Posts Tagged experiment

A fun experiment to try at your desk Marcus Wilson Mar 13


I received the latest PhysicsWorld magazine from the Institute of Physics yesterday. A quick flick through it reveals a fantastic demonstration you can do with kids (or grown-up kids) to show how strong friction can be. Take two telephone directories, and interleave the pages (so every page of book A has a page from book B above and below it, and vice versa). Admittedly this takes some dedication, but that's what graduate students are for. Then try to pull the directories apart. In fact, the photo in the magazine shows two such interleaved directories being used in the centre of a tug-of-war. I have got to try with my students. 

In fact, you don't need the patience to turn page-by-page through two phonebooks to do this. I've spent a couple of minutes interleaving my copy of the 84-page University of Waikato Science and Engineering Graduate Handbook with the slightly larger University of Waikato Science and Engineering Undergraduate Handbook.  (Some might say the two make a lot more sense arranged in this manner….) It didn't take too long to do. I can't pull them apart.

It's simply down to the large surface area that the interleaved books have. They are A5 in size (approx 21 cm x 15 cm), with 86 (84 pages plus inside covers) surfaces. That gives, very approximately 27000 cm2 area of contact, around two or three metres squared of contact. That's pretty sizeable. A pair of telephone directories could come in at about 30 metres squared or so!  Lots of surface are gives lots of frictional force. 

What makes something show on radar? Marcus Wilson Mar 11

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One of the questions on everyone's lips at the moment is "How does a large passenger jet simply disappear from radar without trace?"  It is clearly very distressing for anyone with friends or relatives on board – not knowing what has happened. As I write this, there still seems to be a complete lack of clear evidence pointing one way or another. I'm not an aviation expert so I really can't add anything of value here. But I can turn the question around to one of more physics relevance, which is "What allows a plane to be detected with radar in the first place?"

Radar, in concept at least, is pretty simple physics. It's name comes from an acronym "RAdio Detection AND Ranging". However, it has out-grown its acronym, since it now does more than simply detect and 'range' (tell the distance to), and 'radar' is now a word in itself and not spelled in uppercase.  The basic idea is that a radio wave will reflect off a metal object (a plane, ship, your car…) and some of that wave will return to where it came from. To be pedantic, while we often think about the radio transmitter and detector being in the same place, this doesn't need to be the case. In fact the first radar systems had detectors physically separate from the receivers. Anyway, we know the speed at which radio waves travel (pretty well the speed of light in air) and therefore by timing the delay between transmitting and receiving we know how far the object is away. By also knowing the direction the reflections come from, we can therefore work out a position. 

It gets a bit more difficult in practice, since radio waves don't necessarily travel in straight lines, but can be bent due to atmospheric conditions. And radar can tell us more than just position. For example, we can exploit the doppler effect to measure the how fast an object is travelling. Waves reflecting from a moving object return with a different wavelength – measure the wavelength shift and you measure how quickly the object is moving towards or away from you. 

So why does a metal object reflect radio waves? That's down to its high electrical conductivity ensuring that there must be no electric field at the surface.  The waves simply can't get into the material and are completely reflected. I won't bore you with the analysis of Maxwell's equations to show this – unless you happen to be in my third year electromagnetic waves class in which case I'll bore you with it – whoops, make that excite you with it – in a week's time.  Metal makes a pretty good shield for radio. 

Just what fraction of the power of the incident wave that gets reflected back towards the transmitter can be tricky to calculate. It's encapsulated in a term known as the 'radar cross section' (RCS). The definition of RCS is a little tricky to wrap one's head around, but I'll give you it: The radar cross section of an object, in a given direction and a particular frequency, is the cross sectional area of a perfectly-reflecting sphere that would give the same power return as the object gives in that direction. In other words, imagine a large, metal sphere, that reflects the same amount of power that our plane does. Take the cross-sectional area of it (pi times the radius squared) and that's the RCS. A large RCS means a large amount of power returned.

To some extent the RCS simply depends on how big an object is, but just as important is the shape of an object.  Geometry with right-angles in it will cause large reflections back in the direction of the transmitter (think of a snooker table – if a ball bounces off two cushions it's direction of travel is reversed – it doesn't matter about the angle of incidence). Long edges also give large returns – they can act rather like antennas and re-radiate the incoming radiation. Unless you specifically set out to design an aircraft with a low RCS the chances are that what you'll end up with is something which has a pretty substantial RCS. The tail is at right-angles to the fuselage, it has long straight wings, and is made from highly reflective metal. 

And that means that a Boeing 777 isn't likely to vanish off a radar screen while it remains in one piece. 



