# Physics Stop

## Kinetic and Potential EnergyMarcus WilsonJan 31

I read last week an electronics magazine article about developments in electric aircraft. Yes – for real – the aviation industry is not confined to using truck loads of fossil fuel to get from A to B. The article got off to a bad start by using the word ‘electric’ with the phrase ‘reduced greenhouse emissions’ in the same sentence (it depends on how you generate the electricity, doesn’t it?) but from then on it was very interesting.

There are lots of aircraft (mostly small ones at the moment) being designed and built to run off electricity. (Propellor driven, obviously.) Indeed, there is a team aiming to get a solar power aircraft to fly non-stop round the world. When you think about it, solar power for aircraft is a good idea – you have lots of wing area to put solar cells on, and above the cloud it’s always sunny.

"Yes", I hear you say, "but what about at night? There’s not much sunshine then." True. But with supercapacitors becoming bigger (in terms of ability to store electric charge) and cheaper and smaller, storing the energy collected in the daytime and releasing it at night is now much more realistic.

And there’s another energy storage method too, that is available to aircraft. That’s to gain height. Store up your energy in the day as gravitational potential energy, and then release it at night by losing altitude.

A quick back-of-the-envelope calculation tells me that a plane at 10 km height doing about 800 km/h has about four times as much potential energy as it has kinetic. There’s a lot to give up. Indeed, one of the problems facing a descending aircraft preparing to land is that it has to dump both potential energy (drop in height) AND kinetic energy (slow down) at the same time. The only option to it is the drag force – use air-resistance to get rid of that energy. Another rough calculation for a 777 tells me that the amount of energy it has to get rid of is equivalent to burning about 700 litres of fuel  Not terribly large considering the amount of fuel that can be carried (Wikipedia says about 180 000 litres), but wasted energy nonetheless.

(Based on 32 MJ per litre as energy density of petrol – aviation fuel will be a touch different, and 250 000 kg maximum landing weight for a 777-300ER. Not that I expect Boeing to realease a solar-powered version of the 777 anytime soon. Probably the 777 isn’t a good example to pick anyway because it’s a jet not a prop, but it’s my favourite plane which is why I picked it.)

## Another weight-based challengeMarcus WilsonJan 28

About eighteen months ago I thought about the challenge invited by an airline’s policy on cabin baggage – forget the weight, if it’s smaller than a certain size you can take it on board.

A similar challenge has been set by the company that collects my garden rubbish. I have a ‘green’ wheelie bin, which gets stocked up every month with grass clippings, hedge trimmings, pruned branches and other garden waste.  On the back of my latest bill, in the small print, I read that the company will decline to collect any bin that weighs more than 100 kg. Reassuringly, they point out that in their experience this only happens once in a thousand collections.

I should say so. Given the bin is maybe 1.25 m high, and about 40 – 50 cm in breadth and depth, its volume comes in at a paltry 250 litres or so.  A weight of 100 kg would entail filling it 40% full with water. It’s hard to envisage that I could come close just sticking garden refuse, complete with lots of air gaps, in it.  I’d need to really pack it with moisture laden grass clippings, leaving no time for them to dry out, to get it that heavy, I think.

I’ve been spending some time over the last few days clearing out six years of accumulated junk from our garage. One of the latest discoveries was an empty paint tin, left over from when we attacked the bathroom about three years ago. I like to think that I am moderately conscious of what I do to the environment, so I looked on the tin for instructions on how to get rid of it properly.  In amongst the various warnings on what to do if you accidently drink the contents or paint your eyes with the stuff (one assumes you are meant to read the instructions BEFORE you get it in your eyes) there was the friendly command "Dispose of the empty can safely".  I quite agree, but I was looking for something a little more specific. Just what does ‘safely’ entail?

To be fair, if they do include instructions on what to do with left-over paint ("Consult [company name] for advice on disposal of paint products", or words to that effect), but I don’t have the paint, I just have the empty can and some residue.

The same thing applies to batteries and fluorescent light bulbs, which kindly tell you that they have to be disposed of properly but neglect to tell you what the proper manner is.  Grrrrr

## Fighting global warming with old technologyMarcus WilsonJan 25

Man – the swimming pool was cold today.  Just three days with no sunshine is enough to send it back into penguin territory.

Anyway, that’s not today’s entry. I’ve just been at a very interesting seminar by Bill Redman-White, a visitor to the university from The University of Southampton and NXP Semiconductors UK. He was talking about how semiconductor technology (microprocessors and the like) can play a significant part in  tackling climate change.

The general thrust of the talk was not so much  the development of new technology, though that will happen, but taking existing technology and using it more carefully (using microprocessors etc) so that it consumes less energy. There is a lot of industry drive to do this. To some extent, this has been stimulated by legislation in places like California – which dictates amongst other things how much energy your mobile phone charger is allowed to waste when you’ve finished charging the phone but still have the charger plugged into the wall.

