# Physics Stop

## Sandpile physicsMarcus WilsonJun 22

Yesterday I had a look around the Tate Modern art gallery in London. As is the way with modern art, there was the expected mix of fabulous, bizarre and seriously-stretching-the-definition-of-art exhibits.

One of the pieces on show at present is a pile of porcelain ‘sunflower seeds’, by the artist Ai Weiwei. This piece of artwork is probably best known for being very politically-charged, the manner in which it is currently presented also illlustrates nicely a perplexing physics phenomenon: the ‘sunflower seeds’ form a large but nearly perfect cone.

If we pile up lots of small particles (e.g. grains of sand) we find that there is a maximum possible angle that the pile can take. If you try to make a slope that is too steep, you get an avalanche, until the slope reverts back to what is called the angle of repose. This angle depends on the particle size, shape and frictional properties. As we know, it is easier to build a sandcastle out of wet sand than dry sand, because the wet sand can hold a slope much better: it has a higher angle of repose.

Another example would be the conical nature of many volcanoes, e.g. Ngauruhoe. If you model this (perhaps simplistically) as a source of cinder at the top of the volcano then a cone shape naturally follows, since the slope everywhere on the cone is the same.

Although the effect is very clear, the exact reasons as to why this happens is not; it’s still a subject of a lot of research.

## Force, momentum and umbrellasMarcus WilsonJun 21

Perhaps the most difficult of the ‘big’ challenges facing modern engineering is designing a lightweight umbrella that doesn’t turn inside out with the slightest breath of wind. There you are, walking down a Portsmouth Street with your umbrella up, when a bus comes along and its wake is sufficient to rip the umbrella inside out. And you get very wet.

A brief physics-based estimate reveals the scale of the problem facing the umbrella-technologists. The force exerted by the wind on an object is equal to the rate at which it intercepts momentum from the air. It’s roughly equal to the cross-sectional area of the object, times the density of the air, times the velocity of the air squared.  The velocity gets squared because while  the momentum of a volume of gas is equal to its mass times its velocity,  the mass intercepted per second is also proportional to velocity. That gives a factor of velocity squared.

So, for my mother’s ex-umbrella, with a cross-sectional area of maybe 3/4 of a metre squared (circle of radius 50 cm), air of density 1 kg per metre cubed, and a wind speed of 10 metres per second, which isn’t terribly large, we get an estimate of force of 75 newtons. That’s the force that a 7.5 kg mass has under earth’s gravity.  With a six-spoked umbrella, it’s the equivalent of hanging about bag of sugar on each spoke.  That’s a lot for a lightweight umbrella to cope with. So no surprises that it failed on me.

Just a couple of days now before I return back to NZ, which is probably when the weather here will improve.

## Troublesome TurbinesMarcus WilsonJun 16

I’ve been reading in theUK’s Daily Telegraph newspaper in the last couple of days about the troublesome position the UK’s wind turbines are in. As well as being noisy and (according to some) ugly, there are big problems with managing their power output. Apparently, the average wind turbine is shut down for about 25 days each year because it is too windy. The problem is not that the turbines are in danger of damage – it’s because supply of electricity outstrips demand. On a windy summer night the generating capacity of the turbines can exceed the consumption of power.

Ideally, this extra energy would be stored and then released when it was required, but there is very limited potential for doing this. This is one of the major issues holding back some of the greener forms of power generation. For example, with solar power, there is great generation potential during the middle of a sunny summer day, but, in many countries, the power consumption is going to peak on a cold winter evening. How do we keep hold of that energy in the meantime? On a small-scale it can be done with batteries, fuel cells,supercapacitors, etc., on a larger scale withpump-storage hydro-electric schemes, but on the kind of scale needed to exploit renewable energy fully, we are lacking cost-effective options. In the meantime, the turbines need to be turned off so they don’t overload the grid on windy summer nights.

Then there’s also the issue of justhow much greenhouse gas is produced during the construction of a wind turbine, but that’s another story.

