SciBlogs

Archive March 2010

Goodbye old technology and hello new Marcus Wilson Mar 31

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First the new stuff:

Judging by the excited twittering of the last day or so, there are a few rather excited people at CERN.  http://www.twitter.com/cern . It’s now running and producing collisions at 3.5 TeV per beam. We are well in the realm of new physics.  The Higgs boson might feel a little nervous now – it will find it increasingly difficult to hide from those experimenters (assuming of course that it exists). And the PhD students finally will be getting some decent data to work with.

Now the old stuff:

I came across this blog yesterday, moaning about the UK’s decision to cease broadcasting analogue radio in five years time. http://new.uk.music.yahoo.com/blogs/touchingthevoid/28943/is-this-the-end-of-your-radio/ Every single comment was negative, and I must say I share this view. Digital radio gives noise free reception, because it’s signal is discrete.  In crude terms it’s like broadcasting a series of ’0′s and ’1′s.   Now, the receiver might pick up a ’0.003′ because of some noise, or some event on the way, but the receiver is smart enough to recognize that this is far more likely to have started life as a zero than a one, and sets it back to its original value. The result is crystal clear reception, and that is supposed to be what the listeners want.

Except that analogue radios are, in their simplest form, really very easy to build and very very cheap. Anyone with a small amount of electronics experience could build one. You have a power source (battery), an aerial (a length of wire) a tuner (a resonant circuit with a variable capacitor and an inductor), an amplifier (a transistor and a few resistors), and a loudspeaker.  And hey presto, a working radio.  Who cares if the sound is a bit fuzzy on occasion.

Analogue radios have made a huge impact on communication throughout the world – and going the digital-only route starts excluding those who are happy to pay a few dollars for an analogue radio, but not a few hundred for a digital variety. Sure, noise free gives a good sound, but, when all you want to listen to the cricket commentary while out in the garden one afternoon, why is it deemed necessary?

Things that don’t like water Marcus Wilson Mar 30

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So, my class of students (well, at least one of them) have done the calculations and think that a centimetre of water is enough to shield a mobile phone from communicating with the nearest mast.  Only one way to find out.  I’ll bring along a bucket, lots of glad wrap and waterproofing materials to tomorrow’s lecture, along with my phone, and they can have a go. (Do I trust them?) If a bucket of water isn’t enough, we’ll have to move over to the swimming pool, but that will probably have to wait until after Easter.

On the subject of putting things that aren’t usually considered as water loving into swimming pools, our cat came into the house Sunday night absolutely drenched.  I can only surmise that he’d either had a bucket of water thrown over him by a fed-up neighbour (should hopefully do the trick, but then our cat hasn’t shown a huge degree of intellectual ability in the year or so he’s lived with us) or he’d fallen into someone’s swimming pool. It is reassuring to see from YouTube that cats can actually swim pretty well, and climb out of a pool on their own. It probably wasn’t a near-death experience.

 

What goes up… must come down Marcus Wilson Mar 26

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Yesterday morning while driving into work I was reminded that this week is ‘Balloons over Waikato‘ – the annual hot air balloon festival.  It was hard to miss; I counted 20 balloons making their way gracefully over south-east Hamilton and drifting slowly towards Morrinsville. (NB: I counted the balloons AFTER I had parked the car, not while driving, in case you are worried.)

Then this morning I was treated to the sight of a balloon landing on the University sports fields. I say ‘landing’, but don’t get the impression this was a smooth touchdown. There was a fair wind blowing – in fact I was surprised the balloons were flying at all –  and the balloon came down at about 45 degrees at a fair pace – hit the ground, did a short bounce, hit again, whereupon the basket was tipped onto its side and dragged for several metres before the balloonists manged to deflate the balloon a bit and reduce some of that lateral pull from the wind.  I’m sure it’s the kind of landing that could break bones if you’re not prepared for it.  (Not that I know – I’ve never been in a balloon – and this sight doesn’t encourage me to).

Being the physicist that I am, I think a few estimates are in order.

Wind speed – probably aroud force 3, so about 15 km/h or about 4 metres a second. Cross-sectional area of balloon – maybe about 10 metres by 10 metres, or 100 metres squared. Then, I’ll assume the balloon intercepts in one second a lump of air of size 100 metres squared times 4 metres, that’s  400 metres cubed, which weighs about 400 kg  (density of air is about 1 kg per metre cubed).  At 4 metres per second, the momentum transfered in one second is then 400 kg times 4 m/s or 1600 kg m/s.   The sideways force on the balloon, being rate of change of momentum, is then 1600 kg m/s /s, or 1600 newtons. 

