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Thermodynamics: Entropy

What is Entropy

Entropy is the measurement of molecular disorder within a system. As the temperature of the system increases the entropy will increase, and as it cools the entropy will decrease. This means that a solid will have lower entropy then a gas would. Due this fact the third law of thermodynamics can be defined. "Entropy of a pure crystalline substance at absolute zero temperature is zero."

When people think of thermodynamics, entropy does not normally come to mind. However, entropy is a very important property to consider, since it is the opposite of work. Work can be defined as organized energy, with a known direction and quantity. Entropy on the other hand is chaos, and even though individual particle can have a predefined quantity and direction of energy, this will be canceled out by the randomness of other particles within the system. For example, wind blowing through a windmill is organized energy creating work. On the other hand air inside the room you are sitting in has energetic particles colliding into you right now. However, they are colliding into you at all directions in a chaotic manner, so that their energy is canceled out and you don't notice their presents, which is entropy.

During a thermodynamic process the total entropy of the system can either remain constant, or it will increase. If the process is a reversible process then the entropy of the system will remain constant. However, the entropy will increase if the process is irreversible. Due to this fact the entropy change cannot be negative without violating the second law of thermodynamics. This is a major concern for engineers, because an increase in entropy can prematurely end chemical reactions and other thermodynamic processes. Due to this fact it is speculated that as the entropy of the universe continues to increase it will eventually reach a point that all thermodynamic processes will stop.

Entropy can also sometimes be referred to as the Clausius Inequality, which was first stated by German physicist R. J. E. Clausius and is represented by the equation below.

Clausius  Inequality (1)

Notice that the cyclic integral above states that the change in heat over temperature cannot be greater than zero. If it equals zero then the cycle is reversible, and there is no change in the heat energy being released. However, for an irreversible process the irreversibilities will cause an increase in heat energy causing the change in heat to be negative. This means that if the overall entropy increases then more energy will be converted to heat that cannot be recovered instead of being successfully used as work.

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