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;

energy

__second law defines entropy__

and the third law can be used to obtain absolute

entropy values.The above four thermodynamic laws

and the third law can be used to obtain absolute

entropy values.

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.

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

achieved.

**“the entropy of perfect crystals is zero at absolute**

zero”.

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.

unavailable to do work.

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

system.

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.