Second law of Thermodynamics

Second law of Thermodynamics

There are two classical statements of the second law of thermodynamics

1) Kelvin – Planck statement

2) Clausius statement

Kelvin – Planck statement

“It is impossible to construct a device which will operate in a cycle & produce no effect other than the raising of a weight and the exchange of heat with a single reservoir”

i.e., it is impossible to construct an engine which will operate in a cycle will produce no effect other than the transfer of heat from a single thermal reservoir and the performance of an equivalent amount of work”.

No actual or ideal engine operating in cycles can convert into work all the heat supplied to the working substance, it must discharge some heat into a naturally accessible sink because of this aspect and the second law is often referred as the law of degradation of energy.

The statement implies that it is impossible to construct a heat engine that working in a cyclic process can absorb an amount of heat from a high temperature reservoir and can do an equivalent amount of work.  In other words it is not possible to construct a heat engine having thermal efficiency of 100 percent.

Clausius Statement 

It is impossible to construct a heat pump which operating in a cycle will produce no effect other than the transfer of heat from a low temperature thermal reservoir to a higher temperature thermal reservoir.

That is in order to transfer heat from a low temperature thermal reservoir to a high temperature thermal reservoir work must be done on the system by the surroundings.

Although the Kelvin – Planck and Clausius statements appear to be different, they are really equivalent in the sense that a violation of one statement involves violation of the other.

Although the Kelvin – Planck and Clausius statements appear to be different, they are really equivalent in the sense that a violation of one statement involves violation of the other.

Thermodynamic System

A Thermodynamic system is defined as a quantity of matter or a region in space upon which attention is concentrated in the analysis of a problem. Everything external to the system is called the surrounding or environment. The system is separated from the surrounding by the system boundary. Boundary may be either fixed or moving.  A system and its surrounding together comprise a universe.

Open System: The open system is one in which matter crosses the boundary of the system. There may be energy transfer also. Most of the engineering devices are generally open systems.

 Ex: An air compressor in which air enters at low pressure and leave at high pressure and there is energy transfer across the system boundary.

Closed System: A closed system is a system of fixed mass. There is no mass transfer across the system boundary.  

Ex: A certain quantity of fluid in a cylinder bounded by a piston constitutes a closed system.

Isolated System:  The isolated system is one in which there is no interaction between the system and surrounding. It is of the fixed mass and energy and there is no mass or energy transfer across the system boundary.

Homogeneous and Heterogeneous system:

A quantity of matter homogeneous throughout in chemical composition and physical structure is called a phase. Every substance can exist in any one of the three phases viz. Solid, Liquid or  gas. 

A system consisting of a single phase is called a homogeneous system while a system consisting of more than one phase is known as a heterogeneous system.

Thermodynamics

Thermodynamics deals with heat inter-action and work inter-action with the substances called systems. Work and heat are forms of energy. Transfer of heat or work to a substance brings about certain changes in the substance and whatever change happens is called a process. Thermo means heat. Since work is also a form of energy, thermo is taken to mean heat and work. Dynamics refers to the changes that occur as a result of heat or work transfer.

Biological systems are capable doing work. For example, micro-organism is capable swimming in the body fluid of its host. It needs to do the work. Where does energy for doing this work come from? It is the metabolic activity that converts some form of energy (Nutrition that it takes form host is a form of chemical energy) into work. It is important then to understand how this happens so that we can exploit this to our engineering benefit.

In thermodynamics we have work transfer, heat transfer and then we have a system for interaction which undergoes a process.

Reversibility and Irreversibility

If 100% efficiency is unattainable, what is the max possible efficiency which can be attained and what factors promote the attainment of this max value? In trying to answer these questions, thermodynamics has invented & used the concept of reversibility, absolute temperature and entropy.

Reversible Process:

-for a system is defined as a process which once having taken place, can be reversed and leaves no change in either the system or surroundings. Only ideal processes can do this and restore both system and surroundings to their initial states. Hence an ideal process must be a reversible process.

No real process is truly reversible but some processes may approach reversibility, to a close approximation.

Example:

1) Frictionless relative motion

2) Extension and compression of a spring

3) Frictionless adiabatic expansion or compression of fluid.

4) Polytropic expansion or compression etc.,

The conditions for a process to be reversible may be given as follows:

i) There should be no friction

ii) There should be no heat transfer across finite temperature difference.

iii) Both the system and surrounding be stored to original state after the process is reversed.

Any process which is not reversible is irreversible.

Example: Movement of solids with friction, A flow of viscous fluid in pipes and passages mixing of two different substances, A combustion process.

Every quasistatic process is reversible, because a quasistatic process is of an infinite succession of equilibrium states.