# Physics Stop

## Some thoughts while stuck in trafficMarcus WilsonMay 31

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Monday morning was one of those strange days where for some non-obvious reason there was far more traffic than normal. Maybe there had been an accident somewhere, or there was some event on, but, for whatever reason, it took nearly half an hour to crawl through the Hillcrest roundabouts.

All that start-stop on the car is a massive waste of energy. You get the car going forward, which requires energy, then you stick the breaks on and lose it all. That kinetic (movement) energy turns to heat in your brake pads and does nothing except contribute to global warming. That’s for a conventional car, anyway.  The designers of electric and hybrid vehicles know about this, and that’s why these cars have regenerative braking. The idea is that forward motion of the car is transferred not to heat, but to electrical energy, which can then be used again to get the car moving.

Practically, the concept is quite simple; braking is done in the first instance by the electric motor, which, if driven mechanically, will act as a generator. Ensure that the electrical path back to the battery is in place (otherwise the car will just freewheel) and then motor will act as an electrical brake – the turning motion of the engine will generate electricity and take the kinetic energy from the car.

Easy. So how much energy is involved. Here’s a few quick estimates for a crawling car in a traffic jam. At 20 km/h (or 5.6 m/s), a 1500 kg car will have kinetic energy (calculated by half times mass times velocity squared) of about 25 000 joules. Everytime you brake from this to zero, your brake pads take this away (in a conventional vehicle). If you are doing this say twenty times in a long queue for an intersection, that’s getting on for about  500 000 joules of heat energy created.

What can this energy do? Boiling a litre of water (taking it from say 20 degrees to 100 degrees) would take 4200 J/kg/K  times 1 kg times 80 K = 340 000 joules of energy. Here, 4200 J/kg/K is the specific heat capacity of water – the amount of energy needed to raise a kilogram (litre) of water by 1 degree Celsius or kelvin is 4200 J. In other words you could make several cups of tea for the energy you’ve wasted in the brakes. In terms of the electricity ‘unit’  (One unit, or kWh = 3.6 million J,  your brake pads have consumed about 0.14 units of electricity. That comes to about 4 cents worth at domestic rates.

But one important thing to remember with kinetic energy is that it grows with the square of the speed. So a car doing 100 km/h will have twenty five times the kinetic energy of one doing 20 km/h. Breaking just once from 100 km/h down to zero will also use a similar amount of energy to what’s been calculated above. If you’re one of those people who like to brake at the last moment think about each brake pad boiling water for a cup of tea when you do this.

## Is it cold enough yet?Marcus WilsonMay 25

This week has seen some icy mornings in Cambridge – a reminder that we are sliding into winter. Our heat pumps have been going, especially first thing in the morning to warm the place up a bit, and the cat has relocated his primary sleeping spot from a chair by a window to a rug closer to the unit.

We are able to see the outdoor unit of our downstairs heat pump from the dining room window. Tuesday morning, when I looked at it, there was something odd about it. It looked somehow different. The indoor unit then made some gurgling noises and went into defrost mode, and I realized what – it was iced-up. Instead of seeing the multitude of metallic fins on the back and side of the unit, I was seeing white ice. With the unit in defrost mode, this disappeared very quickly into a pool of water around the pump, the fins reappeared, and the heat pump started heating again.

This unfortunately is a problem with heat pumps. When it gets close to freezing outside, the fins of the outdoor unit drop below zero in temperature. That’s because the pump is pumping heat from the outside to the inside. The many highly thermally-conductive metal fins are there to provide good thermal contact area with the air, so that heat can be drawn from it as efficiently as possible. When they drop below zero, ice is going to form on them. How quickly this happens depends on a number of factors, but it is hard to prevent. (Aircraft have similar problems.)

Once the ice has formed, the efficiency of the machine is severely reduced. That’s because ice is a fair thermal insulator. It doesn’t allow heat to pass through it quickly, so the pump cannot suck heat from the air very well. Also, I noticed the ice forms between the fins, thus massively reducing the effective area of the pump in contact with the air. Overall, it is better for the machine to stop heating the house and instead heat the outdoor unit to remove the ice, then start again.

In the same way it’s better to periodically defrost your freezer – it will mean it will be able to keep the inside colder with less electricity cost. That might seem somewhat paradoxical – getting rid of the ice will help cool the interior – but the ice here is a consequence of the cold, not the cause of the cold – and it is insulating the inside from the cooling elements.

