While all the biology and ESS students were away on a field trip I decided to organise a practical morning for my EE students. It's one thing to have an idea but until you start to experiment its difficult to know what to expect. Here are a few of the questions of the day. Mazen, researching the resolving power of lenses was interested to know how the aperture size is adjusted as a zoom lens is zoomed. Antonio noticed some interesting effects produced when a magnet is held close to a vibrating guitar string. The uneven oscillation produced seems to split the frequency of the harmonics resulting in beats. Tim wants to know which what caused gear systems to lose energy, seems like its all to do with slipping as the cogs mesh. Filip tried to build coupled pendulums with Algodoo but they don't seem to work in 2D, maybe springs are better. Wassim is interested in how a robot is programmed to compensate for the state of the batteries. The first stage is to find out how the terminal pd of the battery changes with time. This involves measuring the pd as the batteries run down. This could take a long time with no load being lifted by the motor, maybe I should've told him.

Luckily our students didn't have to go through the trauma of doing this since they do all their answers on the exam paper but if you have been invigilating or sitting other exams you might have come across this little teaser. Looks easy but it isn't.I wonder how many booklets were wasted worldwide due to wobbly lines. Another IB oddity I noticed was that all the exam paper envelopes have a the "open here" line at the bottom. Who opens envelopes at the bottom?

Last week I did the beer foam decay experiment with one of my first year classes. If a couple of students bring their own lap tops we now have enough computers for one each so instead of using the sticky tape and pen method described they can take a video and analyse it with loggerpro. It means that all the data collection and analysis can be done in class. I usually supply some crisps to go with the non alcoholic beer but since it was right after lunch I thought they wouldn't be hungry. The problem with these analogy experiments is that students can sometimes miss the point, I wonder how many of my students now think that beer is radioactive. Or that it should be poured very quickly and drunk out of tall glasses.

If you are using my text book you may have noticed my interest in hats, The photo shows part of my collection. Actually it shows all of my collection, I'm not doing so well. It started when I hinted to a Mexican student that I would really like a Mexican hat. My dad who was a Baptist minister used to do the same thing from the pulpit, that's how I got my first fishing rod. The student kindly bought me back a hat but it wasn't the sort that I was thinking of. Since then I have added hats from Nepal and of course Panama. Some years ago I made a video about Mexican hats with a friend from the Maths department (also interested in hats) after many years it has resurfaced so I posted it on you tube. Here it is.

As teachers we sometimes have to do things a little bit out of the ordinary, last week was one of those times. Every two years we have an evening when the teachers put on a show for the students. As in most international schools the students of UWCRCN put on a lot of cultural shows throughout the year well, this is the time when the staff pay them back. This year I decided to do a mime to Gloria Gaynor singing I will survive. I didn't mime the words but mimed actions to go with the words. In the photo I am doing the actions that go with "I was afraid". Looks more like " I was surprised".
 

On a totally different subject, if you haven't already got it then download the new version of Algodoo. The full version is now free and it has many great SMARTboard friendly features. For example, anything drawn with the blue pen turns to water. The strings are much better too and its generally easier to operate from the board. Having said that I've been struggling a bit having to re-learn my way around but I guess I'll survive.

Here's something interesting that one of my students (Julia RB) spotted today. We've been doing a design lab using "the worlds smallest solar powered car" and a lamp. It's all related to Earth day (which was yesterday). The idea was that all classes would do something with an environmental theme so I started with an intro to solar power. I found this interesting map on Wikipedia showing the amount of land that would have to be covered to collect enough solar energy to meet today's needs. Surprisingly little. The little cars actually don't use solar power directly they use light from a lamp that is produced from hydroelectricity, (in Norway) which comes from water that landed on top of mountains, due to the rain cycle which is driven by the sun. Anyway that's a bit off topic, the point that Julia was making was that after exposure to the lamp the photovoltaic cells change colour. This can be seen clearly in the photo, the one on the right has gone blue after exposure to light. Here's my theory: The photovoltaic cells are covered with a thin non-reflective layer. This is a thin film, light reflected off the top and bottom of the film interferes destructively cancelling out the reflection. This layer is very thin so the path difference is small however the waves are out of phase because the top wave undergoes a phase change of π on reflection. Now if the film gets hot, as it does under the lamp, it  expands and the path difference becomes bigger until it equals 1/2 λ for blue light. Blue light will now interfere constructively. Of course it could be due to something completely different. Talking of something completely different, in one of my other classes two students found something else interesting about the cars.

