SciBlogs

Archive January 2010

What lightning does to your television Marcus Wilson Jan 29

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   Well, last night’s thunderstorm was a bit of a feeble affair after the fireworks of Wednesday. There were a few flashes, the odd rumble, and a bit of rain, but it cleared away after an hour. Maybe somewhere else got the drenching this time. Still, it makes four days in a row of the same daily weather, almost hour-by-hour. And this morning….is shaping up the same way…

A student of mine sent me these photos of his television screen on Wednesday night (used with permission).  There was a lightning strike close to his house, which seems to have magnetized the television.  The picture is there, but the colour is wrong.

lightning.jpgIn an old CRT television, each point on the screen contains a phosphor – and there are three different colours, red, green and blue. To make up the colour picture, the phosphors are stimulated by the required amount, by a beam of electrons that hit the screen from behind. The beams are scanned across the screen using magnetic fields (charged particles like electrons are bent by electromagnetic fields), and, in order to hit the right phosphor, need to emerge in the exactly the right direction.  What I think has happened in this case is that the lightning has permanently magnetized some part of the television. This is hardly surprising given that lightning carries a huge direct current, and consequently creates a nice magnetic field around it.

So as the electrons pass, they are bent slightly but consistently off course, and hit the wrong colour phosphor. Hence the image looks right, but just with the wrong colours. De-gaussing  the TV removed the colour shift, putting it back how it should be.

what should look like.jpg 

Wild weather in Waikato Marcus Wilson Jan 28

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Well, those of you living in the central North Island will probably have some idea already of what I’m going to say, but, for those of you who don’t, I’ll start by saying that the weather here has been rather predictable this week.  We’ve had three tropical-style days in a row, with a fourth shaping up the same way already.

It goes like this.  6.30 am Wake up to a beautifully clear morning, temperatures perhaps slightly on the cool side, but not a cloud to be seen.   9 am. Beautiful morning, nice mild temperatures. 12 noon. A few clouds around, getting warm.  3 pm. Uncomfortably warm and hot  6 pm.  Yukky sticky feeling to it.  Watch those clouds build. 7 pm or so. Thunderstorms start, along with the rain.  9 pm (or later, as it was last night) Thunderstorms begin to fizzle out.    Not sure what it’s like during the night, but its nice and clear again the next morning.

Last night’s thunder was particularly impressive, if that’s the right word.  It went on for about three hours, with the sky almost constantly lit up by lightning flashes. And lots and lots of rain. The house remained nice and dry, but some of the buildings at the university haven’t – there’s a very damp smell pervading the corridor at the moment and I hear rumours of some offices down the corridor from mine being flooded.

This weather is more reminiscent of the tropics. I’ve only spent a few days in Singapore, but pretty much this is the pattern of events that unfolds there most days.

Say goodbye to the drill… Marcus Wilson Jan 27

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Here’s a nice piece of applied physics research that will excite a significant minority of the population – specifically those who dread going to the dentist. Personally, I have never had any issues with drills (needles are a different story), but I know lots of people who do.

The proposed method uses cold plasmas to kill off bacteria-infected dentin that would otherwise have to be drilled out before a filling applied. A bit like microwaving the tooth. It’s in the reasonably early stages of the commercialization process, but maybe you will see it at your dentist in a few years’ time.

Read for yourself on the physicsworld site at  http://physicsworld.com/cws/article/news/41506

Now, what would be really exciting is if they could develop a method for taking blood samples without sticking sharp metal objects into one’s veins. I would invest in that one.

Heads I win, tails you lose Marcus Wilson Jan 25

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The comment on my previous entry raises a few  issues with the way we feel heat.  (NB for those who normally read this blog on http://www.sciblogs.co.nz , you’ll need to go onto physicsstop to see the comment – http://sci.waikato.ac.nz/physicsstop ) 

How hot we feel has more to do than just what the temperature is.  Anyone who has stood outside in a gale will know that it feels much colder than what the thermometer reads. That’s the windchill.  The temperature is the same, but the rate at which heat leaves your body is much higher when the airflow past you is greater. That’s because in still air, your body heats up the air around your skin, so unsurprisingly it feels warmer to you (because the air next to your skin really is warmer). But in a strong wind, that warm air is just blown straight away.

Humidity plays a key role too. Water requires energy to evaporate, and it takes that energy from what it is in contact with. So when sweat evaporates, it cools the skin. It evaporates more readily in low humidity conditions, so here it takes energy from you at a greater rate than in high humidity. Thus a dry heat might feel more tolerable than a wet heat.

