Second Law of Thermodynamics

What is Second Law of Thermodynamics?

· First law of thermodynamics states the equivalence of heat and energy. It does not state anything about the limitation in the conversion of heat into work or about the condition necessary for such conversion.

· Second law of thermodynamics is generalization of certain experience and observation and is concerned with tine direction in which energy flow takes place.

· This law can be stated in number of ways. Although differently said, they are essentially equivalent.

(i) Kelvin Plank Statement:

"It is impossible to construct a device which, operating in a cycle, has a sole effect of extracting heat from a reservoir and performing an equivalent amount of work".

(ii) Clasius Statement:

"It is impossible for a self acting machine, unaided by internal agency, to transfer heat from a colder body to a hotter body".

· It can be proved that these two statements of second law are completely equivalent and violation of Kelvin Plank statement leads to violation of Clasius statement and vice-versa.

Second Law of Thermodynamics

Significance of second law of thermodynamics:

· The second law puts a limits to the efficiency of heat engine and the co efficient of performance of a refrigerator.

· According to second law,

· The efficiency of a heat engine can never be unity which implies that the heat released to the cold reservoir can never be made zero.

· The co efficient of performance of a refrigerator can never be infinite which implies that the external work (W) can never be made zero.

Limitations of second law of thermodynamics:

· The second law of thermodynamics cannot be proven directly. But its validity has not been contradicted by any machine designed so far.

· It is applicable to a cyclic process in which the system returns to its original state after a complete cycle of changes.

· It makes no predictions as to what will happen under certain conditions but simply states what will happen under a given set of conditions.

Reversible and irreversible process:

What is a Reversible Process?

The process in which the system and surroundings can be restored to the initial state from the final state without producing any changes in the thermodynamics properties of the universe is called a reversible process. In the figure below, let us suppose that the system has undergone a change from state A to state B. If the system can be restored from state B to state A, and there is no change in the universe, then the process is said to be a reversible process. The reversible process can be reversed completely and there is no trace left to show that the system had undergone thermodynamic change undergo reversible change, it should occur infinitely slowly due to infinitesimal gradient. During reversible process all the changes in state that occur in the system are in thermodynamic equilibrium with each other.

Thus there are two important conditions for the reversible process to occur. Firstly, the process should occur in infinitesimally small time and secondly all of the initial and final state of the system should be in equilibrium with each other.

If during the reversible process the heat content of the system remains constant, i.e. it is adiabatic process, then the process is also isentropic process, i.e. the entropy of the system remains constant.

The phenomenon of undergoing reversible change is also called reversibility. In actual practice the reversible process never occurs, thus it is an ideal or hypothetical process.

Conditions for Reversible Process:

There are two important conditions for the reversible process to occur.

· Firstly, the process should occur in infinitesimally small time.

· Secondly, all the initial and final state should be in the equilibrium with each other.

Reversibility in Thermodynamics:

The phenomena of undergoing reversible change is also called reversibility. In actual practice the reversible process never occurs, thus it is an ideal or hypothetical process.

Reversible Process Example:

Although no actual change is completely reversible by the process of liquification and evaporation of a system performed slowly are practically reversible. Similarly slow compression of the gas in a cylinder is reversible process as gas can be expand slowly by decreasing the weight on the piston to reverse the operation.

What is an Irreversible Process?

The irreversible process is also called the natural process because all the processes occurring in nature are irreversible processes. The natural process occurs due to the finite gradient between the two states of the system. For instance, heat flow between two bodies occurs due to the temperature gradient between the two bodies; this is in fact the natural flow of heat. Similarly, water flows from high level to low level, current moves from high potential to low potential, etc.

Here are some important points about the irreversible process:

1) In the irreversible process the initial state of the system and surroundings cannot be restored from the final state.

2) During the irreversible process the various states of the system on the path of change from initial state to final state are not in equilibrium with each other.

3) During the irreversible process the entropy of the system increases decisively and it cannot be reduced back to its initial value.

4) The phenomenon of a system undergoing irreversible process is called as irreversibility.

Irreversible process examples:

· The conduction of heat from a hot body to a cold body.

· Production of heat by the friction

· Producing of heat by the passing of current through an electrical resistance

· Transfer of heat by radiation

· An explosion

· Inelastic deformation

· Magnetization or polarization with a hysteresis

· Spontaneous chemical reactions

· Spontaneous mixing of matter of varying states

Cyclic and Non-cyclic Process:

A cyclic process consists of a series of changes which return the system back to its initial state.

In non-cyclic process the series of changes involved do not return the system back to its initial state.

(1) In cyclic process change in internal energy is zero and temperature of system remains constant.

(2) Heat supplied is equal to the work done by the system.

(3) For cyclic process P–V graph is a closed curve and area enclosed by the closed path represents the work done.

If the cycle is clockwise work done is positive and if the cycle is anticlockwise work done is negative.

Graphical Representation of Various Processes

Heat engine is a device which converts heat into work continuously througha cyclic process.

The essential parts of a heat engine are :

Source : Working substance : Steam, petrol etc.

Sink : ‘‘efficiency’’ η is given by

A perfect heat engine η = 1. Practically efficiency is always less than 1.