I wouldn't want to be accused of favouritism so I've animated my SL class too, not that they aren't already animated of course. Unfortunately the gif ended up with a blank preview so you have to open the post to see it. This was inspired by the class we had today on relativity. I condensed the whole of the relativity in to one lesson. It's not because I ran out of time, the class asked if we could do it for fun. At least it was more entertaining than doing another past paper (for me at least).

Last HL class was today and never got round to taking that class photo so here is an alternative, the class interactive physics mexican dance. Well, not much of a dance really, apart from a little stamp of the foot at the start, more of a parade. I didn't manage to get the girls heads in quite the right place, they all ended with rather long necks which is quite attractive in some cultures or so I am told.

Went to Bergen today with three students who were hoping to pick up a prize in a science competition. The prize didn't materialise but we did get to visit vilvite which I thought meant wild knowledge but that would be a different spelling, it actually means "will know" and is one of those science parks with loads of activities, like measuring the speed of a football. I managed 30kmh^-1, not very impressive. One exhibit that caught my eye was the oscylinder scope. It's a stretched string in front of a rotating drum which has black and white lines on it. When you pluck the string you see a wave pattern as in the photo. The wave pattern isn't the wave in the string as this would have a much bigger wavelength however it is related. I think its something like a double strobe effect. You only see the string when its over a white band which you only see when the lights are shining. Now the lights are flashing on and off at 100 Hz due to the mains supply, (100 not 50 since they are on  for both direction of current). Explanations I've read on line don't mention the flashing light but I can't see how it would work without (see below for what is probably the correct explanation). If you have one oscillating point in a light that flashes with the same frequency as the oscillation you would see it always at the same place, it appears to stop. What seems to be happening here is that each bit of the string is seen a little after the bit above resulting in the wave pattern. I guess one could calculate the frequency of the sound from the number angular velocity of the drum but I haven't got that far yet. I was thinking of making one until I saw this video on youtube. It seems that taking a video with an iphone produces the same effect.

Now I'm having second thoughts about whether the effect depends on flashing electric lights or not. I'll have to get a student to change their EE topic.

Attempt 2: If viewed from one point the drum will be like a strobe enabling you to see the string every time a white band goes past. If this coincides with the string being in the equilibrium position then that point on the string will appear in the middle. If you now consider the point directly below the white bars will arrive there slightly later so this point will be slightly off the central line. Depending on the speed of the drum there will be a point xcm below the first where the string is again in the equilibrium position when the white band passes. So the time difference between the first point and the second must be half a cycle I think. Maybe I should leave the rest for my students to figure out.

Attempt 3: Now I've definitely got it. Forget that there is a wave in the string and think of the string as just oscillating up and down. As one white stipe moves along the string a bit of the string is seen to oscillate up and down like in the animation below.

You can see that the this gives the appearance of a wave. If this was speeded up and your eyes would retain the image (persistence of vision) and you'd see a sine wave. The wavelength of this wave depends on the speed of the moving window. This is pretty much how an oscilloscope works hence the name oscylinderscope, duhh.

A friend of mine sent me an interesting question, “why is the Fosbury flop a more efficient way to jump over a bar”. The friends name is Per which would be quite a good name for a Physicist, meters Per second, Kilogrammes Per meter cubed etc. I wasn’t sure of the answer so looked it up on Wikipedia and discovered that with this technique the centre of mass of the jumper stays below the bar so you don’t have to jump so high. At first I wasn’t convinced, surely you still have to get each part of your body over the bar. Eventually I convinced myself thinking about throwing a stick over the bar. If you through the stick up vertically then to clear the bar the centre of gravity has to get much higher so clearly has to be given more energy than if the stick was thrown up horizontally. So even though in both case the stick got over the bar the position of the centre of gravity is what counts. Now, if the stick could bend as in the diagram below then it could get over without having to be thrown very high at all. According to this snakes should be very good high jumpers. If I could invent a method that kept my centre of gravity about 2m below the bar then maybe I’d be in with a chance.

It was a wet weekend so I went down to the lab to play with the apparatus. Years ago I broke the lamp that is used in the Stefan Boltzmann experiment, never got around to replacing it but discovered that it's just a car side-lamp so found one in my garage and replaced it. The thermopile was still in its drawer so I set the apparatus up to see if it still worked. The Stefan Boltzmann Law is pretty important in several areas of the syllabus so thought this might be a good practical to reintroduce. The only problem is that I only have one and I have rather gone off the idea of circuses of experiments over the past few years. Maybe I can get some cheap IR sensors from Ebay? As you can see I'm using a Pasco set up which comes with a handy manual that includes a table to convert the resistance of the filament into temperature, all you have to do is measure the current and voltage. I quickly ran through the range of voltage offered by my power supply and came up with a nice curve. To test the power 4 relationship I decided to plot a log log graph and got the results shown below.

