With origins dating back to Sadi Carnot’s 1824 paper Reflections on the Motive Power of Fire, the second law of thermodynamics states that, overall, heat cannot flow from a cold object to a hot one. Since heat measures disorder, or entropy, another way of expressing this concept is that entropy always increases for an isolated system.
Entropy
Since heat is really a measure of disorder and, in physics, disorder is often quantified as ‘entropy’, it measures the ways in which a number of items can arrange themselves.
Another way of stating the second law is that, for a bounded system, entropy increases, it never decreases. Temperature is directly related to entropy and cold bodies have low entropy. Their atoms are less disordered than those in hot bodies. So any changes in the entropy of a system, considering all of its parts, must produce a net effect that is an increase.
The second law of thermodynamics is true for an isolated system, a sealed one where there is no influx into or outflow of energy from it. Energy is conserved within it. The universe itself is an isolated system since, by definition, nothing exists outside it. So for the universe as a whole, energy is conserved and entropy must always increase.
When considering the universe as a whole, the second law also implies that atoms gradually become more disordered over time. When the universe has expanded so much that galaxies are torn apart and its matter is diluted, all that will remain is a blended mass of particles. This end state of the universe, presuming the universe continues to expand, is known as ‘heat death’.
The Defeat of Perpetual Motion
Since heat is a form of energy, it can also be put to work powering machines such as steam engines. Indeed, much of the science of thermodynamics was developed from the practical engineering of steam engines rather than being deduced by physicists in a lab. One of the implications of the second law is that engines that run off heat energy are not perfect since, in any process that changes heat into another form of energy, a little energy is always lost and so the entropy of the system as a whole increases.
Effectively, the second law of thermodynamics scotched engineers’ dreams of a perpetual motion engine since it proved that, ultimately, all of the energy providing the machine’s power would be lost.
Sources:
Baker, Joanne (2007) 50 Physics Ideas You Really Need to Know (Quercus Publishing)
Holzner, Steve (2005) Physics for Dummies (John Wiley & Sons)
Kuhn, Karl (1996) Basic Physics: A Self-Teaching Guide (John Wiley & Sons)