Evaporation is what makes you feel cold the second you step out of a swimming pool, especially in nice sunny weather. All that water on you starts evaporating, and it sucks heat out of your body. So there’s a vicious irony here – when you jump in the swimming pool to start your swim, it feels cold (water is better at carrying heat away from you than air), and when you get out of the pool at the end of your swim, it also feels cold.  Why can’t it feel warmer both ways?

Can you feel the cold? Marcus Wilson Jan 21

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Writing the last piece about fridges has reminded me about a comment I heard from a fellow student while I was an undergraduate. I can’t remember the exact circumstances, but it quite possibly had something to do with objects in liquid nitrogen.  Anyway, the comment was something along the lines of ‘The temperature’s so low you can feel the cold radiating from it’.

Hmmm. Yes, we know what you mean, but it’s not quite right, is it?  It is heat that radiates. Hotter things radiate more. Cold is the lack of heat.   If we hold our hand close to something very hot, we can feel the heat radiation. But the hot thing isn’t the only thing that’s radiating, our hand radiates heat as well. What matters is the difference between the heat it receives and the heat it gives off. In my office at the moment, my hands feel to me neither cold nor hot, because what they are pointing at (namely the keyboard) is pretty well the same temperature as my hands themselves.  The amount of radiation arriving on them is roughly balanced by the amount leaving.

So when we ‘feel the cold radiating from something’, what we are feeling is that not enough heat arrives on our hands to balance the heat that is leaving. (Plus probably we are feeling the cold air too, due to convection currents).

But, before I treat my fellow undergraduate too harshly here, I should point out that physicists are quite adept and speaking about the absence of something as being something itself.   When we describe semiconductors (e.g. silicon, as in chips), we talk about n-type and p-type material. ‘n’ stands for negative, and in n-type silicon we consider electrical conduction happening because electrons move. That is a conventional way to think about electrical conductivity.  But in p-type (p is for positive) the mechanism for conduction is slightly different – we talk about the moving of positively charged holes.   A hole is really the absence of an electron, but we can treat a hole as an entity itself – even to the point of assigning it a mass. It’s a bit like those slidy puzzles – slide one tile into the square gap to create a gap where the tile was – the gap (hole) appears to move, though of course it is really the tiles (electrons) that do the moving. When you’ve worked with semiconductors for a while you can forget that a ‘hole’ isn’t a real thing.

So is it then really wrong to say you can feel the cold radiating off something?

 

BBQ Physics Marcus Wilson Jan 19

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Here’s another little bit of physics seen in everyday stuff.  When disconnecting the gas cylinder to our camp stove while on holiday, I got a bit of a shock at how cold it was.  It shouldn’t have shocked me – that’s how it should be. 

When gas is made to expand it cools down. And in the gas cylinder, compressed gas is allowed to exit and expand to atmospheric pressure, so naturally the nozzle will get very cold indeed. The reverse is true of the bicycle pump – here you are compressing gas from atmospheric pressure to tyre pressure, and it gets hot.

The cooling of gas as it expands is a crucial part of the cycle going on in your fridge or air conditioner. The refrigerant liquid is allowed to expand while it is in contact with the cold bit of the fridge, thus sucking heat out of it. The liquid is then pumped to the outside of the fridge where it is compressed again, dumping heat out. That’s why your fridge is warm at the back. The second law of thermodynamics says that more heat has to dumped on the outside than taken from the inside, that is, there needs to be an input of work (in a fridge’s case, electrical energy) to make this process possible. Feel the heat coming out of the outside unit of an air-conditioning system – that is why you get a big electricity bill if you like your house nice and cool in summer.

Though I have to say, we haven’t even thought about switching our units on so far this summer.

Useful origami (and wine bottles) Marcus Wilson Jan 18

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Here’s something silly and not-quite-entirely-useless for a Monday morning.

Think of a deformable material (something solid but something you can squeeze, stretch, dent, etc). Maybe a bean bag, lump of plasticine, football etc. It might or might not return to its original position after you let go of it, but that doesn’t matter. Think what happens if you squeeze it.  It gets shorter in one direction, the direction you squeeze it in,  but it splurges outwards in the other direction. This isn’t surprising, since many things pretty well conserve their volume when you derform them – if you push them in one direction they grow in the others.

Now find a cork (what used to go in the top of wine bottles before screwcaps). Squeeze it on the ends. It gets thinner.That’s pretty unusual in materials.  Cork has a negative Poisson ratio – the Poisson ratio describes the negative of the ratio of transverse strain (the deformation along the non-squash dimensions) to transverse strain (the deformation along the dimension of squash).  That means that a cork is easy to put into a wine bottle – squash it and it gets thinner – but not so easy to pull out, because when you pull it it gets thicker and grips the neck of the bottle.