Dem Cables Marcus Wilson Feb 21

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I've just been shifting around various bits of equipment and computers in our 2nd and 3rd year physics lab, to make way for an item that's shifting in there from a nearby lab. It's gone something like this…(rising in semitones, with apologies to the original performers) 

Da power socket is connected to da extension cord;

Da extension cord is connected to da monitor;

Da monitor is connected to da computer;

da computer is connected to da control box;

da control box is connected to da MRI machine;

da MRI machine is connected to da MRI-machine stand;  

da MRI-stand is connected to da floor*;  

Now why are there so many cables?

Dem cables, dem cables, dem power cables;

dem cables, dem cables, dem ethernet cables;

dem cables, dem cables, dem USB cables;

What a mess of knitting!  [Roll on wireless power transmission!]


*The MRI-stand is connected to the floor because we don't want the thing to move. The unit is calibrated for the position it's currently in; I'm not inclinded to move it in a hurry. The other things, however, might be more sensibly located. 

Nanotechnology, asbestos and measurement Marcus Wilson Feb 04

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Last week I had a very interesting and useful visit to the Measurement Standards Laboratory in Lower Hutt. I went along with my summer scholarship student to discuss the measurement of electrical properties of biological tissue. While the procedure for measuring the conductivity of a piece of solid is pretty-well established, biological tissue is soft, squishy, easily damaged, reacts chemically with what you try to measure it with,  and changes its properties considerably between 'alive' and 'dead' states. There's no clear-cut method here.  

We were also shown around some of the labs. What I found particularly interesting is the progress towards ditching the kilogram. By that I don't mean getting rid of the unit and using pounds and ounces, I mean doing away with the need to have a single, standard kilogram locked away in Paris. One can get away with this problematic beast through using a Watt Balance machine and defining Planck's constant. More on that later, I think.

But today's blog entry is about a discussion I had one evening in a cafe in Wellington train station, with a friend of mine who works for what is now known as Worksafe. As their name suggests, their purpose is to ensure  workplaces are healthy and safe (Not that this replaces the obligation on everyone to ensure a safe work environment, I should add.)  My friend has been having some discussion around the health issues associated with nanotechnology. Engineering tiny things has opened a huge range of possibilities – intensely strong fibres, minature motors, molecular-sized electronics – it's all possible, and it's going to get more common place. But have the risks of such technology been thought about? More specifically, are the monitoring processes keeping track with the development of the technology. My friend refers to nanotechnology as 'The asbestosis of the future'. That might prove to be unfounded, but the point is we simply don't know. Asbestos was a wonder-material that has been used intensively in the 20th century and a huge number of buildings (including the one I'm sitting in as I write this) is loaded with the stuff. It makes a great fire retardant and insulator, with the teeny-weeny drawback that inhaling asbestos dust can kill you. It is a massive headache for Worksafe as the whole country is full of the stuff – cue the story about the arguments between EQC, insurance companies and ACC regarding what to do with the great many earthquake damaged houses found to contain (now exposed) asbestos.

And nanotechnology could follow. By definition, it consists of tiny, tiny particles. What will they do in someone's lungs for twenty years? Who knows? How does one monitor the exposure to nanotechnology? That's maybe a more useful question to ask, and one to pursue properly. We have measures of exposure to radiation, for example, that we can apply to those who work with it, so what about a practical measure of nanotechnology exposure, that can be implemented in a workplace? An open question. 




How big is an atom? Marcus Wilson Jan 08

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I started back at work on Monday thinking that it would be a nice, peaceful day, with no-one else around on campus. Surely, on a beautiful, sunny, 6th January, the entire of Hamilton except for myself would be on the beach at Raglan. Wow, was I mistaken. The campus was buzzing with activity and there was a constant stream of knocks on my door from people wanting things done. I've hardly had time to breathe, let alone write blog entries. Maybe things will quieten down when A-semester starts (!). 

On Sunday Benjamin (now 18 months old) acquired a balloon filled with helium. I've been wondering for a little while about letting him drop a helium balloon just to see the reaction when it fails to hit the floor, and I was given the chance when a friend gave us one. (Inidentally, the frivolous use of helium in party balloons is a subject that is worth debating in itself.) Our child was most impressed and rather excited to see that not everything obeys the law of gravity, or, at least, not in the way  he understood it. 

So, the balloon was up on the ceiling for a few hours. What surprised me, however, was how quickly it lost its helium. The following morning it was back on the floor, rather deflated. This probably shouldn't have surprised me – there's good reason that the helium escapes the balloon quickly – the helium atoms in the gas are very small, and very inert (non reactive). They can simply make their way out through very tiny holes in the rubber of the balloon. If the balloon were filled with air (about four fifths nitrogen molecules – two atoms of nitrogen joined together, and one fifth oxygen molecules) I'd expect it to stay inflated rather longer. The air molecules are so much larger than the helium atoms. 