## Diode FlavoursMarcus WilsonJun 14

A week or so ago, I had an exciting poster session with my second year solid-state physics class. Their main assignment for the term was to prepare, in small groups, a poster on a particular flavour of diode. My intention here was to get the students to learn about the underlying physics of a solid-state diode. Rather than simply tell them about it, and then set some test questions on it, I thought that posters would be much more interesting and motivating.

But the students didn’t just have to make their own posters. Part of the exercise was to look at other posters and comment on them. That way I hoped they’d learn a bit from each other too. Whether they’ve done that I can’t tell just yet – the students are yet to sit their final exam, but overall I felt that the session was really productive. I was impressed by the amount of work students had put into their posters and the way that they had found out, distilled, and presented information. They seemed to enjoy it too – there was a  certainly a lot of talking in the room while we were looking at the posters.

I had groups look at five different flavours of diode, namely the tunnel diode, Zener diode, Avalanche diode, Photodiode and Schottky diode. A standard diode allows electrical current to pass one way, but not the other. It can be made very simply from semiconductor material. The other diodes have subtlely different behaviours. For example, the Zener and the Avalanche diodes will breakdown at high enough reverse ‘bias’ (voltage) so they start conducting if you apply a large enough voltage. My ‘favourite’ though is the tunnel diode. At a small range of voltages, quantum mechanical tunnelling allows a higher current to flow than otherwise would be expected. At the upper end of this voltage, this means that current reduces as voltage increases – quite an unusual effect. That leads to a few specialist uses, some of which the students were able to elucidate. Well done to them.

I’m in the UK at the moment, enjoying the cold wet weather. Just 5 degrees on Exmoor at 4pm on a June afternoon a couple of days ago. Nicer than earthquakes, though.

## Growing as a teacherMarcus WilsonJun 02

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Yesterday I had a lunchtime discussion meeting with some of my colleagues doing the Post Graduate Certificate in Tertiary Teaching.  It was really a social session with the idea of being able to learn from each other with regard to our teaching experiences and the things we’re doing with our classes.

The PGCertTT at Waikato consists of two papers. The first one, which I did last year, involves undertaking and evaluating a couple of teaching initiatives with one or more student classes. One of these is focused on assessment – I think that’s a good idea since so much of what students learn is driven by what they are being assessed on.

The second paper (this year’s) involves preparation of a couple of portfolios.  One is a ‘career’ portfolio, which includes things you might wish to include with a CV or application for a teaching job – e.g. information about your professional development, publications on teaching, feedback from students, examples of teaching initiatives you’ve tried, and so on. The other portfolio is the ‘personal’ one, which is more of what a die-hard scientist would call a wishy-washy touchy-feely thing, which looks at things like your teaching beliefs and how these have changed, important events that gave you insights into teaching, future plans, etc.  The good news for me is that I’ve basically done most of the touchy-feely-personal-portfolio-thing last year as part of applying for a faculty and then university teaching award  (I got a faculty one, but not a university one).  The career portfolio should be rather more straightforward.

Anyway, getting back to what I wanted to say, at this meeting  several of us were enlightened by talking to others about just how much stuff they had available to put in their portfolios.  By talking about the kinds of things we’d done with our students, ways that we’d reported on that at conferences etc, a lot of us realized that we actually had done more than we thought on our own development of teachers.  Perhaps the very-kiwi thing about not shouting about how well you’ve been doing something eventaully leads you to believe yourself that you haven’t done well at it, and it takes others to tease that information out of you. Something to remember when we mark student work – they need to know when they have done it well, not just what they’ve got wrong.  So I realized that there’s a bit of stuff I could feed into my career portfolio that I would tend to dismiss as ‘trivial’, but others would disagree.

Oh, and also, there’s one more task that I need to do this year, which is keep a reflective journal during the process of doing the personal portfolio. A blog counts as a suitable medium, so you’ll get a few posts in the forthcoming weeks about what I’m feeling and recalling about my teaching, interspersed, of course, with more direct physics ones. Hope you enjoy it.

I’ll be away from work for the next three weeks, so blog entries will reduce, but hopefully not stop altogether.

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