Another example are the fluorescent light bulbs that are now ubiquitous in homes. They make a big energy (and cost) saving over the old incandescent bulbs, but, only if they have a long life. The key to making a fluorescent bulb last a long time is to get the tube hot before it is ignited. And the smarter light bulbs now have electronics inside them that ensure this happens. (There is a bit of a problem here though – the electronics resides ABOVE the tube (when the bulb is installed) which means that the electronics gets hot, due to the hot air around the tube rising.  Some components really don’t like this – and there needs to be some thought in the design here.)

Bill also talked about hybrid cars, LCD televisions, computer power supplies, etc.  He ended with a good reason for someone to become a physicist or an electronics engineer: the first step to saving energy is to understand what the problems are – that means science and engineering.

## Measurement beats guessworkMarcus WilsonJan 21

OK – so this story is a touch light on the physics front, but it does demonstrate the power of actually taking the time to measure something, instead of assuming the obvious.

My wife has kindly agreed to help a friend (who has limited mobility) travel from Cambridge to Hamilton for a medical appointment. This of course means getting the friend into a car, which from all accounts isn’t an easy job.   One naturally assumes that the process will be aided if the car door is big, the seat is wide, and if it isn’t too high or too low off the ground.

We have two cars, both rather long in the tooth now, and both from the same manufacturer. One’s a station wagon, with lots of space and a towbar - it’s certainly useful to have but it does guzzle a lot of fuel, and the other’s smaller, more comfortable, and is pretty economical to run. Unsurprisingly, the station wagon spends most days locked in the garage going no-where while we use the other nearly every day.

Now, it was a no-brain decision that I should take the smaller car to work today leaving my wife the large car in which to take our friend to the hospital.  Though, for some reason, last night I felt compelled actually to measure the door sizes, space in the front seat, height from the ground to the seat, etc, of both cars – just to confirm what was patently obvious.

On measuring, I had a bit of a shock. They are actually the same. I reckon I could take the passenger door off one car and fit it snugly on the other.  Perhaps I shouldn’t have been surprised – two cars of similar ages from the same manufacturer, but I just thought that the larger car had to be the one easier to get in and out of. I’ve even ‘felt’ that there is more space in the front seat of the larger car, but the tape measure tells me otherwise. My intuition has been deluding me.

Unlike your intuition, a tape measure, or other scientific measurement, doesn’t often lie to you. A story to recount to my students next time they can’t be bothered to measure something that’s ‘obvious’ in the lab.

## Teach science understanding, not recipesMarcus WilsonJan 19

If you teach science at university (or, I suggest, at school too) and have an hour free (ha ha) this recent lecture by Eric Mazur is well worth a listen.  I’m willing to bet that it will be an hour well invested.

Here’s three points I thought were particularly significant (to the point that I wrote them down) while I listened to it this morning. I’ll say nothing more – just leave it for you to listen to.

1. Where does learning usually take place?   It’s unlikely to be in your classroom. So what on earth is happening in your classroom?  Anecdote – I learnt what Lagrange’s method of undetermined multipliers was all about around six years after I was ‘taught’ it – when I actually had to use it at work for a REAL problem (not a made-up simplistic textbook one – see point 2). The actual ‘a-ha’ moment was when I was walking from my flat to the bus stop, thinking about the problem.  (N.B. Don’t worry about what Lagrange’s method of undetermined multipliers is – you really don’t need to know.)

2. Most ‘problems’ and exercises provided in textbooks unsurprisingly neatly fit the recipes that the textbooks teach. What a textbook teaches is recipes, that may sometimes be useful in the real world, but often they are not. Real-world problems require real understanding, not following a recipe.

3. The longer you teach a subject for, the harder it is for you to grasp why a student might have difficulty understanding something. If it’s second nature to you – if it’s just ‘obvious’, how do you explain it to someone who is having difficulty? It takes effort to tease out how a student is actually thinking and pinpointing their misconceptions. Someone who has only just grasped it (i.e. another student) is probably better equipped at explaining it than you have. (So why not use that for the advantage of the whole class…)

## Wood and waterMarcus WilsonJan 17

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It all started so well…A nice sunny Saturday, a good opportunity to get going on some of those jobs that you’ve put off for too long.  So I put a final coat of paint on the door and, with that task out of the way, I decided to move to tackling the pergola that sits over our deck area. Scrubbing it clean, sanding it before painting it are not really tasks to look forward to, but it was a good excuse to spend the day listening to the cricket so I made a start.

Before I go any further with this story, I wish to emphasize that we acquired this structure when we bought the house. I played no part in its design, the choice of material, or its construction. As far as I am concerned, it has always been there, painted white, showing up the dirt.

