# Second Principle of Thermodynamics

Thermodynamics is the branch of physical science concerned with heat and its relation to other forms of energy and work.

It defines macroscopic variables (such as temperature, entropy, and pressure) that describe average properties of material bodies and radiation, and explains how they are related and by what laws they change with time. Thermodynamics does not describe the microscopic constituents of matter, and its laws can be derived from statistical mechanics.

Second law of thermodynamics: Heat cannot spontaneously flow from a colder location to a hotter location, because according R.J.E.Clausius: “Heat flows naturally from a hot object to a cold; it will never happen spontaneously from a cold to a warm object”.

The second law of thermodynamics is an expression of the universal principle of dissipation of kinetic and potential energy observable in nature. The second law is an observation of the fact that over time, differences in temperature, pressure, and chemical potential tend to even out in a physical system that is isolated from the outside world. Entropy is a measure of how much this process has progressed. The entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.

An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0

FORMULA SUMMARY:

Entropy in reversible processes

(S = Entropy ; Q = the amount of heat exchanged between the system and environment ; T = absolute temperature in kelvin ; The numbers 1 and 2 refer to initial and final states of a thermodynamic system) Bioprofe |Second principle of thermodynamics theory| 01

When there is no temperature variation (isothermal process):

(n = number of moles of substance ; L = Latent heat or molar heat for the change of state) Bioprofe |Second principle of thermodynamics theory| 02

When there is temperature variation (nonisothermal process):

(m = mass of the system ; c = specific heat) Bioprofe |Second principle of thermodynamics theory| 03

Isothermal expansion of an ideal gas Bioprofe |Second principle of thermodynamics theory| 04

R is the universal constant of ideal gases, which links together various thermodynamic state functions, establishing essentially a relationship between energy, temperature and the amount of matter. Bioprofe |Second principle of thermodynamics theory| 05

Performance of thermal machines

(Q1 = heat absorbed from the hot focus ; Q2 = heat transfered to the cold focus ; T1 = Temperature of the hot focus ; T2 = Temperature of the cold focus) Bioprofe |Second principle of thermodynamics theory| 06

Efficiency of refrigerating machines

(Q1 = heat transfered to the hot focus ; Q2 = heat absorbed from the cold focus ; T1 = Temperature of the hot focus ; T2 = Temperature of the cold focus) Bioprofe |Second principle of thermodynamics theory| 07