 Entropy, abbreviated with the letter S, is a fundamental physical quantity in thermodynamics. To understand entropy, we consider a closed system. A closed system is an isolated part of the universe that does not interact with its surroundings. Let us consider the following system, which consists of subsystem A and subsystem B in which four particles can reside. Another concept that is necessary for understanding entropy is the microstate of a system. A microstate of the system is characterized by how the four particles are distributed between the two subsystems. Let's take a small example. There are two particles in subsystem A. The other two particles are in subsystem B. This is one possible microstate of the system. Another possible microstate is when all particles are in subsystem A and none at all in subsystem B. A microstate is therefore a possible arrangement of the particles. Let us denote the number of possible microstates of a system by n. Now we can understand entropy. Entropy is a measure of the number of possible microstates. The more states the system can occupy, the higher its entropy. Let's construct a system in which no particles can be in subsystem B. All four particles must be in subsystem A. In order to be able to make a statement about the entropy of this system, we must consider the number of possible states. The system was constructed in such a way that all particles can only be in A. There is therefore only one microstate of the system, namely the state, all particles in A. Let's look at another system in which there is more than a single microstate. Let us now assume that there can be a maximum of two particles in subsystem B. The first possible microstate is where all particles are in subsystem A. The second allowed microstate is where a single particle is in subsystem B. And the last permitted microstate is where two particles are in subsystem B. Placing another particle in subsystem B is not allowed in this system. This system has a higher entropy than the previously considered system, where particles could only be in subsystem A. Let us now consider a third system. Let us assume for this system that there are no restrictions on how particles can be distributed in subsystems A and B. Let's go through all possible microstates of the system. In the first microstate, all particles are in subsystem A. The second allowed microstate is where a single particle is in subsystem B. The third permitted microstate is where two particles are in subsystem B. The next permitted microstate is where there are three particles in subsystem B. And the last allowed microstate is where all four particles are in subsystem B. This system has five possible microstates. With only four particles, a maximum of five microstates can be realized. Which of the three systems has the maximum and minimum entropy? The first system has a single microstate. This system has the lowest entropy, S1. The second system has three possible microstates and therefore has a higher entropy, S2. And the third system has five possible microstates. The third system has the highest entropy, S3.