The Standing Invitation

Posts Tagged ‘Physics

Cold Power

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I once attended a science demonstration involving a model steam train.

It was quite a neat little toy, with a simple but fully functional combustion engine. You fill the boiler with water, and add a pellet of fuel. The fuel burns, producing heat. The heat causes the water to turn to steam. The steam expands, which pushes against a piston. This motion turns the wheel and sends the train shooting off across the lecture hall.

What we have here is a clear display of energy changing from one form to another: the kinetic energy of the train comes from the kinetic energy of the expanding steam, which ultimately comes from the fuel.

The train doesn’t know or care what fuel goes into it. Under the boiler could be a lump of coal, or burning wood or oil, or a hunk of uranium. All that matters is that the fuel gives off heat. If it can get hot, then it can push the train, and that’s all that really matters… right?

Well, not exactly.

Because then the lecturer filled the boiler not with water, but with liquid nitrogen. This was at about -196 ˚C, and so exposure to air at room temperature caused it to bubble and boil. Inside the train, it gave off huge amounts of expanding nitrogen gas. This, too, pushed against the piston, turned the wheel and sent the train moving again.

So liquid nitrogen is clearly a good fuel for the steam train. But liquid nitrogen is cold. It doesn’t generate heat in the same way that a burning lump of coal does, and when it’s in the train, the train becomes icy-cold even as it moves. Where does the heat that moves the engine come from?

It comes from outside. Energy flows through the walls of the boiler, heating the liquid nitrogen and causing it to boil. While the train moves, the world gets slightly colder.

A fuel can work by emitting heat, in the case of petrol, or by absorbing heat, in the case of liquid nitrogen, and it doesn’t matter in the slightest. What drives the engine is not energy generation, but energy flow. There is a difference in energy between starting state (fuel in the tank) and the end state (fuel consumed), and there is an allowed pathway that allows for the removal of this difference – consumption of the fuel in a way that happens to propel the engine. In thermodynamics, as in so many other things, it’s the differences that make things interesting.

But I’m afraid I can’t foresee a world of high-speed rail travel in trains powered by liquid nitrogen. The little model was only good for a few minutes before the ice caused its wheels to stick. It fell over, the liquid nitrogen spilled, and all the tiny passengers promptly asphyxiated.


Written by The S I

August 13, 2011 at 11:59 pm

A Constant Source of Confusion

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Seven words we all know by heart: “The speed of light is a constant.”

It’s a sentence I learned long ago; since then, I have used this constant speed of light (299,792,458 metres per second) thousands of times in hundreds of different equations – but it is only recently that I have discovered what it really means. Unexamined, it’s just words. After a little thought, however, it explodes into something really remarkable and counterintuitive, rather like the old story of putting one grain of rice on the first square of a chessboard, two on the next, four on the next and eight on the next, and finding that by the end of the board (64 squares) you have rather more rice than you expected.*

The problem comes from having a speed that is constant. Because for something to be constant it must be constant for everybody ­– even people who are themselves moving.

The speed of light is c.

I have a friend, Alice, who is an astronaut. Her space ship’s top speed is half the speed of light ­– 0.5c. Alice’s space ship has headlights. When she is at top speed, she switches on her headlights and a beam of light leaves the front of her ship and travels ahead of her. How fast does the light move? It moves away from her at the speed of light, c. She can check this any way she likes, that is what she will find.

Space travel is not for me. I prefer to stay on Earth, but I do look out of the window and see Alice from time to time.

When Alice is moving away from me at half the speed of light, and switches on her headlights, we know how fast the light moves away from her: c. But how fast does the light move away from me? Surely it should be one and a half times c ­– the speed of light, plus Alice’s speed when she turned on the headlights.

But it’s not. The beam of light moves away from me at speed c, too, because the speed of light is constant, wherever you’re standing.