In context, that’s the same as the force of gravity on a mass of 160 kg.   I’m not sure how mass the basket has, but I’d imagine with a couple of people and some propane tanks it could be around 200 or 300 kg or so. So the lateral force of the wind on the balloon is fairly close to the force of gravity on the basket.  (And, while there’s still buoyancy in the balloon – the net downward force on the basket would be rather less than its weight, bringing them closer still.)

I don’t know what the coefficient of friction is for short grass, but probably not desparately high – so I’d expect that sideways force to be able to move the basket across the ground. Which is exactly what happened.

I have no idea what injuries, if any, were sustained by the crew (I was watching at a distance), but I saw one guy hop out without difficulty before I left to go to my office.

Mind games for physicists Marcus Wilson Mar 24

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Here’s a gem of a paper from Jonathan Tuminaro and Edward Redish.

The authors have carried out a detailed analysis of the discussions a group of physics students had when solving a particular problem. They’ve worked hard (the researchers, as well as the students) – the first case study they chose was a conversation 45 minutes long.

While tackling the problem, the students have ‘played’ several epistemic games – or, put more simply, have used different ways of thinking. There are six different games identified – corresponding to six distinctly different ways of thinking about the same problem.  Students don’t stick to one game though, they can flip between several. Very quickly, they are:

1. Mapping meaning to mathematics.   This is where the students work out what is going on (or what they think is going on) and put it into a mathematical form (e.g. to make an equation) – then the equation can be used for things.

2. Mapping mathematics to meaning.  Kind of the reverse of (1). Here the students start with a mathematical expression they know, and work out what it might mean in practice.

3. Physical Mechanism Game. In this game the students try to draw sense from their own intuition of the physical principles involved.

4. Pictorial Analysis Game.  Here diagrams are used as the major step.

5. Recursive Plug-and-Chug. I’ll quote from the authors here, because they do it so well: "[here the students] plug quantities into physics equations and churn out numeric answers, without conceptually understanding the physical implications of their calculations."   (The emphasis is mine.)

6. Transliteration to mathematics. Here the students draw from a worked example of another, similar problem, and try to map quantities from problem A onto quantities of problem B.

Now, I ask myself, which methods do I see my students doing in my classes, and using in the assignments I set.  I have to say that in many cases I’m not sure – and probably my teaching is the worse for it.  I can say which games I would like to see students using (1 to 4) and which games would make me shudder (5 and 6 – in which the students develop no physical understanding about what is happening),  but do I know?  There are certainly ways of getting students to use the ‘right’ games, notably setting the right kind of assessment questions.

OK, so which games do I play most in my research?  I’d say probably 1, 2 and 3.  I do a lot of physical modelling, in which I represent a problem (e.g. how do neurons in the brain behave in a certain environment) through a series of equations (game 1) and then work out the implications of those equations (game 3). I also draw a lot from my intuition about physics (e.g. if you increase the pressure across a pipe, you’ll get more flow, regardless of what shape the pipe is) – that’s game 3.

Finally, those physicists among you might like to know what problem the students had to solve. It was this. Three electrical charges, q1, q2 and q3 are arranged in a line, with equal distance between q1 & q2, and q2 & q3.  Charges q1 and q2 are held fixed. Charge q3 is not fixed in place, but is held in a constant position by the electrostatic forces present.  If q2 has the charge Q, what charge does q1 have?

    o    q1                   o   q2                    o  q3

The authors say that most experienced physics teachers can solve this problem in less than a minute. I solved it in about five seconds, using game 3, with a tiny smattering of game 2.  The students concerned (3 of them together) took 45 minutes – this massive difference is perhaps interesting in its own right.

Reference (it’s well worth a read if you teach physics at any level):  Tuminaro, J. and Redish, E. F. (2007) Elements of a cognitive model of physics problem solving: Epistemic games. Physical review special topics – Physics education research (3) 020101.   DOI: 10.1103/PhysRevSTPER.3.020101. 

