30 3 / 2012

The last few weeks have been pretty busy in this neck of the woods, as the university term came to an end and there were lots of student presentations to listen to and assessments to gather in. However, peace has now broken out, and last week I was at an ideas ‘charette’ (it’s a kind of brainstorming event…) to do with science outreach.

One of the participants showed a lovely demonstration of how a a Van De Graaf generator, a ping-pong ball and a few other bits and pieces can be used to demonstrate how a particle accelerator works. At this point, I need to wave my hands about in order to explain, but luckily there is a nice video here

Astronomical objects - most likely supernova shock waves and jets of material shot out from black holes - are the space-based equivalents, producing cosmic rays much more energetic than anything a ground-based particle accelerator can manage.

Paula Chadwick, Durham

07 3 / 2012

Further to the marshmallows in a microwve idea for measuring the speed of light, King Size Mars bars are also an excellent alternative. The rotating tray needs to be removed and a platform inserted to prevent the bar from rotating. The principle is the same but the caramel bursts through the chocolate where antinodes occur. The king size bar is to allow a reasonable number of antinodes. Actually any chocolate bar can be used. They are also remakably edible afterwards :)

Paul Edmundson ICHS

02 3 / 2012

Did you know you can measure the speed of light with a plate of marshmallows and a microwave oven? No, nor did I until yesterday.

It goes like this. Spread some miniature marshmallows evenly on a microwaveable plate (it helps if you butter the plate beforehand or I gather the marshmallows are a bit hard to get off afterwards). Take the turntable out of your microwave oven, put the marshmallows in and microwave them. You will see some patches of marshmallow get hotter than others and either melt or burn if you are a bit overenthusiastic. These ‘hotspots’ are spaced by a distance equal to half the wavelength of the microwaves. So you measure the distance between the hotspots. Now, if you look on the back of the oven and see what frequency it’s operating at, you can use

velocity = frequency x wavelength

to work out the speed of the microwaves, which of course is just the speed of light.

This also explains why microwave ovens have to have turntables - without them, your ready meal would be heated up very patchily indeed.

I was wondering what to do with the summer school students on our supported progression scheme later on this year. Now I know - and I am sure they will enjoy eating the the marshmallows afterwards.

Paula Chadwick, Durham

P.S. I haven’t tried this yet - I confess I don’t own a microwave - so I am going to be invading a friend’s kitchen this weekend. Meanwhile, there is a nice video here.

24 2 / 2012

One of the things we are planning to have at the exhibition is a cloud chamber - a classic device designed to show the presence of ionising radiation by its producing condensation nuclei for clouds in a supersaturated vapour - which is a long-winded way of saying each particle produces a rather nice little trail of cloud in the chamber. They are sometimes called Wilson chambers as they were invented by the Scottish physicist Charles Thomson Rees Wilson.

Talking to one of my colleagues on Monday about this, he happened to mention that welding rods are quite good sources of alpha-particles (a.k.a. helium nuclei), and might work rather well in our cloud chamber.

I wandered down to the mechanical workshop here in the Physics Department at Durham to enquire. So, I asked, trying to sound like I knew what I was talking about, is there more than one type of welding rod? I got the sort of pitying look you only get from the lads in the workshop when you ask a stupid question (which I do, all the time). “Yes”, they said, “there are as many types as there are metals to weld.” Time to do some research, which revealed that what I actually needed was a thoriated TIG welding rod. Armed with the right jargon I secured one from the workshop and went up to the first year undergraduate labs, where the ever-reliable lab technician, Sue, dug out a small cloud chamber for me.

Cloud chambers always take a little while to get going, but once I had it working, there they were - the characteristic trails of alpha-particles coming from the welding rod. Great! The only trouble is, the rate was very low, which I guess isn’t surprising; you wouldn’t want to be radiating welders all the time. That means they won’t really be suitable for the exhibition, but it is one of those nice little facts to tuck away for future reference.

Paula Chadwick, Durham

17 2 / 2012

Up here at Durham University, our final year undergraduate M.Phys. students do a research project that takes up half their time in the year. One of my project students, Sian Cleaver, is helping with the testing of mirrors for the Cherenkov Telescope Array, a big international gamma-ray telescope project of which I am a (very small!) part. We are interested in how mirrors behave in cold and damp conditions - basically, how easily they mist over. Sian’s results will help us decide the right type of mirrors to use on the telescopes and even where to put the telescopes.

Anyway, it was rather cold in Durham last week, and Sian thought she had seen mirrors freezing before they misted over. This surprised her and set her thinking, and - well, why don’t I just tell you what she told me in her e-mail?

"I’ve been looking into the formation of frost on surfaces, and yes, frost can form without condensation forming first, i.e. sublimation occurs. The "frost point" is always a couple of degrees higher than the dew point when the temperature is below zero because of the higher binding energy between solid water molecules, which would explain why I’ve been seeing frost even when temperatures have remained above the dew point.

I found a formula, and worked out how to convert from the dew point to the frost point, and plotted it on one of my graphs for a night where the mirror frosted over. It’s fascinating - my graph indicates frost forming at about 2200. I looked at the webcam footage and you can see frost forming at exactly that time!! It seems to form from the centre and spreads outwards over the course of a few hours.

Thought it was quite cool…”

So there you go - I thought it was quite cool too. I enjoy finding out a bit of everyday physics I didn’t know before, and I will be looking at my car windscreen in winter with a lot more interest now.

Paula Chadwick, Durham

13 2 / 2012

We had our exhibition planning meeting up here in Durham University last Friday. If there is one thing that is clear it is that we have far too many ideas. The whole cosmic ray field is very active and touches on so many areas of science - particle physics, astronomy, cosmology, climate science, biology etc. - that it is quite hard to pick out just a few things. I wonder if they would let us take over a whole floor of the Royal Society???

One of my colleagues at the meeting told me something I hadn’t realised before. Victor Hess actually flew in a hydrogen balloon, not a hot-air balloon. These are pretty dangerous things, so Hess and his co-workers really risked their lives performing the experiments. It makes braving the odd snake out at the HESS telescopes in Namibia seem pretty tame…

Paula Chadwick, Durham

06 2 / 2012

We are reliably informed that we need things to give away at the exhibition, so I have been busy looking at what is available to customize. It is amazing what you can get (promotional dashboard mat anyone? Golfing periscope? Wet wipes?) This entails talking to the marketing people of marketing companies. You can imagine. Anyway, now I have several quotes from various companies and a sample box on the way from one of them. I have a feeling that this will become the single most distracting item at our exhibition planning meeting on Friday. If there is one thing scientists can never resist it’s a toy…

Paula Chadwick (organiser)

30 1 / 2012

It might surprise you, but we all spend our lives being bombarded by sub-atomic particles (about 5 per second through the top of your head!). They are actually produced in the Earth’s atmosphere by cosmic rays - high-energy particles which start out life in deep space. For scientists, cosmic rays are important as part of the energy balance of the Universe and in understanding the evolution of its building blocks. They are thought to come from some of the most violent places in the Universe, jets of material launched from black holes and the remains of supernova explosions being two of the prime candidates. Closer to home, they have almost certainly had a role to play in human evolution and the origin of life, may cause lightning and have recently (and controversially) been implicated in climate change.

It so happens that 2012 is the 100th anniversary of the discovery of cosmic rays by Victor Hess, who made his discovery by taking simple radiation detectors up in a hot-air balloon. However, the origin of cosmic rays remains a mystery. We study these particles, their origin and their effects on Earth with experiments up mountains, in space, underground….and in schools.

More posts later when we have got used to blogging!


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