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# Summary of week 3

This week, you learned the second law of thermodynamics. The second law is the entropy principle.

The second law is concerned with the conversion of heat into work and the quality of energy. The quality of energy indicates the amount of useful energy that can do work among the conserved total energy. There are various statements expressing the second law. One is involved with the entropy of the isolated system. The other is for the equilibrium. The last statement present the criteria for reversibility.

Entropy is one of the thermodynamic functions that describe the condition of the system. Entropy is related to degree of irreversibility, decrease in useful energy and degree of randomness.

As a traditional interpretation of the second law, the highest efficiency of the heat engine, refrigerator and heat pump were calculated. The highest efficiency could be estimated by combining the first and the second law of thermodynamics.

Next week, you will consider the application of the second law to real problems. To calculate entropy, the third law of thermodynamics will be discussed. You will look at the mathematical formulation of the dependence of entropy on various parameters, such as temperature at constant pressure, volume or volume at constant temperature.

The actual numerical calculation will demonstrate the role of entropy in determining reversibility of the process.