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March 2014 Archives

At a recent staff meeting here, the topic of students' writing ability came up (yet again)! Why are our engineering students and physics students just so bad at writing in whole sentences, using correction punctuation and using consistent tenses? Why can't they string four relevant sentences together to make a paragraph that actually makes a point? In short, why can't they make themselves clearly understood in written form?

We have lots of stories of despair, and much of it is directed at 'the secondary school system', or lecturers in other departments not doing their job properly, or the rise of txt spk. 

But there's a very obvious point that we have to pay attention to: Have we actually taught the students how to write? Have we shown them how to put sentences together that make a coherent argument? It's very easy to say "not my job - mine's to teach thermodynamics, or materials science, or differential equations, or whatever". But if the assignments a lecturer sets demand that the students have good writing ability, isn't it the responsibility of the lecturer to ensure that the students have been taught how to write? 

I'm not saying every paper that a student does has to contain writing skills in it, but if we want to have students exit their degrees with the ability to communicate about their area clearly in written English, then we must make sure that somewhere they have opportunity to develop those skills. In short, don't complain about students' lack of ability in things that no-one has actually taught them. 

 

 

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So, as I said, it appears that it's awfully hard to hide a commercial airliner from military radar.

But let's backtrack a bit. Why do aircraft carry transponders? (What is a transponder?) There are a couple of reasons here. First, we need to look at a big problem with radar. It has limited range. We can see this quite simply by considering what happens to the energy contained in a radar pulse sent out by an antenna. The pulse spreads out as it travels, and so the intensity of the pulse diminishes. It follows an inverse square law, which is very common in physics. If you are in a plane and you double your distance away from the antenna, the electric field strength you receive is quartered. This pulse is then reflected back toward the antenna. Again, the reflection spreads out, and it follows an inverse square law. By the time it gets back to the antenna, it has undergone two inverse square spreadings.  That makes an inverse fourth power law. In other words, if you double your distance from the antenna,  the radar station will receive only 1 over 2 to the power of 4, or 1/16th of the power. That rather limits the range of radar. 

Yes, there are other things to consider, such as absorption in the atmosphere, and radar ducts (paths of high transmission) due to interesting meteorological conditions, but, basically, if you rely on reflections of the radio waves to detect an object your ability to detect goes down rapidly as the object gets further away. 

That's where the transponder comes in. When the transponder on the plane detects a radio pulse coming in, it calls back. The power it transmits back with is much greater than the strength of the reflected pulse. Thus there'll be sufficient power to get back to the ground station, and the plane is detected. There's only one inverse square law that matters, and that helps considerably with range. 

Secondly, the transponder rather helpfully transmits an identifier that tells the ground station what it is. Rather than simply a blip on a radar screen, it's a labelled blip. 

Until the transponder is turned off, of course. 

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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. 

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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. 

 

 

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So, you spend six days working frantically on a large project proposal in order to meet the ludicrous deadline (see previous post), and just as you think that the it's under control and the goal is in sight, someone walks onto the pitch, picks up the goal posts and deposits them in the vicinity of the dug-out. 

43 hours and 13 minutes before the deadline (the email has a time-stamp on it), we're told that (a) we have another week to get things in order and (b) the template we are using to write the proposal has changed. So I've been jumping up and down frantically on the wrong side of the pitch a week early. Mixing metaphors,  while I don't wish to bite a hand that might feed me, I'm still going to scream at it. Fortunate then that in cyberspace no one can hear you scream. 

 

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Hello everyone. Are you still there? It's not much fun reading a blog that doesn't update for several days, is it? My excuse is two-fold.

1. That the University of Waikato Semester A has just started. Actually, that isn't a real excuse since I've known about it for a long time. It just acts to aggrevate the second reason.

2. A project call for some of the National Science Challenges has been released.

What are the National Science Challenges? Have a read on the MBIE website. The government is putting a lot of money into research focused on addressing ten, grand challenges. There was a bit of debate about these when they came out as being 'more of the same' for New Zealand (i.e. doing just what we've been doing anyway), but, whatever your view, they are here to stay a while. Now, a call for proposals has just been put out for some of these. The timescale isn't terribly long - I learned about it on Wednesday last week and I have till 5pm Wednesday this week (tomorrow!) to get something pulled together. With a team split over different institutions and numbering about eight researchers, this isn't an easy or quick job. But when someone dangles half a million dollars in front of you and asks you to jump up and down for a week (plus fit in some undergraduate lectures while you're at it)  it sometimes pays to do it.

So sorry for no blog entries. Things might get calmer at 5.01pm tomorrow.

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