I would expect Alice to tell me she detected the light crawling away from her at a mere half-c, because that is what I see is the difference between her speed and the speed of light. But she insists she saw the beam move away at c as well.

Which of us is right?

Amazingly, we both are.

In order for the speed of light to remain constant, other things change instead – space and time. If Alice and I measured the distance between Earth and Alice’s destination, we would report different distances ­– and we would both be correct. We would disagree about how long things took to happen outside the space ship, and both be correct.

Space and time distort depending on how you are moving, all so that the speed of light doesn’t have to. It remains, for all observers, a constant.


An excellent introduction to relativity is Bertrand Russell’s “The ABCs of Relativity” (1925).

* 18,446,744,073,709,551,615 grains of rice, or about 9.2 cubic kilometres of rice, according to Wikipedia.

Written by The S I

August 5, 2011 at 8:30 pm

Microwave Cooking

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It much less common these days, but you do still occasionally hear stories about mobile phones causing cancer. They probably don’t, by the way – see references. But the issue is a convenient gateway into another of the unsolved problems of chemistry, a problem to do with microwaves.

Mobile phones are microwave emitters. The question of mobile phones and cancer is really a question about the role of microwaves in biochemistry. Do microwaves interfere with chemical reactions?

We know that infrared radiation, as emitted by hot things, can interfere with or even cause chemical reactions – we demonstrate this every time we cook a meal. The pizza that just burned in my oven underwent several infrared-initiated chemical reactions, most of them undesirable. This happened because photons of infrared radiation are able to break chemical bonds; they forced the molecules in my pizza to reconfigure themselves, generating smoke and steam and ruining my meal in the process.

When it comes to microwaves, what we are looking for is called a specific microwave effect (SME) – some chemical process that occurs only as a result of the microwaves, and cannot be explained in any other way.

Microwave photons are much less energetic than infrared. They do not have the strength to break chemical bonds. They can move electrons around within the molecule, causing them to rotate very quickly, but there simply isn’t enough energy to do any chemistry.

So when you irradiate a sample with microwaves, you would not expect any chemical reactions to take place. But they do. What is happening?

The answer is that the microwaves are causing the molecules to rotate, which in turn generates heat through something akin to friction. This heat does have the energy to break bonds – and so we’re back to chemical reactions caused by thermal effects. So although the microwaves are causing the chemistry, it’s only through normal, boring heating.

But that doesn’t quite explain everything. Sometimes, heating a reaction in the conventional way gives one product, but blasting it with microwaves generates another. Are these examples of SMEs?

Probably not. There is a difference between heating something directly with infrared and heating it indirectly with microwaves: heating with infrared works from the outside in. If you heat a vessel, the surface gets hot first, and a lot of the heat just gets radiated off to the air. Microwave heating works from the inside out, which allows much higher temperatures to be reached inside the vessel. This can have an effect on the chemistry – but it’s still heat that’s doing things; it’s not an SME.

The existence of true SMEs is still much debated. Can all the quirky chemical effects of microwave irradiation be attributed to thermal heating? No, not yet, but we’re working on it. Maybe someday an exception will be found, and that will be very exciting; but for now there is no solid evidence that a microwave emitter like a mobile phone has any more capacity to change your body chemistry than simply being outside on a hot day.


Written by The S I

August 1, 2011 at 8:30 pm

The World In Ten Dimensions

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Just a quick one for you today, folks.

By now people are used to the idea that we need four dimensions to describe the universe: length, breadth and depth providing information about space, and a fourth dimension, time. But in the world of really high-level theoretical physics, people are finding that four dimensions are just not enough to give the full picture. You often hear physicists talking about numbers of dimensions that are just plain silly. Why? Isn’t four enough for them?

Here is a lovely video in two parts that gives an interesting explanation. In order to describe the world fully ­– really­ fully ­– we need to use ten-dimensional space. Make yourself a nice cup of tea, and enjoy.





Written by The S I

July 22, 2011 at 8:30 pm

Posted in Science

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