The Laws of Thermodynamics are the widely applicable generalizations derived on the basis of studying energy ties and the interdependency between various properties of substances.

The "Zero" Law of Thermodynamics: the Law of Heat Equilibrium

J. Black (1728 – 1799) put forward the following wording of the Law:

"All bodies freely communicating with one another and not subject to a non-equilibrium impact of the ambient conditions acquire one the same temperature, as determined with the thermometer. All bodies acquire the temperature of the environment."</dd>

The First Law of Thermodynamics: the First Principle of Thermodynamics

This law is also known as the Energy Conservation and Transformation Law. G. Helmholtz (1847) presented it as follows:

"Energy is neither arise, nor destructed, but only gets transformed from one form to another."</dd>

In accordance with the First Law of Thermodynamics, the efficiency of any heat engine is conventionally defined as the ratio of the effective work output W_out to the heat produced Q_in by fuel consumption:

? = W_out/Q_in, (1).</dd>

The following formulation is also an acceptable restatement of this law:

It is impossible to create a perpetual motion machine of the first kind that is a device capable of producing work without consuming energy.</dd>

In the middle of the 20th century many of the text-books on physical chemistry proposed the following interpretations of the First Law:

• The internal energy of a system is a single-valued, continuous, and finite function of the system's state.
• The internal energy of an isolated system is constant.

Encyclopedia of Physics (1998) gives the following version:

The amount of heat Q absorbed by a body is, along with the work A performed on the body, a measure of change in its internal energy U. </dd>

The Second Law of Thermodynamics: the Second Principle of Thermodynamics

W. Thompson's interpretation (1854):

Heat of the coldest of the bodies taking part in the process cannot be a source of work.</dd>

R. Clausius's interpretation (1879):

The only result of any set of processes cannot be a transition of heat from a less heated body to a more heated one.</dd>

The S. Carnot - R. Clausius Theorem:

The efficiency of a heat engine reversibly operating by the Carnot cycle does not depend on the nature of the machine's working medium, but only depends on the temperatures of the heater and the cooler.</dd>

In accordance with the Second Law of Thermodynamics the Efficiency of Power Plant (?) is calculated through the temperature difference between the hot (T_hot ) and the cold ( T_cold) thermal reservoirs

? = (T_hot - T_cold)/T_hot = 1 - (T_hot/T_cold) , (2) </dd>

The work is determined in accordance with the First Law of Thermodynamics only from the initial and the final states of the system, and not from the way of transition from one state to the other. With this approach, the efficiency of a heat machine should be calculated in terms of the drop in heat content of the working medium in its hot and cold states, or, in Kamerlingh Onnes's terms proposed in 1909, through the enthalpy ( h) drop:

? = (h_hot - h_cold)/h_hot = 1 - (h_hot/h_cold), (3) </dd>

The heat content can be defined by

h = Cp x ?T , (4)</dd>

where Cp is heat capacity, also known as the specific heat.

When as the working medium an ideal monoatomic gas is used whose heat capacity is, as following from statistical physics,

Cp = (3/2) kT, (5.1)</dd>

where k is Boltzmann constant.

And its enthalpy in a quasi-equilibrium process with a temperature change of 10K is

h = (3/2) kT x 1 (5.2)</dd>

And then, after substituting Eq. (5.2) into Eq. (3), we obtain the very Eq. (2) for calculating the efficiency "after the S. Carnot - R. Clausius Theorem".

There also exist the following interpretations of the Second Law of Thermodynamics:

• It is impossible to create a perpetual motion machine of the second kind that is a device completely transforming heat energy into work.

• Unique result of any set of processes cannot be transformation of heat into work.

• Entropy of a closed system increases (more exactly, does not decreases) with time.

Encyclopedia of Physics (1998) gives the following version:

Thermodynamic processes are irreversible. At the contact of two bodies heat spontaneously passes from the more heating body to the less heating one. (See the Law of Heat Equilibrium.) To achieve a reverse transit, it is necessary to perform work. </dd>

The Third Law of Thermodynamics: the Third Principle of Thermodynamics

W. Nernst's theorem (1906):

Entropy of any system becomes zero at the absolute zero of temperature.</dd>

M. Planck's postulate (1911):

Entropy of an individual crystalline substance at the absolute zero equals zero.</dd>

Encyclopedia of Physics (1998):

Entropy of any equilibrium system tends to zero, if T ? 0.</dd>

Absolute zero of temperature is experimentally unattainable.</dd>