# Physics Stop

## A light puzzleMarcus WilsonJul 13

Here's a puzzling photograph that Hans Bachor showed me at the end of the NZ Institute of Physics conference last week. It comes from his public lecture on lasers a week ago. And we don't have the answer to it, so maybe you can enlighten us (pun intended).

The photo is of a demonstration of total internal reflection with a laser. Hans is holding a container of water, which has a small hole at the bottom. Consequently there is a jet of water emerging. A laser is held up to the container, and with careful orientation it can be made to shine down the stream of water. The light follows the water, due to total internal reflection at the boundary between the water and the air (rather like a fibre-optic). Actually, it's not TOTAL internal reflection – if it were we wouldn't see the light escaping from the stream of water, but a great proportion of it is contained within the water stream.

Now, in this case, Hans didn't quite get the hole the right size and shape. Consequently the stream breaks up into discrete droplets, which you can see in the photograph. Now, here's the puzzle. Look at the droplets and you can see that a couple of them are shining green – i.e. they appear to have laser light in them.

But how does that work? Light moves so much faster than water one can consider the water to be 'frozen' in space as far as the light is concerned. While the laser light will happily travel along the water stream, when the stream breaks up into drops there is no total internal reflection anymore. The drops should not be glowing. Perhaps the light is jumping from drop to drop to drop. Unlikely – each drop will scatter the light considerably so that very little will jump from one drop to the next – let alone across many drops.

As you think about this, you should bear in mind the conditions the photograph is taken over. It's a flash photograph, but it's likely that the shutter is open for longer than the flash illuminates the scence. This might (or might not) be significant, since the flash will capture the position of the water stream, but the shutter will still be letting in light from the laser even after the flash has stopped. So the capturing of the 'green' laser light in the photograph is not completely synchronized with the capturing of the rest of the image.

Our best hypothesis is that the light that is that drops are illuminated directly by light that is emerging from the end of the stream – that is, the light leaves the stream, travels though the air, and hits a drop. In the spirit of Eugenia Etkina's ISLE approach then, are there other hypotheses and what experiments can we formulate to test them?

## NZIP2015 HighlightsMarcus WilsonJul 07

So the NZ Institute of Physics conference is in full swing. I have a bit of a break between the end of the last session and tonight's conference dinner, so there's time to give some highlights so far.

Well, first, the low-light: Like the rest of my family and half of Hamilton I've had a horrible cold. On Sunday morning I was wondering whether I'd be able to make any of the conference. But I've managed to hold things together and now I've stopped sneezing I'm rather less infectious than I was at the weekend. So I've been able to get to some of the sessions.

So what's been going on? We've heard from Hans Bachor that after decades of international scientific research into getting lasers to work, the world's first funding application for using lasers was for a 'death ray'. Fortunately, applications have grown well beyond this one (which is still, thankfully, not in place) and far beyond the ideas of the original researchers (i.e. 'blue sky' research can have real value). We've seen edible fibre-optics (basically jelly), and heard from Jenni Adams about the ICE CUBE detector at the South Pole for detecting high-energy neutrinos.

The speed talk session last night gave us a rapid-fire mix-and-match bag of physics research from across the country – from Kannan Ridings' simulations of the melting of metal nanowire's through to Inga Smith's (unanswered) question of why do so few women do physics?

But the real highlight for me has been Eugenia Etkina's inspiring talk yesterday and workshop this morning, on physics laboratory experiments. The basic idea here is that experimental science is done by experts in a particular way (and she has evidence for this), including a cycle of observation, hypothesis, experimental design, prediction-making, experimental testing, then judgement. Experiments  by experts are done for particular reasons – either to observe, to test, or to apply. Give a group of scientists a practical problem and they will tackle it in a very systematic way, that usually allows them to get to the bottom of what's happening. Give the same problem to first-year university students, and it's a mess of hypothesis, tesing, judgement, observation all rolled into one. So it then makes sense for us to give students opportunities to carry out the same scientific processes as real scientists. Too often we give them a series of instructions to follow. This isn't how real science works. It simply doesn't help them learn science.

