Fundamental laws of Thermodynamics

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
; first law defines internal
; second law defines entropy
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
, 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.

Example of zeroth law
Fig. Example of zeroth law

The First law of

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
of first kind. 

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


Mathematical expression of first law
Mathematical expression of first

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

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
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

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

Mathematically, Kelvin-Planck statement can be written as:

Kelvin-Planck statement

Third law of thermodynamics:  

This law gives the definition of absolute value of
and also states that absolute zero cannot be
Another version of this law is that
“the entropy of perfect crystals is zero at absolute

Definitions of Entropy :

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

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

3. a measure of the disorder of a

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.