Amazingly close with a gradient of 3.793, not bad considering the time spent taking measurements. At first I was surprised by the large negative gradient but of course the constant of proportionality is very small making the log of it -ve.

I distinctly remember saying to the class, "look at the light not the camera". They ignored me of course and although I got a nice photo of my class it didn't really illustrate the point I was trying to make. Next time I'll wear my glasses, reading glasses that is not spectrum glasses. Actually now I look at the photo I can see why they weren't looking at the light source. It wasn't switched on and wasn't even pointing towards them. The point was that these spectral glasses are great when introducing line spectra. I used to pass round the 4 proper diffraction gratings but these are cheaper and work just as well. You can get them from eBay for under a pound.

As in a lot of international schools we celebrate many different cultural days and last weekend was Holi. I'm not sure what Holi is all about except that the celebration of it involves throwing paint at people. Looking at the picture it seems that the the students opted for a clean version of Holi using water rather than coloured powder paint, maybe they didn't want to ruin the snow? The photo was taken by one of my students (Mette), she posted it on facebook and I stole it (actually I did ask but it was after the event). The thing that caught my eye in the photo was of course the parabolic path of the water. Pishun is attempting to soak Mazen although from this angle it looks like he might miss. However Pishun, being a HL physics student, has taken into account the air resistance and knows that the water will drop more sharply than one might think. Interesting how the water doesn't come out of the bucket in one go but flows out as if coming from a hose pipe. I think there is a good EE in there some where. I thought I'd try some analysis with LoggerPro to see how parabolic the path actually is. We normally analyse video with logger pro but you can use the same method for analysing stills. The equation for the trajectory is

The angle of projection is approximately 45° so this simplifies to

Here's what I got from LoggerPro

The answer to the question "did Mazen get soaked?" is yes he did but not because the trajectory was correct (sorry Pishun). The water arcing in a nice parabola isn't made of a series of particles with the same initial velocity. The water particles don't follow each other like a series of balls. The first bit is thrown with greater force than the last so the last bit of water has a much flatter trajectory. This was pointed out by Mazen and he should know.

You can see what I mean in this video (at 30s):

It's revision time again. Well at least it is with my SL class, won't have any time for class revision with my HL. Just got time for the second option before the end, must admit we're pushing it a bit this year but I think it'll be fine. I always let my class think they are choosing the option but always manage to get them to choose astrophysics and relativity. Today my class rebelled and demanded a re-vote so we're doing Medical instead of relativity. Threw me a bit having to suddenly try to teach medical physics with no preparation. At one inspired moment I told the class that ears are for hearing, oh well, never mind. My SL class have been working through some exam papers, tomorrow I'm planning to give them a paper 1 but can't bring myself to print out all those copies just for one lesson (I'll be joining Greenpeace next). So I've thought of a cunning plan. I'm going to split the class into groups and get them to do a paper between them. Turn it into a group competition,might even be fun. We'll see how it goes.

Worked quite well. Students interacted more than when working on individual papers (obviously). They were also much more eager to offer explanations to solutions when speaking on behalf of a group. The only thing that didn't work was the competition aspect, I forgot to ask the team scores so no one will ever know who won. Also came across a question that was different on the original to the bought pdf version I projected. The original didn't have a solution so was changed before sale, thought I was going bonkers for a bit.

Ever since last July, when I bought this magic wand, I've been dying to use it, now at last I'm doing electric fields with my first year classes. I almost forgot that I had it so they almost missed a big treat. I'm not sure how it works but it probably some sort of piezo electric device like a cigarette lighter. A piezo electric crystal become charged when it's squashed, when I press the trigger something inside starts clicking and the wand gets a static charge. The wand comes with some very flimsy Mylar shapes which when charged repel themselves into spheres like the one in the photo. To get them to this state is a bit tricky as they tend to stick to the wand, a quick flick and they drift up into the air repelled from the wand. I'm not sure how long this will keep working since once used the Mylar strips are impossible to store flat so after a while it will need some real magic to get them aloft. Today was quite a day for demos, not only did I get out the magic wand but I did a repeat performance of the hairy hand and got a student to try the hairy lip (you don't expect me to put magnets in my mouth do you?).