The Second law of Thermodynamics

As has just been discussed, when a hot body is placed in contact with a cold body then the cold body heats up and the hot body cools down: heat flows from the hot body to the cold body. The reverse process is never experienced. Of course, with a refrigerator heat is extracted from the cold contents and flows into the warm room. However this is only achieved by using energy via the refrigerator motor; the process does not happen naturally work has to be done. The foregoing is essentially a simple formulation of the second law of thermodynamics, which states that heat always flows from a high temperature body to a low temperature body unless work is done by an outside agent. More insight into the nature of this law is obtained by looking at the microscopic situation. Starting with two bodies at different temperatures, the component molecules in the hotter body have higher average energies than those in the colder body; molecules with different average energies are separated. This means that there is some order in the system in that the molecules with higher average energy are separated from molecules with lower average energy. It is similar to the order achieved when you have one box containing red buttons (say) and another containing blue buttons. When the two bodies are placed in contact with each other the flow of heat means that eventually they reach a common temperature and all the molecules have the same average energy; there is less order (more disorder) in the system. With our button analogy, it is equivalent to mixing all the buttons in one box. In other words, the second law of thermodynamics can be taken to state that the flow of heat in a system is always in such a direction as to increase the disorder of the system. A physical quantity that expresses the amount of disorder in a system is what is called its enrropy. Entropy is simply related to the probability of a given state of the system existing the lower the entropy the less probable is the state. For example, consider a single box containing both blue and red buttons. If it is shaken, it is possible, but very improbable, that you will end up with an ordered (low entropy)’state in which the blue buttons are on one side and the red buttons on the other. It is, in contrast, highly probable that the buttons will become very mixed up and highly disordered (high entropy). Therefore, another formulation of the second law is that in all physical processes taking place in a closed system, i.e. with no external influences, the entropy (disorder) always increases. This law is a statistical law and so it must be recognized that it is possible, although negligibly so, that a system might end up in a more ordered state with a resultant decrease in entropy. The likelihood of shaking a button box and finding the blue and red buttons completely separated is extremly small; how much more so for interacting pieces of matter containing 10²⁴ or more particles. The whole universe is subject to the second law, which means that its entropy and therefore its state of disorder is continually increasing. Of course, order is all around us in the form of furniture, machines, roads, buildings, indeed ourselves etc, but this order is only achieved by vast expenditure of energy which leads to ever increasing disorder elsewhere. Eventually (but an inconceivably long time ahead) it might be thought that there will be complete disorder everywhere and a common temperature throughout the universe; the so called ‘heat death’ will be upon us. That being said, there are residual uncertainties about this conclusion when it is put into the context of the expanding universe and the possible eventual contraction of the universe leading to the ‘big crunch’. This continual increase of entropy with time in the universe has another implication, namely that it indicates the direction in which time is flowing the ‘arrow of time’. Make a video of a china plate (a very orderly state) being dropped on the floor and breaking up into smithereens (a very disordered state) a process in which the disorder, and therefore the entropy, clearly increases. Then run the video backwards. Clearly the reassembly of the plate shown in this backward run does not accord with our everyday life; time just does not run in that direction. Our experience of time runs in the direction in which entropy increases. However, this discussion is in the context of the behaviour of very complex entities. At a fundamental level involving elementary particles the direction of time is obscure and, we shall come back again to the nature of physical laws when time is ‘reversed’. However, let us for the present stay with everyday matter and its behaviour, which is governed by, among other things, the two laws of thermodynamics just discussed. In the following some of the important properties of solids, liquids and gases will be briefly described and explained.