# Physics Stop

## Simple MachinesMarcus WilsonDec 26

There’s a lot to do while driving.   Look at the road – watch the speedo (98 kmh – OK there), watch the road – look in mirrors – check fuel gauge (half – OK there) – watch road – watch that car at the intersection ahead – check temperature gauge (where it should be) – eyes on road – glance in mirror – eyes on road – check fuel gauge (quarter – OK there) – car ahead slowing, might need to break – check mirror -

Whoaa. Hang on. How did the fuel gauge drop a quarter of a tank in the space of a couple of minutes?  Watch gauge. Visibly dropping. Where’s my petrol going? Car isn’t on fire – can’t see a trail of fuel behind me in  the mirror. Watch gauge again. Stable, no – wait – it’s going up again. Back to half a tank again.

Now, I assume this is a problem with the fuel gauge, not the fuel system itself. Probably with the float in the fuel tank, but I haven’t checked it out. The fuel gauge is a pretty simple machine, but one that works quite well. It’s a bit like what’s in the toilet cistern – there is a float that sits on top of the petrol in the tank; as the level drops the float drops. It is connected to an arm that goes to a variable resistor that controls current to the gauge in the car. The gauge itself is probably a bimetallic strip – the more current flows the more it heats up – and the more it heats up the more it bends, due to the different expansion rates of the two metals. And generally the whole thing works reasonably well, though I know it can fail, particularly in older cars like mine. Low-tech solutions are often quite good enough.

One interesting project a couple of students in my department have been working on recently is a ‘battery gauge that works’. Have you noticed that your battery gauge on your mobile phone, or your digital camera, isn’t really to be trusted. Might say half charged, and suddenly you find yourself out of power. Or switch it off for a few minutes, and suddenly it appears to have charged up a bit?

Making a gauge that is accurate for a battery is actually not a simple task. The way they release their energy isn’t quite as simple as that of a fuel tank.  But, my question is, does it really matter? Given that car fuel gauges are a bit iffy, but every driver seems to cope OK, do we need any high-tech battery gauges?

NB – students – you know who you are – I’m not trying to rubbish your work – just get you thinking about the need for it. As any engineer should.

## The We(s)t CoastMarcus WilsonDec 23

Back online briefly – blogging is a bit tricky from campsites – not all of them have wireless broadband connections yet – particularly on the West Coast.

For those non-New Zealanders, ‘West Coast’ refers to the west coast of the South Island, which I have now driven the length of. Or as much as is accessible by road, anyway. Having done this, I can agree with the guidebooks on a couple of things.

1. It is stunningly beautiful.

2. It is wet.

The two are closely related. It is the proximity to both the towering Southern Alps and the sea that makes the coast wet. It’s quite simple physics; a good example of relief rainfall. The westerly wind blows over the Tasman sea, picking up lots of water vapour on the way. It arrives over the coast, and runs smack into the mountains. It has no-where to go but up. As it goes up it cools. Cold air can hold less water than warm air, so the water has to condense out, and it falls as rain. Lots and lots of it. Unfortunately the presence of the clouds tends to hide the mountains from view – so the best place to see them is generally from the other side, which, no surprises, tends to be very dry.

Driving over the Haast Pass is a great example – just in the space of a few kilometers the weather and vegatation turn from damp and prolific, to dry and scrubby. And, after several days of rain, the chance to dry out a bit was very welcome.

Wishing you all a Happy Christmas.

## LHC happeningsMarcus WilsonDec 15

A couple of weeks ago the Large Hadron Collider became the world’s highest energy particle accelerator, reaching a beam energy of 1.18 TeV. It also breaks the 1 TeV barrier for the first time, taking the record of the Tevatron at Fermilab, near Chicago.

The equipment is gradually being ‘tuned up’ and hopefully we will see some very interesting things emerge in 2010.  I look forward to following the news.

## Summer holidays…Marcus WilsonDec 12

It’s summer (well, on paper anyway) which means holiday, which means blogging will certainly ease off until the new year. I’ll try to get the odd post up now and then so you don’t feel neglected, but please don’t expect three or four a week. Happy Christmas and New Year.

## Climate changeMarcus WilsonDec 10

I feel that, as a physicist, I should be making some reasonable and informed comment on the Copenhagen summit. After all, climate is immensely physicsy. We have fluid flow, conduction, convection and radiation of heat, interaction of electromagnetic radiation with electrons in molecules, scattering of light by small particles, solar activity (on second thoughts, scrub that one, WAY too controversial). Actually, all of these I’ve mentioned in my blog over the past year, in some form or other (follow the links if you don’t belive me.) And then there’s the issue of whether fish mix the oceans of not.

But, to be honest, the Copenhagen summit appears a white elephant to me. Don’t misunderstand me, I’m not at all suggesting that climate change isn’t something that we should tackle. Rather, the key is to actually get on and do something about it. Talking about it is OK, but what matters is action. My recollection was that climate change was talked about seriously at the Rio Earth Summit in 1992. What has happened since then? No, I don’t mean the Kyoto protocol, and legally binding or non-binding targets etc, I mean by how much have global emissions actually reduced. You can set what targets and agreements you like, but it is the action that solves any problem.

