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Second Law of Thermodynamics


The laws of thermodynamics summarize the properties of energy and its transformation from one form to another. Thermodynamics, like much of the rest science, takes terms with an everyday meaning and sharpens them.

The second law of thermodynamics is one of the most fundamental rules of physics. While the first law clarifies the nature of energy, the second places limits on the direction of energy transfers. The figure depicts the second law of thermodynamics.
Second Law of Thermodynamics

Second Law of Thermodynamics Definition

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Second law of Thermodynamics defined as "It is impossible to get a continuous supply of work from a body which can transfer heat with a single heat reservoir".
This is a negative statement. According to this statement, a single reservoir at a single temperature cannot continuously transfer heat in to work. It means that there should be two reservoirs for any heat engine. One reservoir (called the source) is taken at a lower higher temperature and the other reservoir (called the sink) is taken at a lower temperature.

Second Law of Thermodynamics Equation

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According to the concept of second law of thermodynamics, internal energy of a given system has a tendency to diffuse to a larger area. If a reaction is said to be spontaneous, the entropy change is positive. This entropy helps to dissipate the large amount of energy to the surroundings, this energy is known as activation energy.

The mathematical expression of second law is given below :
$\Delta S$ = $\frac{(\Delta H - \Delta G)}{T}$
S is the entropy
H is the enthalpy
G is the Gibbs free energy

What is Entropy?

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Entropy indicates the disoderness of a system. In the case of an isolated system, entropy should not decrease. This is because isolated system suddenly attains the thermodynamic equilibrium, equilibrium state is the maximum entropy state.

Second Law of Thermodynamics Examples

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Some of the examples are given below:
  • Consider a hot and a cold body. If they are in contact with each other, the heat will flow from hot body to cold body. The reverse process will not exists.
  • Operation of a refrigerator. Here external reservoir is used for the operation.