Marcus Wilson

Dr Marcus Wilson is a lecturer in the Engineering Department at Waikato University and author of the Physics Stop blog. His current research involves modelling of the electrical behaviour of the human brain during natural sleep, focussing particularly on the transitions between sleep states. Previous research interests include infra-red physics and signature control (stealth) and quantum Monte Carlo methods. He graduated from Cambridge University in 1992 (BA Hons) and completed his PhD at Bristol University in 1995.

High-tech, Low-tech, planetary observations. - Physics Stop

Jul 01, 2015

First the low-tech:  The conjunction of Venus (the brighter one) and Jupiter as recorded by my very lousy cellphone camera  just after sunset yesterday. 


Now the high-tech: A day before that Pluto occulted a star. It moved in front of the star, rather like an eclipse. The significance of the event was that it allowed Pluto's atmosphere to be studied - by looking at the way the light moved through and around the atmosphere, various properties of the atmosphere can be inferred. The SOFIA project was in action, capturing the event, at the other end of the cost spectrum to my mobile phone.

There'll be more Pluto excitement coming as the New Horizons probe flies closeby in just a couple of weeks.  


P.S. I should add in the conversation I had with my son (just turned 3) yesterday, after showing him the planets outside. 

Benjamin: "I don't like planets"

Me: "Why not?"

Benjamin: "Because they're quite noisy"

Me: "How are they noisy?"

Benjamin: "Because Grandad says they're quite loud, actually."

Umm.... Work that one out!

Tips on organizing a conference - Physics Stop

Jun 23, 2015

With the NZ Institute of Physics conference rapidly approaching, I thought I'd share my thoughts and experiences on how to organize a good conference. Or maybe on how not to organize a good conference. Time will tell.

1. Don't try organizing a conference when you have 450 exam scripts to mark. 

2. Employ a professional conference organizer. They are worth their weight in free conference alcohol. 

3. Remember that participants at your conference will read 'deadline' as 'guideline'. Don't expect more than half your abstracts to come before the advertised deadline. While it's tempting to reject abstracts that are late, the reality is that we can't afford to do that. So it's a good idea to work out when the deadline really is, and advertise it for two weeks before then. One could say that exacerbates the problem, because your contributers will learn to take your deadline (whenever you set it for) and add two weeks to it. 

4. Remember, that any 'extended deadline' will be read as being 'extended guideline'. There will still be abstracts coming in well after the two-weeks extra that takes you up to where your deadline really should have been in the first place. And you can't afford to say no. And they'll be asking to be placed in premium oral slots, too. 

5. Inter- and intra-organizational politics is just as complex as parliamentary politics. 

6. Don't be treasurer of the organizing body (in this case NZ Institute of Physics), a key conference organizer, and an employee of the host institution (in this case University of Waikato) all at the same time. Way too many conflicts of interest to manage all at once. 

7. All the above notwithstanding, the conference will probably go very well and the average attendee will have no knowledge of points 1-6 above. So don't tell them. Whoops. 

And, finally, if you're a member of the physics community, remember it's not too late to register. And if you're not, we have two excellent public lectures lined up for you on the evenings of Sunday 5th July and Monday 6th July. See you there!


The equation of time strikes again - Physics Stop

Jun 17, 2015

Some of us are rather looking forward to getting to 22 June. That's when the days get longer again. Yes, the reality is that no-one's really going to notice much difference for a while, but it's encouraging to think that the days will be getting lighter again, if only by a little bit. Don't confuse that with temperatures getting warmer - the coldest day (on average, of course) lags the darkest day quite considerably. Here it's around the end of July

But there's an interesting effect going on with sunrise and sunset. We've already had the darkest evening (hooray!) yet the darkest morning is still to come. Look at the sunrise and sunset times (for Hamilton) on the MetService website: today we've had sunrise at 7.32am and with cloudless skies the sun may stay out all the way to sunset at 5.07pm. But tomorrow sunset is recorded at 5.08pm (later!) and on Saturday sunrise has shifted to 7.33am (also later!). How can that be?

The point here is that the length of a day, meaning now the time between when the sun is at its highest to the next time the sun is at its highest, is only 24 hours on average (for some periods of the year its greater, for some periods is less), and isn't equal to the time it takes for the earth to spin once on its axis. 

Let's take this last point first. It's solar midday, meaning that the sun is at its highest. Now, let the earth rotate exactly once on its axis. Do we get back to solar midday the next day? No. That's because, in the time taken for the earth to rotate once, it has also moved along its orbit (about 1/365th of the way around). That means it's got to spin a little bit more before the sun reaches its higherst point. The time to spin once (the siderial day) is about 23 hours 56 minutes - four minutes less than the mean solar day. Note that 4 minutes x 365 = 24 hours - which means one more revolution than you might expect  - the earth actually does 366 and a quarter revolutions each year. 

However, the movement along the earth's orbit in a day is only on average 1/365th of the orbit. When the earth is closest to the sun (called perihelion - 3 January at present) it moves faster. That's Kepler's second law. When it's further away (at this time of year) it moves slower. That would mean that in January, we should perceive that solar midday gets later every day by our watch (since the earth needs extra time to spin that extra bit more), but that in July, the solar midday should be getting earlier. However, that's not what is observed. Our prediction for January is true, but for July it's the other way around - solar midday actually gets later as measured by our watches. 