To be honest, I don’t know the answer to that question (how much have global CO2 emissions changed since 1992) – the internet is no help to me here. I’m sure someone can tell me. But I don’t think it’s downwards.

So here’s my point – Copenhagen is all well and good, but as a burning current story it is just not doing it for me. So I shall leave the comment to people who have more idea what they are talking about, like Gareth Renowden.

## Small TransistorsMarcus WilsonDec 09

How often do you get attracted to an article somewhere because of its outrageous headline, and then discover on reading the article that its headline, if not an outright lie, doesn’t quite represent what the article is actually about?

This is one that got my attention earlier this week on physorg.com.  The headline "Scientists build ‘single-atom transistor’ " certainly promises a lot. Getting ever-decreasingly small transistors is what has driven computer and electronics technology forward at such a blistering pace. But there is (probably?) a fundamental limit – when a transistor is the size of an atom it can’t be made any smaller. A single atom transistor (or just a few atoms transistor) is one of the holy grails of nanotechnolgy. So are we there now?

Hmmm. No, not really. The clue was in the ‘…’ quotation marks around ‘single-atom transistor’. What is really meant is that researchers have studied the effect of single phosphorous donor atoms in silicon. That is not the same as building a single-atom transistor – although it might exhibit measurable effects due to single phosphorous atoms, the device itself is much bigger than a single atom. After a bit of trawling with search engines I recovered the original paper on which this headline is based. Unsurprisingly, at no point in it do the researchers claim to have build a single-atom transistor. Kuan Yen Tan et al, Nano Lett., December 1, 2009 (Letter), DOI: 10.1021/nl901635j.

Moral: In case you didn’t know, don’t believe everything you read.

## A boost to my vocabularyMarcus WilsonDec 07

I learnt a new word recently, courtesy of PhysicsWorld. Yoctosecond. That’s a period of time equal to ten to the power of minus twenty-four of a second. Or 0.000 000 000 000 000 000 000 001 seconds. As in "I’m just popping out – I’ll only be a yoctosecond."

Physicists (or anyone else for that matter) haven’t needed to delve into this short a time-frame before. There are femtosecond lasers (ten to the power minus 15 seconds) but this is still a long way off a yoctosecond.

But it’s thought that the quark-gluon plasma soupy thing that the ALICE experiment at the Large Hadron Collider will look at could emit light pulses that last just a few yoctoseconds.

## Waves in a plasmaMarcus WilsonDec 04

This post follows from a comment I had yesterday from Robert McCormick on the www.sciblogs.co.nz version of PhysicsStop. (Unfortunately the mapping of PhysicsStop onto the sciblogs website doesn’t combine the comments – so if you read my blog through The University of Waikato website you won’t have got his comment, so I attach the link below)

Plasmas provide a wonderful medium for waves. A plasma, in simple terms, is a gas (often very high temperature) consisting of charged particles (e.g. electrons and positively-charged molecules). As well as bouncing around like your normal gas molecules will do, the particles in the plasma feel the electric and magnetic fields that each other create, leading to a great array of possible ways for waves to travel. So plasmas can carry sound waves, electromagnetic waves and bizarre combinations of  both.

I studied a little plasma theory while I was an undergraduate and found in fascinating. I can’t say I’ve had much to do with plasmas since, but the theoretical techniques I learnt about for analyzing waves in plasmas have proved really useful for other physical situations, such as waves of neuronal activity in the brain. It’s an example of how the same underlying phenomena can manifest themselves in very different physical systems. And it’s an example of how to answer the undergraduate student how says ‘Why am I studying this stuff – I’m never going to use it’.

## RainbowsMarcus WilsonDec 03

1 Comment

The sun poked out from behind the clouds on my way in to work this morning just long enough to produce a beautiful rainbow for a few seconds. Not sure when we’ll next see it.

I guess most of us studied rainbows at school, but I’ll throw in a couple of physics words with this comment that you may not have heard of. The light from the sun is, as we know, made up of a whole range of different colours. Each point in the spectrum is made up of light with a different wavelength (the distance between peaks of the wave); red is longer wavelength (I say long, but I mean about 700 nanometres, where one nanometre is a millionth of a millimetre); violet is shorter, about 400 nanometres.

Now, the key to making a spectrum (e.g. a rainbow) from white light is a dispersive material. Water is one example, as is glass (e.g. your prism). Physicists describe something as dispersive if it has different properties at different wavelengths. That means the red light interacts with it differently to the blue, so they get split up. One of the key things physicists like to know about waves of some kind moving through a medium (e.g. light in air, sound in water, waves on a violin string) is the dispersion relation. This describes the relationship between wavelength and frequency (the number of oscillations a second).  A non-dispersive medium has wavelength times frequency equal to a constant (the wave velocity). But, what they forget to tell you at school, is that, in general, this isn’t true. And it’s materials where it isn’t true that are way more interesting to work with.

No dispersion, no rainbows. That simple.