There are four laws of thermodynamics that describe basic principles of heat and work transfer that occur in thermodynamic systems. These laws were formulated during scientific research and are proven by experiments which may be repeated to confirm the statements postulated. Laws of thermodynamics are accepted as postulates to enable axiomatic architecture of thermodynamic knowledge only. The principal law of thermodynamics, which is also called zeroth law, postulates that a closed thermodynamic system will come to the thermal equilibrium regardless of initial conditions of components of that system.

When thermal equilibrium in a closed system will be established, temperatures of all its components will be identical. In thermal equilibrium, neither overall temperature of a closed system, nor temperature of any component of that system would change. Zeroth law of thermodynamics can also be expressed as following: “If A and B are both in thermal equilibrium with C, then A is also in thermal equilibrium with B”, or in equation: If T(A) = T(C); If T(B) = T(C); then T(A) = T(B).

First law of thermodynamics states that energy can be neither created nor destroyed, but can be transformed (changed from one form into another).

This law is a thermodynamic equivalent of more general law ow physics – the law of the conservation of energy. The first law of thermodynamics can be stated also as the following: “Changes in the internal energy of a system are equal to the algebraic sum of energy received through heating and work done by an external force. Amount of internal energy change is independent from either initial and final conditions of the system or from way that system changed. ” Mathematical statement of the first law of thermodynamics is: dU = ? Q + ? W, where dU stands for increase of internal energy of a system, ?

Q is energy received by a system through heating, and ? W is work done on the system by an external force. Another mathematical statement of the first law of thermodynamics is: dU = ? Q – ? W, where dU and ? Q have meanings similar to previous equation, and ? W stands for work done by the system. Positive ? Q means that system receives external heat, negative ? Q – that system produces heat. If amount of ? Q is equal to zero, the process is called isocaloric, or adiabatic (which means absence of heat transfer between system and external space, and between system components both).

It is important to consider forms which mathematic statement of the first law of thermodynamic took under certain conditions, when one of thermodynamic parameters is held constant while others change: Q = ? U + W – when the pressure is constant (isobaric process); Q = ? U = – when the volume is constant (isochoric or isovolumetric process); Q = W – when the temperature is constant (isothermal process). Second law of thermodynamics defines main principle of heat transfer between bodies and systems.

This law is a thermodynamic reflection of universal law of increasing entropy. Several formulations of this law exist. One of existing formulations – the well-known Clausius statement: Heat generally can not flow spontaneously from a material with lower temperature to a material with higher temperature. The main conclusion from this formulation of second law is that transfer of heat from cold material to material with higher temperature is possible only when work is applied. Another formulation is by Lord Kelvin. This one is known also as “heat engine formulation”:

In a cyclic process complete conversion of heat into work is impossible. Formulation of the second law that is referred directly to entropy is the following: Every process that occur in a system will tend to increase total entropy of the universe. This can be stated mathematically as: , where ? S is change of entropy over period of time ? t. The second law in any formulation expresses tendency of any closed physical system to even out all differences in parameters that characterize components of that system (temperature, pressure and density).

The third law of thermodynamics defines entropy and its characteristics under conditions of temperature reaching absolute zero. The third law can be stated as following: When temperature of a system is reaching absolute zero, all processes stop and total entropy of that system is reaching its minimum value. Mathematical statement of the third law is: , where x stands for any thermodynamic parameter. Since second law defines not entropy but only changes of entropy, third law allows to state that under the absolute zero conditions entropy reaches zero:

, therefore if T > 0, value of ? S also > 0. 2. Entropy of a system is a measure of a free energy in that system. This free energy can not be used by the system to perform external work. The higher is amount of entropy in a system, the less energy is available for external work and vise versa. Amount of entropy in a system depends on absolute temperature of that system and is directly proportional to the quantity of heat in a system. The higher is initial temperature of the system to which heat is added, the lower is increase of entropy in that system because of additional heat.