Chapter 2 – Is a calorie a calorie?

The second law of thermodynamics

We cannot place any faith in the first law of thermodynamics alone. The first law can only be applied in isolation to closed systems, in thermal equilibrium. Living organisms (human beings) are open systems, far from equilibrium (although continuously trying to achieve this state); carrying out continual metabolic reactions and so we must also take into account the second law of thermodynamics.

The second law of thermodynamics is effectively the law of common sense. This second law of thermodynamics came about while looking into optimal efficiency of machines during the Industrial Revolution. The first law alone implies that perpetual motion could be possible (if energy is neither created, nor destroyed, could a system go on for ever?) The second law introduces the pragmatic fact that energy will be lost to its surroundings and energy will be used to convert energy and the quest for perpetual motion was thus ended.

To give an everyday example, when we put the kettle on, electricity or gas energy is converted into heat energy to heat the water. Some energy is used up heating the filament to then heat the water (that’s energy used in making available energy) and some energy is lost out of the spout and some more is ‘wasted’ as the sides of the kettle inevitably heat (that’s energy lost because the system is not closed).

The second law also introduces the irreversibility concept. We can turn coal into heat, but we can’t turn the heat back into coal. Where we can turn something back into the form it once was, we can’t turn it back into the same amount. E.g. electricity can produce steam and steam can produce electricity, but the energy lost throughout the process is such that the electricity at the end would be a fraction of the electricity with which we began.


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