Entropy

We all have heard sometimes about this rare concept of entropy, its understanding and interpretation cause confusion even among graduates and scientists. Today we will try to explain what entropy is and present a relation to our daily lives. Let´s begin.E

Entropy is a concept introduced within the field of thermodynamics. We must state that classical thermodynamics describes a system (a portion of the universe that is isolated for its study) in terms of macroscopic variables such as volume, temperature, pressure and surprisingly entropy as well. Entropy may not be seen as easily as volume or pressure though.

The idea of irreversibility is key to understand entropy. We have certain notions of what reversible and irreversible processes are. For example if you pour cream into a cup of coffee you would see an irreversible process because the cream would never came out of the coffee. If you move a chair from one place to another you could think of a reversible process because you can just simply put the chair in its original place.

The truth is that in reality all processes are irreversible and the processes we see as reversible have a very small amount of entropy change. Entropy is an extensive property of a system, we can approach it by thinking of it as an amount of energy disorder and/or energy degradation. In the previous examples the energy of the systems is always being dispersed.

In classical thermodynamics we can only calculate entropy change (ΔS) of a system. To a system containing a sub-system which undergoes heat transfer to its surroundings (inside the system of interest). It is based on the macroscopic relationship between heat flow into the sub-system and the temperature at which it occurs summed over the boundary of that sub-system.

Following the formalism of Clausius, the first calculation can be mathematically stated as:

Where δS is the increase or decrease in entropy, δq is the heat added to the system or subtracted from it, and T is temperature. The equal sign indicates that the change is reversible, because Clausius shows a proportional relationship between entropy and the energy flow, in a system, the heat energy can be transformed into work, and work can be transformed into heat through a cyclical process. If the temperature is allowed to vary, the equation must be integrated over the temperature path.

The second law of thermodynamics states: for an irreversible process in an isolated system (a system not subject to outside influence), the thermodynamic state variable known as entropy is never decreasing. Therefore if we think of the universe as an isolated system, the entropy will always rise.

In the upcoming notes we will discuss another interesting approach to explain entropy, which is the microscopic approach and the third law of thermodynamics.