Defining the size of an atom is a bit like defining the size of Auckland. Putting aside artifially drawn boundaries, where exactly does the urban development stop? Nonetheless, there are some practical definitions of atomic size. Helium has a diameter of about 60 picometres (1 picometre is ten to the power of minus 12 metres, that is, 0.000 000 000 001 of a metre). Contrast this to the size of a nitrogen molecule, which is (again, vaguely), about 100 picometres wide. The smaller helium is simply better at getting out through the holes in the rubber balloon, and so the thing doesn't stay inflated for long. Instead, some precious atoms are lost into the upper atmosphere.  








Heat and water and making nappies Marcus Wilson Dec 06

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In the lab, my summer student has been working on a small device to keep a small piece of equipment at a stable temperature. It uses a Peltier device – in essence it's a solid-state heat pump. Pass through current one way, and heat is drawn from the top surface to the bottom; pass current through the other, heat is drawn from the bottom to the top. Therefore, by putting the equipment on the top surface of the peltier device, we can control how much we heat or cool it by via how much electric current (and in which direction) we pass through. 

There are a few things we need to consider, however, to get this to work well. One is the thermal resistance between the peltier device and the object. We need there to be a good thermal contact between the two, otherwise the flow of heat is going to be hampered. It would be rather like putting insulation around radiators in your house. It will keep the radiators nice and warm but it won't do much to the temperature inside your house. We need to ensure that the glue we use to hold our equipment to the Peltier has high thermal conductivity.

But also we are interested in knowing how quickly the equipment changes its temperature in response to heat input. This is quantified by its heat capacity – how much energy (heat) is required to raise its temperature by a given amount. Something with low heat capacity will change its temperature quickly, something with a high heat capacity will change its temperature only slowly.  A large lump of something, like the water in the university swimming pool, has a large heat capacity, and therefore takes a long time to heat up once it's been filled (and consequently remains very cold until January). Do we want our equipment to have a high or low heat capacity? That's not  entirely obvious. Our aim is for something that remains at fairly stable temperature – that neither heats up nor cools down quickly. Otherwise controlling the temperature becomes very difficult. That would suggest a high heat capacity for it. But we don't want it too big or our Peltier Device would never be able to bring it up to the temperature that we'd like. There's a bit of a balance to be had here.

What struck me this week was the obvious parallel with nappies. Well, I guess it's obvious to any physicist who changes nappies on a regular basis. The perfect nappy needs to take urine away from the skin quickly, and also have a high capacity to hold it. The first task is equivalent to the thermal conductivity, but with water. The fluid needs to be able to flow quickly from the skin to the absorbing bit of the nappy. The second task is the equivalent of the heat capacity – we need the material to absorb lots of water while not getting very wet (equivalent to absorbing lots of heat but not raising its temperature very much). The cloth nappies we use have a two different material textures. The first bit, that is in contact with the skin, sucks water away very quickly. The second part holds onto the water very well. Working together, they keep baby dry for longer, which sounds like a rather corny tag line for a nappy brand. 

And, yes, I've taken a clean dry nappy to the bathroom with a measuring jug and slowly poured water in to see exactly how much one would hold. Could you expect a physicist to do anything else?



Who is doing the observing? Marcus Wilson Nov 20

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Last week I watched again the highly amusing film "Kitchen Stories". It's hardly a mainstream affair – in fact I feel like editing Wikipedia's meagre entry on it. The scenario is amusing because it's so ridiculous – a group of Swedish scientists is sent off to Norway to observe single men use their kitchens, in order to optimize the kitchen layout for them.  I detected a bit of Norway-poking-fun-at-Sweden in this film (or was it the other way around?) but also quite a lot of everyone-poking-fun-at-scientists-and-their-stupid-studies. 

Observer Nilsson gets the short-straw and has to observe the intensely unco-operative Bjorvik. The observers have been instructed that it is of paramount importance that they do not communicate with their subjects or interact with them in any way. The research is from a positivist perspective – that means the subject 'does' and the observer 'observes'. Any mixing of the two would jepoardise the whole undertaking. 

In protest at this stupidity Bjorvik refuses to do anything in his kitchen. He then decides that if he is being observed, he can observe back, and starts observing Nilsson. There are long periods in this film where nothing else happens. No dialogue, no interaction, just one observing the other. 

What happens from then I'll leave you to find out. Be warned – this is not fast-paced  Indiana Jones-style entertainment.

