The Standing Invitation

Archive for August 13th, 2011

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