Cleaning the thing started well enough. In fact, I got thinking that perhaps I didn’t need to paint it after all, just a good clean would be sufficient. Then the problems started. At the top of a pillar, where no-one usually touches, chunks of the structure started coming away with the scrubbing brush. A bit of unenthusiastic prodding with a screwdriver showed me that there were large patches of wood that were anything but solid anymore. In one place I could push the screwdriver through a supporting pillar and see it come out the other side. I’m not a builder, but I don’t think that is meant to happen. The ‘wood’ inside is very, very wet. I think what has happened is that water has got in at the top of the pillar, and worked its way, probably for years, down inside slowly destroying the structure.

Methinks the whole structure is riddled with patches like this, and probably the only thing for it is to come down. Water and wood, not a good combination.

But that’s not completely true, is it?  Boats can be made quite happily of wood, so long as the wood has been looked after properly. In fact, wooden boats have a couple of nice features. First, they don’t sink – (most) wood is less dense than water – and, secondly, damp wood swells and that helps to seal the gaps between the boards on a wooden boat.

Several years ago in Portsmouth, in the UK, I helped re-launch an old wooden yacht that had been out of the water for quite some time, for maintenance. Within just a few seconds back on the water, it was clear that she was leaking. The wood had shrunk, which had opened up small gaps in the hull, and, when back in the sea, the water started coming in. We watched from the shore as she got lower and lower in the water, with the solitary crew member assigned with the task of rowing her to her mooring getting very, very wet. But of course she was never in any danger of sinking, and, once she was nicely damp, the boards swelled, plugged the holes, and, after the water had been bailed out she was back as she should be – nicely watertight and floating happily.

So wood and water can work quite well together,  in the right place.

## Electricity from water – the exciting wayMarcus WilsonJan 14

Forget conventional hydroelectric installations – if you want to have fun generating electricity in the lab then the completely static tin can and bucket generator is for you. They are all the rage at the moment – at least in our lab here, where we’ve sidetracked a student from his summer project into making one to wheel out on Open Day. (It is, actually, vaguely related to what the student is doing for his project, so it’s not a complete sidetrack.)

Anyway, there are lots of clips on YouTube of these devices. Walter Lewin does a good job (of course) of demonstrating one.   Basically, you have two dripping water streams (from the same ultimate source – e.g. a bucket) Each stream falls through a metal ring and into a metal bucket. But – the bit that makes it work is that you electrically-connect the left hand ring to the righ-hand bucket, and vice versa.  This leads to a nice build up of charge in the buckets and sparks can fly between them.

## DNA sequencing in one easy step?Marcus WilsonJan 12

DNA Sequencing has been around for a while now. It’s success is down to a wonderfully unlikely collaboration between molecular biology and computer science. Basically, in simple terms (apologies to molecular biologists if I get this wrong – feel free to correct me) to sequence your genome, you take your DNA, chop it into bits, replicate those bits, attach clever fluorescent molecules to the bases (a different colour attaches to each base) then read of the sequence of bases by looking at the fluroescent colours. This gives you lots of little bits of the whole sequence – you then throw these into your computer programme which works out what the complete sequence has to be.

Is there a quicker way?  What about making a DNA reading machine – just feed your entire DNA strand through the machine and it reads it automatically – rather like swiping your credit card through the magnetic reader. The physicists and chemists are working towards that point, as described by Philip Ball in December’s PhysicsWorld magazine, and blogged about here.

A way of doing this is with the nanomaterial graphene.  Graphene is a form of carbon, and can be considered to be a single layer of graphite. It’s a sheet of carbon atoms, arranged hexagonally - just one atom thick. The idea is to punch a hole into a graphene sheet, just large enough to feed a DNA strand through. The strand almost blocks the hole, but not quite, so it is possible for small ions to pass through the hole as well, when an electric field is applied. This flow of ions constitutes an electric current, which can be measured. But here’s the clever bit: each base of the DNA has a slightly different size and shape, which means it blocks the hole to a slightly greater or lesser extent compared to another base. This change in the unblocked hole area means an increase or decrease in the amount of current that can flow through it. So by measuring the current that flows, one can determine which base is in the hole. Just pull the strand through the hole, measure the variations in the ionic current, and, hey-presto, you have read the entire strand.

That’s the idea, anyway. Why use graphene, not another membrane with a small hole? Graphene offers extreme strength and is very thin, and low-cost too. It’s so thin that only a single base can get into the hole at one time, meaning it really does have the potential to read a strand of DNA, base by base.  It’s a candidate to watch for the X PRIZE Foundation’s \$10 million for a machine that can read 100 human genomes in 10 days (at less than \$10 000 a genome).