At the end of her talk, Eugenia asked a very simple but really telling question. "How do you know that Newton's third law is true?" My initial answer, to be honest, was: "because the text-books say so". Not the answer of a scientist.  Thinking about it a bit more, I can say "because that's what I experience…if I hit something hard it hurts…i.e. if I exert a large force on something it exerts a large force on me". But here's (roughly) what one of Eugenia's students said when given the same question:

"I have carried out many independent tests of this law and have not found a single case where it is violated." Now, that is the response of a real scientist.

## Lenz’s law – at 3 teslaMarcus WilsonMay 20

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

## A blatant plug for the NZIP2015 conferenceMarcus WilsonMay 06

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.

## Static friction is something sticky (as is Scholarship physics)Marcus WilsonFeb 13

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In January I had a go at the 2014 Scholarship Physics Exam, as I've done for the last couple of years. Sam Hight from the PhysicsLounge came along to help (or was it laugh?) The idea of this collaboration is that I get filmed attempting to do the Scholarship paper for the first time. This means, unlike some of the beautifully explained answers you can find on YouTube, you get my thoughts as I think about the question and how to answer it. Our hope is that this captures some of the underlying thinking behind the answers – e.g. how do you know you're supposed to start this way rather than that way? What are the key bits of information that I recognize are going to be important – and why do I recognize them as such? So the videos (to be put up on PhysicsLounge) will demonstrate how I go about solving a physics problem (or, in some cases, making a mess of a physics problem), rather than providing model answers, which you can find elsewhere. We hope this is helpful.

One of the questions for 2014 concerned friction. This is a slippery little concept. Make that a sticky little concept. We all have a good idea of what it is and does, but how do you characterize it? It's not completely straightforward, but a very common model is captured by the equation f=mu N, where f is the frictional force on an object (e.g. my coffee mug on my desk), N is the normal force on the object due to whatever its resting on, and mu (a greek letter), is a proportionality constant called the coefficient of friction.

What we see here is that if the normal force increases, so does the frictional force, in proportion to the normal force. In the case of my coffee mug on a flat desk*, that means that if I increase the weight of the mug by putting coffee in it, the normal force of the desk holding it up against gravity will also increase, and so will the frictional force, in proportion.

Or, at least, that's true if the cup is moving. Here we can be more specific and say that the constant mu is called 'the coefficient of kinetic friction': kinetic implying movement.  But what happens when the cup is stationary? Here it gets a bit harder. The equation f=mu N gets modified a bit: f < mu N. In other words, the maximum frictional force on a static object is mu N. Now mu is the 'coefficient of static friction'. Another way of looking at that is that if the frictional force required to keep an object stationary is bigger than mu N, then the object will not remain stationary. So in a static problem (nothing moving) this equation actually doesn't help you at all. If I tip my desk up so that it slopes, but not enough for my coffee mug to slide downwards, the magnitude force of friction acting on the mug due to the desk is determined by the component of gravity down the slope. The greater the slope, the greater the frictional force. If I keep tipping up the desk, eventually, the frictional force needed to hold the cup there exceeds mu N, and off slides the cup.

What this means is that we when faced with friction questions, we do have to think about whether we have a static or kinetic case. Watch the videos (Q4) you'll see how I forget this fact (I blame it on a poorly written question – that's my excuse anyway!).

*N.B. I have just picked up a new pair of glasses, and consequently previously flat surfaces such as my desk have now become curved, and gravity fails to act downwards. I expect this local anomoly to sort itself out over the weekend.

P.S. 17 February 2015. Sam now has the videos uploaded on physicslounge   www.physicslounge.org

## Modes of a square plateMarcus WilsonJan 15

Alison has drawn my attention to this video. It demonstrates vibrational modes of a square plate by using sand. At certain frequencies, there are well defined modes of oscillation, in which parts of the plate 'nodal lines' are stationary. The sand will find its way to these parts and trace out some lovely pictures.

