Enthalpy Entropy and Second Law of Thermodynamics
The objective is to develope the basics understanding of following concepts:
- Internal Energy and First Law of Thermodynamics
- Cyclic and arbitrary process of a system
- Reversibility and Irreversibility
- Entropy and Entahlpy
- Second Law of Thermodynamics
Internal Energy and First Law of ThermodynamicsWhen energy of a molecule with in a system is associated with the property of the system, then it is termed as Internal Energy(u). Energy neither be created nor be destroyed and based on this principal system internal energy(u) changes whenever energy-crosses the system boundary. Thus the first law of thermodynamics can be expressed as given below when heat/work interacts-with-the-system.
In the above equation u is the internal energy per-unit-mass and q and w are heat and work per unit mass respectively. The sign convention adopted in the above equation is: dq > 0 (considered as positive) ⇒ Heat transfer to the system dq < 0 (considered as negative) ⇒ Heat transfer from the system dw > 0 (considered as positive) ⇒ work done by the system dw < 0 (considered as negative) ⇒ work done on the system
Cyclic and Arbitary Process of a SystemOne of the important form of First law of thermodynamics is obtained when We integration above equation for a cyclic process. A system said to be in cyclic process, when after undergoing random changes on account of heat/work returns to its original state.
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Points to ponder are:
- Integration of any state property differential is the difference of its limits.
- Final state is same as the original state and there is no change in internal energy of the system.
Arbitrary Process of a SystemIt is outcome of First law of thermodynamics and is related with the equation (1) if a system involves arbitrary process. In this equation q and w are the net heat transferred and net work for the process respectively, while uf and ui are final and initial values of internal energy(u). In a rigid and isolated adiabatic system (w = 0, q = 0), then its internal energy(u) remains unchanged. Then from eq(2) of a cyclic process.
Reversibility and IrreversibilitySystem is said to be undergone a process when it initial state changes to final state. Properties like pressure, volume, enthalpy, temperature, entropy etc changes during a thermodynamic processes. Second law of thermodynamics categories the processes under two heads
- Ideal or reversible processes
- Natural or irreversible processes
If temperature (t) and pressure (p) variations are infinitesimal in a system, which is undergoing-a-process, then the process can be termed as near equilibrium states or approaching reversibility. The process is said to be reversible-internally if the original state is re-stored in reverse direction. The process is said to be externally-reversible environment accompanying the change can also be reversed-in-sequence. Reversible-process is one that is reversible both internally and externally. In order to measure the success of a real processes, professionals uses reversible process as the measure for comparing and bringing the real and actual processes closer to reversibility by lowering down losses in order to increase the efficiency of the processes.