There are some interesting parallels with science here. An underlying assumption in the 'positivistic' approach is that there is a real reality out there that has no relation to what's looking at it. Doing an experiment on it doesn't change it.  However, just as Nilsson's presence changed how Bjorvik behaved, so it's not always the case in science that we can do this. In some experiments, it is pretty hard to carry out the experiment without fundamentally changing what it is you are experimenting on. The extreme example is quantum mechanics, where it is impossible to observe a system without making a drastic change to it. The observer cannot be isolated from the system.

But even in more classical situations, the issue may still be there. I have a project in which a student is tackling the thorny problem of measuring the electrical conductivity of biological tissue. In order to keep the tissue 'alive' (in the sense that any isolated piece of tissue can be alive), it has to be bathed in a solution that contains what the tissue needs to function. But the solution has an appreciable electrical conductivity itself. If we measure the conductivity of the tissue on its own, it will be dead tissue – but if we measure that of the alive tissue it won't be the conductivity of the tissue alone. Tricky. To measure it, we have to change it. 

It's an issue that's worth more than a passing thought in the design of a good many experiments. 





The 2013 Nobel Prize in Physics goes to…. Marcus Wilson Oct 09


….Well, what do you think? No surprises this year.  Francois Englert and Peter Higgs have been awarded this year's Nobel Prize in physics for the theoretical 'discovery' of the Higgs mechanism. The citation, however, I find very interesting:

for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted funamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider.

First of all, can one 'discover' something theoretically? Sure, one can predict the presence of something theoretically, but can it be discovered by a piece of theoretical analysis? I'll let you debate the semantics of 'discovery'. 

Then, note how the prize isn't given for the discovery of the Higgs Boson.  The word 'boson' doesn't get a mention at all, in fact, though it is implied by the words 'predicted fundamental particle'.  The boson is merely a piece of experimental evidence  - a rather key piece, it has to be said, given it's the only thing about the Higgs mechanism that is really observable – but still only a piece of evidence for the Higgs mechanism. It is the explanation of the origin of mass that is the notable thing here.

Well, actually, not quite. Note how the citation is for "…a mechanism that contributes to our understanding of the origin of mass…" It stops short of saying that the Higgs mechanism explains it. Is there more to come?

Then finally the experimental credit is given. The Nobel Prize isn't generally awarded to large teams of people. The ATLAS and CMS teams are vast indeed (see the list of authors on the ATLAS and CMS Higgs Boson discovery papers here and here) but these teams are rightfully given credit for their part in confirming the Higgs mechanism.

So, well done to you all. 

What is physics? Marcus Wilson Oct 04


One of the many good education-focused talks at the NZ Institute of Physics conference last week was by Kerry Parker of Te Aho o Te Kura Pounamu and the University of Otago. She described her own and  her students' experiences of attending the International Young Physicists' Tournament. The tournament stretches students far beyond the confines of the secondary school curriculum by using really open-ended questions (so open-ended that the 'right' answer is actually still a matter of debate and research.) Kerry showed a few film snippets of the tournament and interviews with students, both during and after it. One student quote I wrote down since it explained beautifully what physics is about:

Physics is not a set of laws inscribed in stone but rather a means to explore and understand the world around us.

I would challenge anyone to put it more succinctly than that. 

Precision Cosmology – Yeah, Right! Marcus Wilson Sep 27

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We've just had our first session at the NZ Institute of Physics Conference. The focus was on astrophysics, and we heard from Richard Easther about 'Precision Cosmology' – measuring things about the universe accurately enough to test theories and models of the universe. We ablso heard about binary stars and supernovae, and evidence for the existence of dark matter from observing high energy gamma rays.

Perhaps the most telling insight into cosmology was given in an off-the-cuff comment from one of our speakers, David Wiltshire. It went something like this. “In cosmology, if you have a model that fits all the experimental data then your model will be wrong, because you can guarantee that some of the data will be wrong.”

Testing models against experimental observation is a necessary step in their development. We call it validation. Take known experimental results for a situation and ask the model to reproduce them. If it can't (or can't get close enough) then the model is either wrong or it's missing some important factor.(s). Of course, this relies on your experimental observations being correct. And, if they're not, you're going to struggle to develop good models an good understanding about a situation.

The problem with astrophysics and cosmology is that experimental data is usually difficult and expensive to collect. There's not a lot of it – you don't tend to have twenty experiments sitting in orbit all measuring the same thing to offer you cross-checks of results – so if something goes wrong it might not be immediately apparent. And if you can't cross-check, you can't be terribly sure that your results are correct. It's a very standard idea across all of science – don't measure something just once, or just twice, (like so many of my students want to do), keep going until you are certain that you have agreement.

Little wonder why people have only very recently taken the words 'precision cosmology' at all seriously.

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