Vibrational modes are often illustrated through waves on a guitar string. Here, the string is held stationary at both ends, but is free to vibrate elsewhere. There is a fundamental frequency of oscillation, where the distance between the ends of the string is half of a wavelength (this ensures the displacement of both ends of the string is zero since they are clamped).  Since wavelength is related to frequency (frequency = speed/wavelength) that means if the wavelength is 2 L where L is the distance between the ends of the string, we have frequency = speed/2L.

But that's not the only possible mode. Another one would have L equal to a whole wavelength (equals two half wavelengths). Or one-and-a-half wavelengths (equals three half-wavelengths.) This gives us, rather neatly, frequency = n speed/2L, where n is an integer. We see that our 'harmonics' are just integer multiples of the fundamental frequency. Rather neat.

However, if you look at the frequencies given in the video, they appear to be all over the place. I challenge you to pull out the relationships between these (I've tried). There are a few reasons why the case shown on the video is considerably more complicated than the waves on the string.

1. The boundary conditions. The edges of the plate aren't clamped in place. This makes it less straightforward to define the modes geometrically.

2. The plate is square, giving rise to 'degeneracy' in the modes. This term refers to two or more distinct modes having the same frequency. You can see it rather well with the 4129 Hz mode. Basically, there are horizontal stripes shown. But equally, with the same frequency, you could get vertical stripes. Why don't the two occur together? They do. You can see the effect of having a little bit of vertical stripe most clearly at the far end of the plate, where the pattern becomes more square-like. Also, with a square, you can get two completely different types of mode with the same frequency. This occurs because what matters are the sums squares of pairs of integers. Broadly speaking (at least for a square clamped on the edges, which I must point out this ISN"T), our modes follow the relationship:

f = C sqrt(n^2 + m^2)

where C is a constant, 'sqrt' means square-root, and n^2 is n-squared. So, for example, not only is 50 equal to 5-squared plus 5-squared, it is also equal to 1-squared plus 7-squared (or 7-squared plus 1-squared). This gives us three  modes all competing to appear at exactly the same time. What happens then isn't easy to tell.

3. Non-linear effects. This a physicist's code-word for 'it's all too difficult'. That's not quite true – arguably most of the interesting physics research happening in the world is looking at non-linear effects. What this really means is that, if A and B are both solutions of a problem, then some combination of A and B is NOT a solution. A lot of physics IS linear – Maxwell's equations in a vacuum is a good example – but a whole lot isn't. With waves, the speed of the wave usually depends on frequency (i.e. is not constant) which means we lose the nice, integer-multiple relationship of our waves-on-a-string mode.

So, enjoy the video for what it is, and don't try to analyze it TOO closely.

## Archimedes principle: think carefullyMarcus WilsonNov 14

Benjamin has recently acquired a 'new' book from Grandma and Grandad: Mr Archimedes' Bath (by Pamela Allen – here's the amazon link – the reviews are as interesting as the content). The story-line is reasonable guessable from the title. Mr Achimedes puts water into his bath, gets in, and the water overflows. What's going on? So we've been doing some copycat experiments – not by filling the bath right up and having it slosh all over the bathroom floor (Waipa District Council – you can rest easy about water usage)  but filling up rather more sensible-sized containers and dropping objects in.

Archimedes principle is actually a little more involved than simply saying that putting an object in the water will raise the water level. It says that the weight of water displaced is equal to the force of buoyancy acting on the object.  This picture summarizes it. That is, if an object of 2 kg floats, then 2 kg of water will be displaced. If an object is unable to displace enough water for this to be the case, it will sink. That still should be pretty easy to get, especially if you've done some experimenting. However, it can still be the basis of some really hard questions. I had one in my third year  physics exams at Cambridge. In our 'paper 3', as it was called then, the examiners had free reign to ask about ANYTHING that was on the core curriculum from any of our years of study – plus ANYTHING that was considered core knowledge for entry into the degree (which meant basically anything at all you were taught in physics or general science from primary school upwards). This paper was feared like anything – it was basically impossible to revise for*.

Here is a question then, as I recall it from the exam.

An ice cube contains a coin. The ice completely surrounds the coin. The cube is floating in a container of water.  The cube melts. Does the water level rise, fall, or stay the same?