## Have you checked your tyres recently?Marcus WilsonMay 23

A couple of days ago I checked the tyre pressures on our new car for the first time since buying it (several weeks ago). Obviously we want about the right pressure in the tyre to get it to do its job properly and to help get the best fuel economy out of the car. However, getting the pressure right isn’t an easy thing.

For a start, I’m sure every garage’s air machine reads differently. Certainly the ones with an analogue scale can be all over the place in their readings. Even two readings of the same tyre on the same (analogue) machine can be way different (I’m thinking about the one at our local garage).  That might partly be because I’m not using it correctly – but I think I’m following the instructions.

But also the temperature of the tyre is going to make a reasonable difference to the pressure. Air is going to follow the ideal gas law – roughly anyway, which says that for a given mass of a given gas, pV/T is constant, where p is the pressure, V is the volume, and T the (absolute) temperature. For constant volume (OK, the tyre isn’t exactly constant volume) pressure would increase linearly with temperature. On a cold morning, before you’ve gone very far in the car, the air inside could be sitting at 0 Celcius, or 273 kelvin absolute temperature. On a hot day after an hour on the road, it might be more like 50 Celcius, or 323 kelvin. That’s a difference in temperature of 18%, and will correspond to a similar change in pressure.

However, there are other factors too. A well inflated tyre – on a hot day for example – will stretch the tyre and make it more likely to lose air through the tyre wall. So in summer, tyres will lose more air than in winter.

The petrol-head websites (e.g. this one) say, unsurprisingly, that pressure should be checked with a cold tyre – i.e. early morning with little driving done to get to the air hose. If you have the right pressure then, you won’t be under-inflated at other times. Plus, it’s as close to a practicable standard as you’re likely to get with a functioning car – no-one’s going to take them to a physics lab and leave them overnight in controlled conditions for an accurate measurement every month!

## A refreshing approach to scienceMarcus WilsonMay 17

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A few weeks ago when I was visiting Dunedin I was in conversation with a new PhD student and her supervisor. The PhD student was saying how she felt everyone should help each other in their research – share all their data, share all their methods and know-how, to make the world a better place (or words to that effect.) Her supervisor, rather tongue-in-cheek, replied with the comment something like "You poor misguided thing – you’ll learn. We all started off thinking like that but in a few years you’ll be just the like the rest of us."

OK, so I haven’t remembered the words, but that was the gist of the exchange. The fact is, despite good intentions and probably a belief that we should share our skills and know-how more widely, there is a strong element of keeping-it-in-house when it comes to research. We all want to land those big funding grants, attract more PhD students, and so on, and to do that we need to be the best people in the country or world to do our particular job – so why tell others your trade secrets?

Well, the PhD student isn’t completely misguided in her wish. It does happen. On Tuesday, for example, I visited BrainResource – a company with their main office in Sydney. One of their core businesses is BRAINnet:  collecting together a huge database ofelectroencephalograms – from healthy people, from people with various conditions (epilepsy, depression etc) – in various situations – e.g. resting, sleeping, carrying out particular tasks – for the purposes of facilitating research. This data is made available (in certain forms) to others. The idea is that the database helps bring in research grants – for both the company and for other institutions. Overall, the winner will be the governments and health services that fund these projects, as they’ll get a better outcome, as well as the BrainResource employees who get employed to do a fascinating job.

It’s an interesting model of doing business, but it appears to be working.

## Why going downhill is hard workMarcus WilsonMay 14

On Saturday I took the train out to Katoomba, in the Blue Mountains. The train journey certainly gave me a feel for just how vast Sydney is. An hour out of central station, and one is still in the Sydney suburbs. But suddenly the end of the city comes abruptly – the train is suddenly amongst the trees and starts climbing. Another hour gets it to Katoomba, a small town perched on top of precipitous sandstone cliffs.

I quickly got the impression that the way many (most?) tourists see the Blue Mountains is on a coach tour. The coach pulls up in the vast carpark at Echo Point, the tourists pile out with their cameras, take lots of photos of ‘The Three Sisters’, grab an overpriced icecream, then pile back into the coach to go to their next viewpoint, wherever that is. Certainly Echo Point has a feel of an international airport about it.

