Entropy is a term often associated with randomness and disorder. In the language of physics, it is the measure of how disorderly a system is, an actual quantity, shown the symbol ‘S’. Furthermore, it is used in statistical mechanics, as it is related to the number of microscopic configurations that a thermodynamic system can have when in a state controlled by macro variables. The entropy of a system is the natural logarithm of the number of said configurations. This is all very complex and heavily advanced, that’s why I will focus on the simple meaning of entropy, along with its relation to the second law of thermodynamics.
The second law of thermodynamics says that in any cyclical process, entropy stays the same or increases. In terms of heat transfer, it states that heat flows from an area of high concentration to an area of low concentration, i.e. heat goes from a hot place to a cold place until it reaches equilibrium. Think about putting a hot cup of coffee on a table, the heat from the cup warms up the air and table beneath it, not the other way around. The second law of thermodynamics is an observation of such phenomenon, as we see that heat will never flow from cold to a hot space, within an isolated system.
You might think that a fridge violates this law, but this isn’t the case as the fridge isn’t isolated. To have a net flow of energy from a cold to a hot area, you need to have work done. External energy is required for heat to flow to a hotter place; hence a fridge running on electricity. When the electric supply is stopped, no work is being done; therefore heat will start to flow to the colder area. The link to entropy is that in terms of the physical world, a concentration of heat symbolizes high levels of organization, and an absence of heat shows disorder.
An alternative definition of the second law of thermodynamics is that it is impossible to have 100% efficiency; you can’t extract an amount of heat, Q, and use it all to do work, W. A proportion of the heat is lost to the environment. It is this law that forbids the perfect heat engine. There is a limit to how efficient a heat engine can be, as it is bound by the second law; and this is the Carnot efficiency.
However, it is important to note that the Carnot efficiency is just a theoretical figure, real like machines are far less efficient. Theoretically, the most efficient heat engine cycle is the Carnot cycle. In order to approach the Carnot efficiency must be reversible and no change in entropy. This statement shows how the Carnot cycle is an idealization since no real engine processes are reversible and all real physical processes involve an increase in entropy. Furthermore, Carnot engine cycle isn’t practical because the heat transfer in the isothermal process is far too slow to be practical. The engine whilst being extremely efficient would be so slow that there really is no use to it.
We have seen how entropy has many meanings, from being a measure of disorder to being a measure of the amount of energy which is unavailable to be used for work. But its most impressive definition is its use of a timescale. Since entropy gives information about how an isolated system has changed, concerning its particles and their energy, it gives us the direction of time. If we are given two snapshots of a system from two different time scales, we can tell which one came last, as it will be the most disordered one. Any isolated system in the universe, ‘flows’ with disorder, in the sense that events will take place on a course that leads to higher entropy.
Other examples of entropy are all around us in our everyday lives. An ice-cube sitting on a table top will melt into water. Ice is a state of order because the structure of the molecules is organized and fixed, whereas water represents a state of disorders, as water molecules aren’t fixed and can flow wherever.
Cleaning up your room is an example of how you need work done to get to a state of high order. In your everyday actions, you place objects randomly around your name, because it is the easiest thing to do, in terms of energy used. This increases how disorderly the room begins. When you decide to clean your room, you use a large amount of energy and work done to achieve a state of order. Similarly with a car, if you don’t drive it for a number of years, it will break down and achieve a state of disorder.
Entropy is all around us and governs what we can and can’t do, and how much energy we need to carry out such actions. It is the universe’s natural timeline and is a cruel master; because it tells us that we will always lose out when using energy. There will never be a 100% efficient isolated system because the universe simply won’t allow it.