Now, the icecube melting question is one that is often banded about. A floating icecube will displace its own mass of water (so says Mr Archimedes). When it melts, this water will occupy the 'space' that is displaced by the cube. Consequently, the water level will stay the same. A practical example of this is in the estimation of sea-level rises due to global climate change. When the ice floating on the Arctic Ocean melts, it does not cause a sea-level rise, since it is already displacing its own weight. However, the icecap on Greenland will cause a sea-level rise as it melts, since it is currently not displacing any of the sea (since it is sitting on land.)

However, that is not the question that is asked. Our icecube has a coin inside it. What difference does it make? Well, the icecube-and-the-coin will still displace its weight of water since it floats. However, when the icecube melts, the coin sinks and no longer displaces the same amount of water as it did when it was frozen into the cube. Therefore the water level falls. That's quite a subtle application of Archimedes principle. After the exam, a group of us sat arguing about it, till we collectively worked out what the right answer was (see – exams can be good learning experiences!). Unfortunately, at this point I realized my answer was wrong. Even still, I managed to get out of the degree with a first-class honours, so I couldn't have done too badly on this exam overall.

*The other question I remember from this paper is 'What is Cherenkov Radiation?' I didn't have a clue what Cherenkov radiation was when I sat the paper – I made up some waffly words and wrote them down and almost certainly received zero for the question.'  Later, one of my friends found a single, incidental sentence in a handout that was given out by our nuclear physics lecturer that identified what it was. That's how nasty this exam was.

## Toddler does physics-artMarcus WilsonOct 29

As we all know, a scientifically-minded toddler plus a piece of technology can lead to unexpected results. This is the result of Benjamin playing with a retractable steel tape measure at the weekend. How we came to break the case apart I don't know, but the results are pretty (the cellphone shot in poor light doesn't do justice to the artwork):

I like the koru-shape made by the end. The measure has curled itself into a complicated form rather reminiscent of a protein structure, with sections of helices and straighter lengths. Although the mechanisms are different (protein structure has a lot to do with the intricaces of chemical bonding) the physical process is similar –  the structure works itself to a local minimum of energy. Just how this happens  is all rather complicated from a physics perspective. Perhaps the most obvious example of twists of this form is in telephone cords. The phenomenon has even lent its name to a type of structure seen in thin films – the 'telephone cord buckle'. Unfortunately Benjamin didn't give me any warning about what was going to happen – otherwise I'd have filmed it (and he would probably have retreated to a safe distance – the whole unravelling was pretty energetic).

BUT…since Karen is an occupational therapist and has accumulated large numbers of free tape measures as corporate freebies in her career, we could maybe spare a few for high-speed filming.

## Robot racingMarcus WilsonOct 22

The Engineering Design Show is currently in full swing here, with the competitions for the various design projects. The white-line followers kicked off proceedings. They were pretty impressive, with all but one team successfully being able to follow the (very squiggly) line without mistakes. There were traps to confuse the robots – the line got thinner and thicker, crossed over itself, had abrupt corners and so on, but the robots were well programmed and coped with this easily. The winning group was impressive indeed. They had some very carefully optimized control parameters, meaning that the robot was (a) really straight and fast on a straight-line section but also (b) precise round the turns, slowing down just enough to take each turn at about the right speed. I think anyone would struggle to get something going quicker than this one.

On show at the moment are the third year mechanical engineering students who have designed a pin-collecting machine. The idea is that the vehicle pulls still pins (about 5 cm in length, maybe 5 mm in diameter) out of a board – the one that collects all the pins in the quickest possible time and drops them back in the collecting bin is the winner. The most striking conclusion from this exercise is the emphasis on the old adage "To finish first, first you must finish". A good proportion of the entries have died part way through the process – pins have jammed the mechanisms, the motors have failed, or, in one disappointing case, the machine collected the pins in lightning quick time and then failed to go back to deposit them in the collecting bin. Also, we've seen one machine disqualified for being downright dangerous – its first run saw it pulling pins out of the board and firing them across the room causing spectators to beat a hasty retreat.

