What Is The Second Law Of Thermodynamics?
The second law of thermodynamics introduces a term called entropy. So, what is entropy? Entropy is the measure of the unavailable energy in a thermodynamically closed system. Alternatively, an easier way to understand entropy, it is considered as the measure of disorder or randomness of a system. Entropy is represented by the letter S.
The second law states the total entropy of an isolated system will not decrease over time and will remain constant only if all processes are reversible. For a process to be reversible, Sf = Si. Converting these words into an equation gives:
dS = delta Q / T
dS = Change in Entropy, Delta Q = Heat transferred to the system, T = Temperature
The second law also states, that if a process is irreversible then the joint entropy of the system and environment must increase as it instinctively develops towards thermodynamic equilibrium; the state with maximum entropy. As most processes in practice are irreversible, this is what we commonly see.
A process is irreversible when the entropy of the final state is greater than the entropy of the initial state, Sf > Si. The second law is entirely independent of the first law; hence it is not possible to infer one from the other. The first law centres around energy transfer, the second law centres around the direction energy travels.
Examples
For example, if we look at the heat transfer between two contacting bodies, one hot and one cold. If heat transfer took place such that the cold body transferred its heat to the hot body, making the hot body hotter and the cold body colder. This process abides by the first law because the energy isn’t being destroyed its merely being converted from one form to another.
In practice, this is not what we see, giving the need for the second law. We would see that the hot body transfers its heat to the cold body until the two are in equilibrium. If we remove the two bodies from each other, they will remain at the same temperature and not return to their initial states. This is an example of an irreversible process.
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