The Second Law of Thermodynamics
The First Law of Thermodynamics is that energy cannot be created or destroyed. With peak-oil, we are only too aware of the restrictions that this conservation law for energy imposes on us. But what of the Second Law of Thermodynamics?
In its popular form, the Second Law of Thermodynamics states that the amount of ‘disorder’ in the Universe, the total entropy, is always increasing. But the original form of the Second Law, concerning the efficiency of heat-engines, was also highly suggestive.
Improved engine-efficiency was the motivation for the shy, young engineer, Sadi Carnot, son of scientist and French Revolutionary, Lazare Carnot. Sadi Carnot wanted to improve steam-engines “for the glory of France” (the year was 1824). He looked into the bare essential theory behind any heat-engine and found that they all work by having heat flow from a high temperature to a lower temperature. He also found that there was a maximum to the efficiency of any heat-engine. This maximum efficiency was determined solely by the initial and final operating temperatures, in other words, by the temperatures of the source-heat and the waste-heat. The lower the temperature of the surroundings or heat-sink, the better the engine would run.
But now here come the bombshells. First: every physical (and biological) process is a heat-engine of sorts – this arises from the amazing universality of the Law; second: the atmosphere, the rivers, lakes and seas, and the very earth – all these are the sinks for our human activities. With global warming, as the sinks become warmer, so the efficiency of every single engine, nay, every single process on Earth, will be compromised.
For example, consider the process of photosynthesis. This process appears paradoxical in as much as gases, in the presence of chlorophyll and sunlight, are tamed and made to reside quietly in large and complicated plant molecules. Surely, the total disorder has decreased? No, what ensures that entropy still always increases is that, at the same time as plant material is being formed, waste heat is thrown out in the form of (chaotic) evaporated water molecules (this is called ‘plant-cooling’). Up to 500 times (!) as much water must be lost to the atmosphere for plant-cooling as is required for plant growth. But, in tropical regions, the ambient temperature is already so high and the air already so humid, that plant-cooling and hence photosynthesis is at the limit of efficiency – any increase in temperature and plant growth will decrease or stop.
Humans, fridges and air-conditioners also work by throwing out waste heat, also mostly by water evaporation. The efficiency of all is lessened by global warming.
Why hasn’t this devastating consequence caused alarm bells to ring? One reason is that, as so many of our man-made engines operate very inefficiently, any reduction in the theoretical maximum efficiency doesn’t show up. The internal combustion engine, that most prevalent man-made engine, is a good example. It is incredible to reflect upon how inefficient a typical car-engine is. My station wagon operates at a mere 13% efficiency. After only a few minutes of driving the engine block is far too hot to touch. Most of the fuel goes simply to heat the engine rather than to move the car from A to B. The car should really be re-named a ‘cooker-on-wheels’. In the summer, when the air-conditioning is on, the car is then a ‘fridge-on-a-cooker-on-wheels’.
But make no mistake, an increase in the background temperature is a bad thing. Even wasteful car-engines run better when the ambient temperature is low: for example, the drive from Bendigo to Sydney consumes less fuel at night than in the day. Also, humans and other animals work better when the air temperature is less than 37ºC.
The German physics professor, Herman Helmholtz, in the middle of the 19th century, predicted the ‘Heat Death’ of the Universe – a time when everything would be at the same temperature and so, even though there would be heat energy galore, this energy would be completely unusable, and everything would stop (apart from the ceaseless microscopic motions).
Global warming will not take us to this dire extreme. While the efficiency of every single process on Earth will be compromised, there will still be one process that will, in fact, work better as a result of global warming: the Earth itself will lose its excess heat to the cold of outer space at a faster rate than before.
Dr Jennifer Coopersmith is an honorary research associate at La Trobe University, Bendigo and a tutor for Swinburne University’s Astronomy Online. She broadcasts occasionally on ABC Radio National. Her book “Energy, the Subtle Concept” was published by Oxford University Press in 2010. She is writing another book, on the Principle of Least Action.
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