But the winner (or so it looks) has pushed their luck to the limit.  The "…first you must finish" line is actually not quite correct. More accurate would be to say "…second you must finish. First, you must start". They've admitted to putting 5 volts over a motor rated at 3 volts in practice just before the event, and frying the motor. They then had to hurridly locate a replacement and install it while the competition was in progress. Missing their first two rounds, they appeared looking hot and sweaty just in time for their run in round 3 out of 4 and simply destroyed the rest of the competition. (Presumably it won't be long before they destroy their new motor too, but it's survived long enough to win, according to the rules, and that's what counts.)

Overall the design show has been great fun to be a part of and has really demonstrated the skills that the students have acquired. Well one everyone involved!

Postscript 29 October 2014: We're a hit with the Waikato Times!

## Telepathy breakthrough – great science, not science fictionMarcus WilsonSep 08

The 'Science' news hitting the media at the weekend was Guilio Ruffini and Alvaro Pascual-Leone's demonstration of 'telepathy'. There's been a lot of media coverage on this – for example the neat little interview of Ruffini on the BBC's 'Today' programme.

Their article on this can be read here. It's not a long one, and, for a piece of science, I reckon it's pretty clearly described.

But, I'm afraid, you can forget The Chrysalids – the messages sent from India to France are of a rather more humble nature. But the science behind it is great.

Essentially, the work has linked together two existing technologies, via the internet. The first is long-established – namely monitoring of the electroencephalogram (EEG). If you place electrodes on the surface of your scalp, you can detect electrical signals that originate from the electrical behaviour of the neurons in the cortex of your brain. The signals aren't large, just a few microvolts, but they are fairly easy to pick up. I get students doing it in the lab. Different kinds of brain activity lead to different signal patterns. A 'thinking' brain has lots of small amplitude, fast activity, whereas someone in deep sleep shows an EEG pattern that has a large, approximately 1 Hz cycle to it. The two patterns are very different. EEG is routinely used for monitoring sleep patterns and as a tool for an anaesthetist to monitor the depth of anaesthesia in their patient – one wants to make sure the patient is well anaesthetised, but on the other hand one doesn't want to head into Michael Jackson territory. The EEG can help.

So the EEG is a way of 'reading' the state of the brain. To go from an EEG recording to working out what the subject is thinking about is a long, long way off, if indeed it's possible at all, but one can certainly say something about the brain state.

If EEG is about reading the state of a brain, then the other technology, transcranial magnetic stimulation (TMS), does the reverse. This is rather newer, and our understanding of it is much poorer (I'm involved with a TMS research project at the moment).  In TMS, pulses of magnetic field are applied to the brain. The effect depends on what area of the brain the pulse is applied to, and in what orientation. At a simple level you can make an arm 'twitch' by applying the pulse to the correct part of the motor cortex. I've seen this done at the University of Otago (on a brave summer student of mine). In Ruffini's work, they used the magnetic pulse to 'create' the perception of a flash of light by stimulating the visual cortex. The subject 'sees' the light, even though there's no such flash on the retina, since the sensory circuits in the cortex that usually interpret what's going on on the retina are activated remotely.

So what did the experiment do? The person in India sending the message imagined a particular activity (hand or foot movement), and their EEG changed depended on whether they imagined the hand or foot. A computer interpreted the EEG, decided on which it was, and communicated with the computer in France. The French hardware system then zapped the human receiver in such a way as to either trigger the flash or not trigger the flash. The receiver then reported orally whether they'd seen a flash. In this way the 'message' (a string of 1's (hands) and 0's (feet) ) has been sent from one to another without using the senses of the receiver.

In that sense this is telepathic. The receiving person had no communication with the transmitting person in a visual, oral, or any other way. True, one might ask, why didn't they just phone/Skype/email each other to send the message, and of course you wouldn't want to communicate with your family members overseas with an EEG/TMS system. But that's not the point. The point is that it is a great demonstration of science.

Will it lead to small telepathic headsets? Rather than fuss with phones and email, we could just have a conversation with anyone in the world just by thinking about it. (You'd want to be sure you'd switched it off afterwards!)  Don't get excited – we're not in Chrysalids territory yet. That's a long, long, long, long way off. But it is good science.

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