There's another effect going on.  This is because the earth is tilted on its axis. However, it's quite tricky to explain why that makes a difference.  Consider the transition from winter to summer, in the southern hemisphere. If we look at the position of the sun at sunrise and sunset, we see it move southward from one day to the next. What is significant is that at sunset the sun is further southward than for the previous sunrise. That gives us a shift in the measured time between solar midday and the next solar midday. A better explanation is given here.  This effect is 'zero' at the solstices and equinoxes, and does two cycles a year. Add this to the effect of Kepler's second law, and we get the odd-looking curve that is called 'the equation of time', and means that, at present, each solar day is slightly longer than 24 hours, giving both ligher evenings and darker mornings. 

You can see a net result displayed on the ground under the sundial in Hamilton Gardens. The elongated figure-of-eight is called an 'analemma'. It will show you the position of the tip of shadow of the pole at different times at different times of year.





Two great talks coming up in Hamilton - Physics Stop

Jun 11, 2015

As part of the forthcoming NZ Institute of Physics conference, and to celebrate the International Year of Light, we have arranged two fantastic and very different public talks for the evenings of Sunday 5th and Monday 6th July. 

First up we have Richard Easther, from the University of Auckland. In "Dawn's Early Light" he'll be talking about cosmology and the early universe. (Sunday 5th July, 7.30pm, Price Waterhouse Coopers Lecture Theatre, University of Waikato),

Then the following evening Hans Bachor, from the Australian National University will enlighten us (pun intended) on the everyday use of lasers. "Lasers are part of your life." This will include some exciting laser demonstrations, all done safely. (Monday 6th July, 7:30pm, Concert Chamber, Gallagher Academy of Performing Arts, University of Waikato). Note the different venues for the two talks. 

So if you're local to Hamilton, do come along and see (another pun) cutting-edge physics from those that do it. Both talks are free.


Why you should clean your heat pump filters (the physics) - Physics Stop

May 28, 2015

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. 

Lenz’s law – at 3 tesla - Physics Stop

May 20, 2015

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. 

Special relativity – not so easy to grasp - Physics Stop

May 18, 2015

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. 


Developing assessment literacy - Physics Stop

May 12, 2015

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.

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. 

A blatant plug for the NZIP2015 conference - Physics Stop

May 06, 2015

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. 


Messing up the test: the next installment - Physics Stop

Apr 29, 2015

In the last few years I've been experimenting with the way I test our 3rd year mechanical engineering students in their 'Dynamics and Mechanisms' paper. I've chosen this paper because (a) it has more than a handful of students, and (b) I am in charge of it. When  I've suggested to my peers that I do something similar with other papers I teach on (but not in charge of) a "Don't you dare" tends to ring out rather clearly in response. So Dynamics and Mechanisms it has to be. 

I've tried 'tests you can talk in', with mixed success. This year, I tried an oral test. That involved giving every student a personal, fifteen minute interview. I had the idea from an article I read which talked about the problems with traditional written assessments, and discussed other possibilities* It's not a new article, but then universities have a lot of inertia, so it's no surprise that the troublesome written assessment still seems to stand as the perceived 'gold-standard' for assessment at university. 

But an oral test was a big risk for several reasons.  First, I had to declare what form the test would be on the paper outline (an official summary of what the paper involves) well in advance of it starting. This meant that I didn't know how many students they'd be, and how much work it would involve. I was expecting, based on previous years, something around forty students. I watched in horror in the week before semester as the student enrolments climbed well beyond this. In the event I had 55 interviews to do, last week and early this week. That was a high workload, fitting all that in amongst my other lectures and commitments. That said, preparing and marking a written test is a pretty demanding exercise time-wise as well. But I wouldn't want to repeat the exercise with a bigger class. 

Then I had to convince students that this was actually a reasonable thing to do. I spent an entire lecture session on discussing how the test would run. It was the best attended of any of the lectures in this paper! If that's not proof that students are motivated by assessment, I'm not sure what is. The feedback I've had so far has been mostly positive, which is reassuring, although there are some things that in hindsight I could have got better. 

Then, what if it all fell apart? What if I were sick? (I had no back-up plan here). Or students, for whatever reason, got the wrong idea of what was required? (I had given them a task to do beforehand which we'd talk about as part of the interview).   In the end, there were no such problems, but there could have been.  

So how did students do? I've had a number of positive comments (plus some negative ones) relating to how students felt that the oral test actually got them better prepared and engaged with their learning beforehand than a written test would. That was part of the plan! Also, from my point of view, I got to learn just what it was that the students had learned. The breadth of the learning took me by surprise. My first question to all of them (which they were expecting, because I'd told them) was "tell me about something you've learned in this paper". I had my own pre-conceived ideas about what they'd all choose, but I was wildly mistaken. Between them the students covered just about the whole paper, sometimes in accurate detail, including bits that I thought I'd glossed over. Aspects that I thought were really difficult were in fact grasped really well. Conversely, when I started to ask some probing questions, some things that I thought were straightforward, proved to be mis-understood. That feedback to me is more useful than years' worth of student appraisal questionnaires, and that reason alone is enough for me to view the oral tests as a success.

So what happens if I get 70 students next year?

*Biggs, J. (1999) Teaching for Quality Learning at University (pp. 165-203). Buckingham, UK: SRHE and Open University Press.