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May 2015 Archives

A couple of days ago I cleaned the filters in our heat pumps. What prompted me to do this wasn't the cold weather, but the visible build up of dust on the casing of the indoor units. It looked horrible. On opening the unit up, it was clear that the filters were well overdue a clean. Eryughhh. But it doesn't take long to do them, and in just a few minutes they're back inside the pump and its throwing out warm, toasty air again. 

Aesthetics is just one reason to attend to the filter. The second is that dust clogs up moving parts, which means the fan and the louvre on the front. Getting rid of that dust has to be a good thing in terms of mechanical performance. 

But there's also a third reason - one driven by physics. Your heat pump will be more efficient. How does that work?

The basic idea of the heat pump is that it takes heat out of the outside air and shifts it inside. It does it with an expansion-compression cycle, rather like a fridge. Although the air outside might be 0 degrees, it still has heat in it, which can be extracted and shifted inside. The result is that, outside, the air leaving the outdoor unit is lower in temperature than the air entering (to the extent that there isn't a lot that will grow in front of an outdoor unit - event the most stubborn of weeds get frozen out of existence once winter starts), while, inside, the air leaving he indoor unit is of higher temperature than the air entering. Hence the indoor temperature rises. 

But pumping heat from something cold to something warm comes at a cost. It's not the natural way that heat will flow. The bigger the temperature gap between indoors and outdoors, the harder it is to pump that heat. That means more power usage in the form of electricity. Heat pumps work really well for small temperature differences (e.g. the outdoor air is 15 C and you want to heat the house to 18 C) but not so well for large differences (e.g. -5 C to 18 C). The unit may still work at -15 C, but it's less efficient - you'll be getting fewer kWh of heat for every kWh of electricty. 

What has that got to do with the filter? Well, a dust-clogged filter starts restricting the air-flow through the indoor unit. That means there is less volume of air passing the heating element every second, to take away the heat.  If the heating element is still putting out the same amount of heat as before,  it means that it must get hotter. It's rather like the fan on a car radiator. The fan doesn't stop the car engine producing heat, but by increasing the air flow it brings the temperature down.  So a clogged filter means that the heating element inside the indoor unit is going to run hotter, if it's putting out the same amount of heat.  That's bad, since it means the heat pump now has a larger temperature difference to pump heat over, and therefore is less efficient. 

(I'm sure the reality is complicated by the control systems that heat-pumps use - so rather than running hotter it may simply pump less heat - but you don't want that either if you want to heat your home.)

So, cleaning those heat pump filters is a good idea, for a good physics reason. 

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When I was at school, and introduced to magnetic fields in a quantitative sense (that is, with a strength attached to it), I remember being told that the S.I. unit of magnetic flux density (B-field) is the tesla, and that 1 tesla is an extremely high B-field indeed. Ha! Not any more. Last Friday night  I got to see a MRI machine in action - at Midland MRI at Waikato Hospital - this particular one is a 3 tesla affair. One of my PhD students was making some measurements with it. It needed to be at night - such is the demand for MRI scans we'd never get to play with it during the day. But well worth extending my day's work for. 

Now, what does 3 tesla do? First you are advised to check pockets very carefully and remove keys and the like. No pacemakers? Good. Now enter the room. Interestingly, I didn't really 'feel' anything until very close to the machine - then there was just a hint of something slightly 'odd'. Things a little tingly, but nothing really significant. 

Two events, however, confirmed that there was a sizeable field indeed. First, my belt unbuckled by itself. That prompted a quick retreat outside to take that off, before bits started flying through the air. Then our host demonstrated what 3 tesla does to a sheet of aluminium. 

It's important to remember that alumunium is not ferromagnetic. It is not attracted by a magnet. But it is, most certainly, very conductive. When a conductor moves through a magnetic field, electric currents are induced. These in turn generate magnetic fields, which are such that they oppose the movement. This is Lenz's law. Consequently there is a force felt by the conductor that opposes its motion. And at 3 tesla, that's some force. You can stand the sheet of alumnium on its end. Normally, you'd expect it to fall over, pretty quickly. But not at 3 tesla, it doesn't. Very, very slowly, it topples, taking several seconds to move from vertical to horizontal. I could feel the effect of Lenz's law by trying to flip the sheet over. It was like trying to turn a rapidly spinning gyroscope. Pretty impressive stuff. 

You can see a movie of this experiment (not ours, I should add), here. https://www.youtube.com/watch?v=JNUVfmy-iqM 

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I've just given a couple of lectures on special relativity to a class of first years. It's clear that grasping the key ideas is going to take some time. The results are so far removed from everyday experience that there is a certain air of bewilderment in the classroom. Here's an example of what I mean. 

Suppose two students are travelling on skateboards, both at 10 km/h, but heading towards each other. In the frame of reference of one, what is the velocity of the other? 

The answer is simple: Add up the speeds - one sees the other coming at a speed of 20 km/h. 

Now make the skateboards a little quicker. To be precise, make them both travel at 0.8 times the speed of light. Now what does one of the skateboarders experience?

Our immediate reaction might be to say 0.8 + 0.8 = 1.6  -they see the other approaching at 1.6 times the speed of light. But that would be wrong. At high velocities, it just doesn't work that way. The correct answer would be (from the Lorentz addition formulae) 0.98 times the speed of light. That is hard to grasp. There are at least two reasons. First, we never experience skateboards going at 0.8 times the speed of light, so the question is not physically meaningful. Secondly, it is so far removed from our physical experience it just doesn't make any intuitive sense. 