 

 

The invisibility cloak Marcus Wilson Mar 22

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Yes, the headline writers are at it again, talking about those crazy scientists designing invisibility cloaks. As usual, the articles I’ve seen in the papers (e.g. the front page of The Waikato Times) and popular internet sites are high in ‘Harry Potter’ and ‘Star Trek’ and low in science.

Research into this kind of thing is not new, but it has lept forward recently with development of ‘meta-materials’, particularly those with negative refractive index. Refraction is what causes a stick to look bent when you put it in water. Negative refractive index materials, amongst other things, bend the light the ‘wrong’ way when the light hits them. They can be used to steer light in otherwise unavailable directions. That’s what you need to do if you want to hide an object – you want the light coming from one side of it to somehow work its way around the object and leave again from the other side as if the object had not been there. I heard a great talk on meta-materials by John Pendry, one of the key players in this field, at the Australian Institute of Physics conference at the end of 2008.

There’s a huge range of potential applications – though of course an invisibility cloak is perhaps the most exciting for your average person in the street.  In this case what has been cloaked is a bump (not a particularly large one, mind you) on piece of gold. A ‘cloak’, has been placed over the bump, and the effect of the cloak is that light reflects off it in the same manner as it would if the surface were flat. A far cry from hiding an entire Romulan armada, but a significant step forward nonetheless.

A reasonable summary of the gold bump experiment is on the PhysicsWorld site, http://physicsworld.com/cws/article/news/42043 , though, in my opinion, it doesn’t go into huge detail. For that you’ll need to go to the original article in the journal Science,  http://www.sciencemag.org/cgi/content/abstract/science.1186351 , which you will probably have to pay for.

Mobile phone physics Marcus Wilson Mar 19

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Just occasionally, I have a crazy thought regarding a physics demonstration.   This is one that I’m thinking about inflicting on my third year electromagnetism class.  

We’ve been discussing the way electromagnetic waves travel (or rather, do not travel) through electrical conductors. Basically, conductors allow electric currents to flow in response to an applied electric field (in simple terms this just means applying a voltage). Electromagnetic waves such as visible light, radio and X-rays contain electric fields, so when one hits a conductor electric currents flow. Flowing currents heat up a material. Where does this heat energy come from? From the wave. In other words, conductors suck out energy from an electomagnetic wave, and, broadly speaking,  the wave can only penetrate so far into the conductor. This distance is what’s known as the ‘skin depth’.

Skin depth depends importantly on two things – the conductivity of the material and the frequency of the wave. The higher the conductivity, or the higher the frequency, the smaller the skin depth.  Thus, if you consider the waves to/from a mobile phone (frequency of around 1000 MHz) travelling through aluminium (a very good conductor) the skin depth turns out to be small indeed – microns in size.  That means wrapping a phone in aluminium foil will prevent it from picking up a signal. I’ve already shown this in class.

But – here’s the crazy thought – what about water?   Distilled water is a pretty non-conductive, but what comes out of the tap is loaded with dissolved salts and has a moderate conductivity, albeit several orders of magnitude below aluminium foil.   What’s its skin depth for  mobile phone frequencies?  I’ve done some quick back-of-the-envelope, and I reckon something of the order  few centimetres.  So….I predict that if we put the phone in just a few millimetres of water (YES, it needs waterproofing first!) it will still receive a signal, but suspend it in the middle of a swimming pool and there’s going to be no reception at all.

 I reckon that getting my class to estimate how much water would be required to shut out the signal, and then design an experiment (that might or might not need to include ‘borrowing’ the university swimming pool for a short while) would be a great way to get them to think about the various issues themselves.  There’s plenty of literature to back up that assertion – e.g. Etkina et al., American Journal of Physics 74(11), p979  (2006). The best thing is that I can’t be tempted to tell them the answer –  because I don’t know it – I haven’t done the experiment myself. Though I have found this YouTube…

ET home phone Marcus Wilson Mar 17

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This month’s feature article in PhysicsWorld is a plea by well-known science (particularly physics) writer Paul Davies to relaunch (or rather, expand) the search for extra-terrestrial life.  The Search For Extraterrestrial Intelligence (SETI) has been around for nearly fifty years, focusing on analyzing data from radio telescopes.  But Paul Davies thinks there are other places where we could and should look for signs of alien activity, not least our own backyard.

