Although we have an intuitive understanding of heat, the way in which energy flows has historically not been well understood, although it has a profound impact at all levels from microsystems through to the Universe!
Thermodynamics is about the behaviour of energy flow in natural systems, and the three “Laws of Thermodynamics” summarise some of the fundamental truths of thermodynamics. The First Law of Thermodynamics is often called the “Law of Conservation of Energy”. It has been stated in a number of ways:
The First Law is the one that shows that the vision of limitless free energy is just a dream, because it is not possible for any machine to produce work without consuming energy. The First Law also says that energy cannot be created or destroyed, and thus implies that the total amount of energy1 available in the Universe is constant.
The Second Law of Thermodynamics can be formulated in several different ways, of which the most useful to us is that “heat can never pass spontaneously from a colder to a hotter body”. As a result of this, any natural processes that involve the transfer of energy inevitably go in one direction, and all natural processes are thus irreversible.
An alternative expression of the Second Law is that “the entropy of an isolated system always increases with time”. Entropy is a measure of the disorder or randomness of energy and matter in a system. At the level of the Universe, the Second Law demonstrates that both energy and the matter are becoming less useful as time goes on, having effectively “gone downhill” ever since the Big Bang, when energy and matter and all forces of the universe were unified.
Clearly the detail of the Second Law of Thermodynamics has a universal level of application that is way beyond this module, but if you want to follow this up there is a good resource at http://www.secondlaw.com/, as well as an application to real life at Frank Lambert’s paper Shakespeare and Thermodynamics: damn the second law! At a more mundane level, the Second Law determines why chemical reactions behave the way they do, and also explains why energy spontaneously tends to flow from being concentrated in one place to being diffused, dispersed and spread out. It is thus the Second Law of Thermodynamics that we use to our advantage in managing thermal issues.
In case you are wondering, the Third Law of Thermodynamics states that if all thermal motion of molecules could be removed, so that they had zero kinetic energy, a state called “absolute zero” would occur. This is at a temperature of 0K (zero Kelvin) or −273.15°C. Eventually the universe will attain absolute zero when all energy and matter is randomly distributed across space. Fortunately the current temperature in the universe is about 2.7K, so we have some way to go!
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