is measuring the charge-to-mass ratio of an electron, "e/m", using an electron beam in a magnetic field. It's a lovely, historical experiment, and the theory behind the method is understandable just from school physics. An electron moves in a magnetic field. It has a force on it perpendicular to both the velocity of the electron and the magnetic field. A force always perpendiular to a velocity is what happens in circular motion - and this therefore results in the electrons moving in a circle (or, more accurately, a helix). The diameter of the circle depends on the strength of the force, which in turn depends on the strength of the magnetic field. So, in this experiment we change the magnetic field by changing the current through some coils of wire, and measure the resulting diameter of the circular electron beam. From the measurements we can calculate e/m. 

It's a classic experiment. Historically, the ratio of the charge of the electron to its mass was measured before either the charge or the mass of the electron was measured individually. Several years later Millikan measured the charge on the electron with his famous oil drop experiment, and then the charge of the electron, and hence its mass (since e/m was already known), two very important values in physics, were known for the first time. Incidentally, we also have Millikan's experiment in the lab, but I tend not to inflict it on students as it is exceptionally tedious and frustrating. It is true that a good scientist needs to be patient, but it is also true that there are limits on the numbers of students we can afford to lose from our courses due to extreme boredom and frustration. I have great admiration for Millikan's patience and persistence. 

 How does one generate and 'see' electrons? We use a vacuum diode. A heated cathode emits electrons. An electric field accelerates them and a beam emerges from a slot in an anode. The electrons are 'visible' because the equipment is inside a sealed glass vessel containing a low density gas. This gas fluoresces as the gas molecules are hit by electrons. We see a nice circular beam. 

Or, in the case of this afternoon, we don't. It appears that the tube has blown. Possibly the cathode has got too thin and has melted, rather like a 'blown' lightbulb filament. There's the right voltage across it, suggesting there's not a problem with the power supply, but there's no glow coming from the filament. After a bit of investigation I realized I wasn't going to solve the problem in a few minutes and I canned the rest of the class, which was rather unfortunate.  

A lovely example of "biology experiments wriggle, chemistry experiments smell, and physics experiments don't work."

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First, sorry for the lack of activity. Simply put, there's been a lot going on. I've taken on the role of treasurer for the NZ Institute of Physics which has eaten up rather more time than I hoped for. I'll get into the swing of things soon though.  In that role, here is a shameless plug for the upcoming NZIP conference in Nelson. 

B-semester has started here today and the last couple of weeks I've also had to spend putting our 2nd and 3rd year lab experiments into some kind or order. It's amazing how something that has been untouched for several months can cease to work when it gets turned on again. 

In the little bit of spare time I've had, I've been reading through Mike Parker's lovely little book 'Map Addict'. As a fellow addict, I know where he is coming from. Love of all things cartographic - be it Google Maps, the floor plans of F-block here at Waikato, or Middle-Earth. The book prompted me to do a bit of background reading on enclaves and exclaves, and this led to 'discovering' the most ridiculous place on the planet, politically speaking.

Go to Google and fly to loc: 26.14987,88.76217 and you'll see what I mean.

From satellite, it looks like an innocuous piece of farmland in India. And, in some respects, it is. But, to be more precise, it is a piece of Indian farmland entirely surrounded by Bangladesh. 

Kind of. That's because the bit of Bangledesh that surrounds it is itself entirely surrounded by India. It's a piece of India in Bangledesh in India. 

Sort of. To be more accurate, one also needs to point out that the piece of India that surrounds the piece of Bangledesh that itself entirely surrounds Dahala Khagrabari #51 is itself surrounded entirely by Bangledesh. 

So we have a third-order enclave - a piece of India surrounded by Bangladesh, surrounded by India, surrounded by Bangladesh.  This is the only third-order enclave in the world, but there are some second order ones, such as in the vicinity of  Baarle-Nassau in the Netherlands. (Or should that be Baarle-Hertog in Belgium?) Or Baarle-Nassau in the Netherlands? 

I can't help thinking that there must be some deep physics analogy to draw from here, but I haven't come up with a good one yet. Perhaps some infinite series of Feynman diagrams in quantum electrodynamics might come close - where to calculate the interaction between two electrons we need to consider all possibilities of exchange of photons and electrons, including all interactions between electrons that might appear as part of this exchange process. Or perhaps an infinite series of reflections in a laser cavity. Any ideas, anyone?

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