The article mentions the neat argument by Enrico Fermi as to why space-faring life elsewhere in the Milky Way is unlikely.   The galaxy’s age is currently thought to be about 13 billion years, give or take (essentially, the age of the universe), whereas its diameter is only a piddly little 100 000 light years.  Fermi’s argument is basically that if life capable of intergalactic travel has evolved elsewhere there is bags of time for it to have travelled across and colonized the whole galaxy by now. Since we don’t see any evidence of it, the conclusion is that it isn’t there.  

There are counters to Fermi’s argument – for example, an alien civilization may have arrived in the past and left again.   In which case, do traces of its existence still remain on earth?

I’ll leave you to read the article, but I will argue now that I would be firmly against such a project.  I’ll give two reasons:

1. It is a waste of our time and resources. Humanity has enough problems to contend to at the moment, such as feeding itself while not inducing major destructive climate change. (I will add, of course, that I do not like to dismiss ‘blue-sky’ research out-of-hand – it is from this kind of research that often new technologies are developed that will benefit in ways that are unforeseen – but in my opinion there is plenty of other ’blue-sky’ stuff (e.g. the Large Hadron Collider) which is well ahead of this in likely benefit to humanity.)

2. What would we do if we discovered that there was space-faring extra-terrestrial life somewhere out there? It seems to me inappropriate to look for it if we do not know how we would respond if we found it.  Is the best thing to do to keep quiet? Or to signal it?  (Though I feel this question would be irrelevant - whatever governments and the UN might advise, someone, somewhere, would send out a signal to it – that is pretty inevitable.) The world does not have a great track record on agreeing on anything, and this, surely, would be a question that would need a unified, world response.

Paul is giving a free online presentation on this on 31 March (early 1 April morning for us NZers) – for those interested

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  • My kind of blog Marcus Wilson Mar 15

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    This is what I like to see – a fellow blogger (Brian Clegg) extolling the virtues of physics blogging and tweeting.

    What’s interesting about Brian’s entry is that he talks about how a blog can trigger a discussion that increases the quality of the original posting.  Like peer review for a scientific paper, but informal, instantaneous, and more widespread. 

    Read the comments attached to his posting, and you will see a great example of what he means.

     

    Copper Conducting Considerable Current Marcus Wilson Mar 12

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    The latest kitchen acquisition (no, we don’t spend all our money on buying things for the kitchen) is decent frying pan. We’ve spent too long with frying pans that are about as flat as the Southern Alps.  It’s a copper-based pan, which probably accounts for its expense, with a stainless steel surface.   The reason for the copper is that it conducts heat extremely well, meaning that the surface of the pan will respond nice and evenly and quickly to the heat from below. 

    Copper is probably better known, however, for conducting electricity rather than heat.  Its electrical conductivity is extremely high, and, coupled with its ease of working, there is no surprise that electrical wiring accounts for a huge amount of copper. 

    These two facts (high thermal conductivity and high electrical conductivity) are not unrelated.  This is because the processes by which they occur are very similar. Electricity is carried by movement of electrons, and electrons are also a major carrier of heat. In copper, there are a lot of electrons that are highly mobile, and hence it has both high electrical and thermal conductivity.

    In fact, in metals, the two follow (approximately) a simple relationship – the ratio of the thermal conductivity to the electrical conductivity is approximately proportional to temperature.  This is called the Wiedemann-Franz law. One can ‘derive’ this relationship using some fairly simple hand-wavy physics arguments, though to do it properly is not so easy. I’ll be doing the hand-wavy approach (for those that want to know, it’s the Drude theory) with my second year students soon.

     

    The LHC ticks onwards Marcus Wilson Mar 11

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    Last month, CERN took the decision to run The Large Hadron Collider for the next eighteen months or so, up to a maximum energy of 3.5 TeV per beam, before having an extended shutdown period to prepare to take it up to its design maximum of 7 TeV per beam.

    I am sure this will come as welcome news to many researchers, particularly the many PhD students who have had their student loans accumulating quietly while they have had no results to process.

    You can follow all the action on www.twitter.com/cern

    P.S. Utterly unrelated to the above, I accidently discovered earlier in the week that if you hit the ‘Windows’ button and the ‘M’ key at the same time on your computer keyboard it minimizes all your windows.  Maybe I was the only person in the world not to have known this, but, in case I’m not, I thought I’d share this essential piece of knowledge with you…

     

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