Such materials can also be made with origami. A work colleague once demonstrated one to me. Pretty bizarre to play with. But it has its uses, like the collapsible stents described in the link here . If you’re good with your fingers, have a go at building one.

 

Dark Matter and statistics Marcus Wilson Jan 14

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While I was on holiday, news broke (e.g. see the piece in The Guardian) about the possible detection of WIMPs.  Weakly Interacting Massive Particles are what many physicists think makes up ‘dark matter’.  (What is dark matter? – basically, if you analyse the way galaxies move, you discover that the amount of matter you can ‘see’ with conventional techniques, be it visible, infra-red or whatever isn’t enough to provide the necessary gravitational attraction. There is some other kind of invisible matter out there – called ‘dark matter’)

The CDMS research team has undertaken a series of experiments deep underground to look for the rare event of a WIMP interacting with normal matter.  The experiment has to be underground to shield it from cosmic rays and other particles incident upon the earth from outer space. And they report that they have possibly seen two events.

Now, if you do a science experiment well, you can quantify your ‘possiblies’.  In this case, they say that the chance of two events like this occuring due to other (background) effects is 23%.  Is this low enough to say that dark matter has been discovered?

That’s an interesting question. Often, people work with a 5% limit.   If the probability of the results being due to chance alone is less than 5%, it is commonly assumed that the experiment has shown a positive result.  But is this true?  Five percent is an arbitrary boundary. It still says that one in twenty of every experiment which reports success at a p=5% limit is actually due to chance. How low do you have to go before you are justified to put out a press-release saying that your WIMP has been detected?  There is no clear-cut answer to it, though prudence would suggest that the more sensational your result, the more certain you should be of it (and the lower your limit.)

Twenty-three percent is pretty high and the authors of course are very sensible in saying that they have seen hints of WIMPs, rather than the WIMPs themselves. To be more certain about it, the experiment needs to be run for longer – and more events seen – or a better experiment produced.

Large, heavy objects Marcus Wilson Jan 13

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Our cat got the shock of his life a couple of days ago when the washing machine got up and chased him out of the room. It’s not often that inanimate objects start walking on their own accord. Poor cat is probably so traumatized he’d never set foot in the laundry again except for the fact that’s where his food bowl lives.

I got a bit of a shock too, but I could see it (or, more accurately, hear it) coming.  It was simply a very unbalanced load that the machine was beginning to spin. The effect is basically conservation of momentum and angular momentum. The machine on its own has no external forces on it (ignoring those  through its feet). Therefore its total momentum should remain zero.  Inside the machine there is a large mass of damp clothing that is spinning off centre, and the reaction this has on the machine makes it wobble – overall the momentum stays the same.

The forces on the machine due to the floor rather complicate things, and  exactly what happens depends on things such as the coefficients of friction between each foot and the floor. But all that wobbling may be enough for the machine to start walking, which is what it did in this case.

The easy fix is of course to stop it and redistribute the load more uniformly around the drum.

Ice and the freezer Marcus Wilson Jan 11

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Last Saturday I got around to doing one of those long overdue jobs in the house – defrosting and clearing out the freezer. There are numerous reasons why this was a good idea – it’s not just about getting rid of the food that has been there rather too long, but also about making sure it works properly. 

There was probably about a centimetre’s worth of ice over the walls and ceiling (it’s an upright freezer – below the fridge) – thicker in patches. I collected 2 kilograms of it – (I weighed it) – and maybe another kilogram ended up lost in puddles across the floor. Now, for a start, that’s three litres of space that couldn’t be used for storing stuff that now has been recovered. But more significantly, it’s going to make the freezer rather more efficient.

Ice isn’t a particularly great conductor of heat. That means to remove the heat from the inside of the freezer, the pump has to work a bit harder, because there is a natural layer of insulation (the ice) around the food.  Ice in a freezer isn’t a sign of a cold freezer – it’s probably a sign of one that isn’t cold enough.   That’s also why a snowhole can save you if you are caught out on a mountain in bad weather – the snow contains lots of air and quite a good insulator – plus you also get yourself out of the wind. A cold place,  yes, but not one that will suck heat from you terribly quickly.

The ice comes from the air. Warm air carries water vapour (it certainly does in damp Waikato). When you open your freezer door, the damp air hits the cold surfaces, cools down, and releases its moisture. This deposits as ice crystals. Try leaving the door of your freezer open with it switched on, and see what happens.  I did this accidently once and the result (after I got back from holiday) was pretty impressive. It didn’t do the food much good though.

Now, with a nice clean freezer, and the door firmly closed, we shouldn’t have any trouble with our ice cream.

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