However, escape the crowds, and it becomes rather more pleasant. The way I did that was to head to the bottom of the cliffs. There’s three ways of doing this. 1. Take the ‘railway’ (I assume this is a funicular – I didn’t get a look at it) or the cable car down. 2. Walk. 3. Jump.  I chose option two. That involved heading down the ‘Giant Staircase’ – about nine hundred steps in total down 250 metres or so of nearly sheer sandstone cliff.

One of the surprising experiences going down this number of steps is just how hard going it is. Near the bottom, my legs were burning – rather like having done a REV class in the gym. What’s happening is that I’m exercising muscles that don’t usually get a good workout, and it hurts. In physics terms, I need to provide a force against gravity to slow me down as I descent. The force of gravity alone would cause an acceleration downwards;  in order  to descend at a constant speed I need a force that balances this (Newton’s first law), and that comes from under-used muscle groups within my legs. After several minutes of this, it hurts.

Incidentally, in energy terms this force is not supplying energy to you, unlike the reverse case where it takes energy to climb the steps back up. That’s because the force is in the opposite direction to your movement – therefore it doesn’t do work on you (transfer energy to you). In fact, it is doing the opposite – your legs are absorbing the potential energy lost as you head downwards. No wonder they are burning at the bottom.

Once at the bottom, it was a lovely walk through the bush (though I did get a little worried about what zero- or eight-legged nasties might be lurking in the undergrowth). I went back up a different route – past many lilttle waterfalls. I lost count at 1200 steps. And at 1000 m altitude, or thereabouts at the top, it’s quite an effort for a near-sea-level-dweller like myself.

## Electromagnetic fields and the brainMarcus WilsonMay 10

Yesterday afternoon I visited Westmead Hospital and talked to a couple of psychiatrists on the use of electromagnetic fields in treating certain conditions. Treatments like the controversial but effective electroconvulsive therapy for depression, and transcranial magnetic stimulation for Parkinson’s and stroke, are becoming well used. However, there is little understanding of why they work. It’s a case of zapping a brain with an electric or magnetic field  improves a problem a patient is having, but no-one knows why.

The gap in knowledge here are at least three-fold. First, how does the electric field interact with neurons and what does it get them to do? There’s experimental data on how they respond, but the mechanisms of this are unclear. And then, when they do respond in the way they do, how does that lead to a change in neural connections? And finally, what is it about the neural connectivity that causes improvements in symptons of depression or Parkinson’s?

These questions are a mix of many different disciplines: molecular biology, neuroscience, mathematics, and, from my perspective, physics, and probably a good deal more.  Making progress on this question is going to require all of these skills. That’s why a cross-discipline collaborative approach is so important to making advances in science. Being on study leave gives me a good opportunity to explore some of these possibilities. Yesterday’s visit was certainly very enlightening in this regard.

## VirgaMarcus WilsonMay 08

I learned a new word today: virga. It was used in a short article in one of the Sydney newspapers, discussing yesterday’s weather. Virga is simply precipitation that evaporates before it hits the ground, and we had some here yesterday. So it was raining, but we didn’t get wet.

Virga can rapidly cool the air around it, causing downdrafts in the atmosphere which then heat as the pressure increases nearer the ground. Personally, I didn’t notice anything particularly bizarre, but then again I was indoors for most of the day.

There’s a nice video of virga on youtube.

## Different place – same old problemsMarcus WilsonMay 07

I arrived in Sydney yesterday. I’m based at the University of Sydney for the next two weeks while on study leave. My first morning here was pretty-well taken up by walking round getting administrative things done like getting a key to my office and getting a computer account and internet connection for my laptop.

It was very reminiscent of a day a few years ago when a new PhD student in our research group turned up at Waikato and needed to get keys, out-of-hours-access card, library card, computer account, etc etc. I spent half the day taking her around the campus. Now at Waikato, this problem should now be much diminished by our student centre in the library building. A lot of the administrative things new students need to do are now located in one place. That’s logic for you. Does it work? I’ll have to track down a few new students when I get back and interrogate them.

Now, at Sydney this morning, in order to get a key for my office in the Physics building I had to walk close to a kilometre, through the campus, across a main road, to the security office, which for some reason is located right on the periphery of the campus. Apparently, all keys are issued centrally – departments don’t issue keys for their own buildings. Then I had to walk back again. An easy experience? Hardly. At least getting the computer access could be done through the physics department. One positive thing from the trip, however, was that I’ve now located the swimming pool, which is large and indoor and cheap and available to short-term visitors like me and open early morning. Yay.