One can do the correct relativistic calculation on our first example - two skateboarders each heading at 10 km/h (or 9.26 billionths of the speed of light). This time we end up with 19.999 999 999 999 998 3 km/h. It is little wonder that calling it 20 km/h is an approximation that works for us! Putting it in context - if we travelled at this speed for an hour (thereby covering 19.999 999 999 999 998 3 km), we would be about 2 picometres short of 20 km. That's something much less that the size of an atom (but rather larger than the size of a nucleus). Little wonder we get away with calling it 20 km without any trouble. 

A consequence of special relativity is that time and space are 'relative' - meaning that different observers will disagree on the time between two events, and the distance between two events. This is measurable - put an atomic clock on an aircraft and one on the ground, and fly the plane around for a few hours. On landing, the two clocks will be different, showing that time has been experienced (very slightly) differently. 

There is, however, one very readily measurable consequence of relativity - one for which we are all familiar. That is magnetism. Magnetic fields and electric fields are part of the same entitiy. Just as observers will disagree on the time and distance between two events, so two different observers will disagree on the strengths of magnetic and electric fields in a system. A magnetic field becomes an electric field to a different observer. The reason we experience magnetic fields at all is down to the extreme neutrality of matter - the number of electrons and protons in a sample of material being incredibly well balanced. I didn't try to explain that one - but it makes a nice bit of analysis for third-years. 

 

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I just love the word 'obfuscate'. It means (in my words) to take something that is perfectly clear, and render it incomprehensible. As in "Using the word 'obfuscate' in a sentence will obfuscate its meaning". 

I say this because I've just been reading an article on which (clearly) a statistician has been let loose - thus rendering an otherwise wonderful article incomprehensible - at least in places. Now, I'm not saying that the statistical analyses aren't necessary, rather that in my opinion the article would be much more readable if most of the statistics were presented neatly in an appendix, rather than being liberally splatted across the second half of the article. I mean, seriously, how many of us actually know what the "Kaiser-Meyer-Olkin measure of sampling adequacy" involves, what "Bartlett's test of sphericity" is (something FIFA use to assess the roundness of a football, maybe?), or what "Mahalanobis' Distance statistic" measures?  Or am I just dumb in this regard?

That aside, the article I think is a little gem (though it's not often such a blatant apostrophe error finds its way into a journal title, especially one in pedagogy):

C.D. Smith, K. Worsfold, L. Davies, R. Fisher & R. McPhail. Assessment literacy and student learning: the case for explicitly developing students [sic] 'assessment literacy'. (2013). Assessment & Evaluation in Higher Education, 38(1), 44-60.  http://dx.doi.org/10.1080/02602938.2011.598636

Here the authors talk about the need to educate students in what assessments are there for and how to interpret them. The over-arching message is clear (that is, unobfuscated).  With reference to Francis (2008)*, they say

...first-year students in particular are likely to over-rate their understanding of the assessment process and ... there is a disjuncture between what they think they are being assessed on and what the marking criteria and achievement standards require of them

The authors go on to describe a simple intervention - a workshop in which students get to undertake peer discussion on examples of submitted work - and how they shape up against the marking criteria. Such an intervention results, I believe  (the paper is rather obfuscated here), in a good improvement in the quality of  students' submissions in a similar assignment. In particular, two areas show marked improvement.

First, students develop the ability to judge for themselves what makes a good response to an assignment. By implication, then, it means they develop the ability to judge the quality of their own work. That is a skill required by any professional. Imagine you have an electrician do some work in your house and she's unable to say for herself whether she's done a good job of it. A frightening prospect!

Secondly, students develop the idea of 'assessment for learning' (as opposed to assessment of learning), that is, they can see that they are able to learn while doing the assignment, Moreover, they begin to grasp that assignments can be set with the very purpose of developing student learning as opposed to simply providing a summative measure - in other words their lecturers are using the assessment process in a carefully considered manner with the primary purpose of achieving student learning. 

Also increased, though not by as much, was student understanding of the actual assessment at hand, and their desire to put effort into the assessment. 

All this has me thinking about what we commonly ask on physics assignments, tests and exams, and whether the students really know what we mean. The answer, I am sure, is 'no'. We use words and phrases such as "show that...", "evaluate...", "discuss..." and "from first principles..." These might be clear to experienced physicists, but I wonder whether  students find them not unobfuscating. 

Time for a bit of research. 

*R. A. Francis (2008). An investigation into the receptivity of undergraduate students to assessment empowerment. Assessment & Evaluation in Higher Education 34(4), 481-489. 

P.S. Thank you to Dorothy Spiller of our Teaching Development Unit for drawing my attention to thie Smith article. 

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There's no hiding my conflicts of interest here. I'm on the New Zealand Institute of Physics 2015 conference organizing committee. I'm also the NZIP treasurer. And I'm a staff member at the host organization.  So, to contribute to the New Zealand physics community's biennial event  in Hamilton on 6 - 8 July, click on this link. 

But why? Pick from the following

a. Because you get to meet colleagues and actually talk with them. 

b. Because you get to hear and discuss first hand about some of the exciting physics work that goes on in New Zealand

c. Because you get to meet, talk to, and learn from Eugenia Etkina, who is one of the most honoured and respected physics educators in the US. She's researched in particular student learning through practical experiments, and how to maximize it. But also she's looked at the modern physics curriculum more generally. And she'll be here with us to share it all. 

d. Because you get to celebrate the International Year of Light (which, by the way, was designated by UNESCO following lobbying from a handful of countries including New Zealand)

e. Because you get to experience practical examples of Bessel Functions.  (You may need to click here for an explanation). 

So, no excuses. See you in The Tron in July. 

 

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