Classical thermodynamics is based upon four empirical principles called zeroth, first, second and third laws of thermodynamics. These laws define thermodynamic properties, which are of great importance in understanding of thermodynamic principles.

__Zeroth law defines temperature__;__first law defines internal energy__;__second law defines entropy and the third law can be used to obtain absolute entropy values.__The above four thermodynamic laws are based on human observation of natural phenomena; they are not mathematically derived equations. Since no exceptions to these have been observed; these are accepted as laws.**Conservation of mass is a fundamental concept**, which states that mass is neither created nor destroyed.

**The Zeroth law of thermodynamics**states that when two systems are in thermal equilibrium with a third system, then they in turn are in thermal equilibrium with each other. This implies that some property must be same for the three systems. This property is temperature. Thus this law is the basis for temperature measurement. Equality of temperature is a necessary and sufficient condition for thermal equilibrium, i.e. no transfer of heat.

__The First law of thermodynamics__
It is a statement of law of conservation of energy. Also, according to this law, heat and work are interchangeable. Any system that violates the first law (i.e., creates or destroys energy) is known as a Perpetual Motion Machine (PMM) of first kind.

For a system undergoing a cyclic process, the first law of thermodynamics is given by:

Mathematical expression of first law |

**Second law of thermodynamics:**

The second law of thermodynamics is a limit law. It gives the upper limit of efficiency of a system. The second law also acknowledges that processes follow in a certain direction but not in the opposite direction. It also defines the important property called

**entropy**.
It is common sense that heat will not flow spontaneously from a body at lower temperature to a body at higher temperature. In order to transfer heat from lower temperature to higher temperature continuously (that is, to maintain the low temperature) a refrigeration system is needed which requires work input from external source. This is one of the principles of second law of thermodynamics, which is known as Clausius statement of the second law.

__Clausius’ statement of second law__**It is impossible to transfer heat in a cyclic process from low temperature to high temperature without work from external source.**

It is also a fact that all the energy supplied to a system as work can be dissipated as

**heat transfer**. On the other hand, all the energy supplied as heat transfer cannot be continuously converted into work giving a thermal efficiency of 100 percent. Only a part of heat transfer at high temperature in a cyclic process can be converted into work, the remaining part has to be rejected to surroundings at lower temperature.*If it were possible to obtain work continuously by heat transfer with a single heat source, then automobile will run by deriving energy from atmosphere at no cost.***A hypothetical machine that can achieve it is called Perpetual Motion Machine of second kind. This fact is embedded in Kelvin-Planck Statement of the Second law.**

__Kelvin-Planck statement of second law__**It is impossible to construct a device (engine) operating in a cycle that will produce no effect other than extraction of heat from a single reservoir and convert all of it into work.**

Mathematically, Kelvin-Planck statement can be written as:

**Third law of thermodynamics:**

This law gives the

**definition of absolute value of entropy**and also states that**absolute zero cannot be achieved.**Another version of this law is that**“the entropy of perfect crystals is zero at absolute zero”.**####
__Definitions of Entropy :__

**1. is a state variable whose change is defined for a reversible process at T where Q is the heat absorbed.**

**2. a measure of the amount of energy which is unavailable to do work.**

**3. a measure of the disorder of a system.**

For imperfect crystals however there is some entropy associated with configuration of molecules and atoms even when all motions cease, hence the entropy in this case does not tend to zero as T → 0, but it tends to a constant called the entropy of configuration.

The third law allows absolute entropy to be determined with zero entropy at absolute zero as the reference state. In refrigeration systems we deal with entropy changes only, the absolute entropy is not of much use. Therefore entropy may be taken to be zero or a constant at any suitably chosen reference state.

Another consequence of third law is that absolute zero cannot be achieved. One tries to approach absolute zero by magnetization to align the molecules. This is followed by cooling and then demagnetization, which extracts energy from the substance and reduces its temperature. It can be shown that this process will require infinite number of cycles to achieve absolute zero. In a later chapter it will be shown that infinitely large amount of work is required to maintain absolute zero if at all it can be achieved.

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