The 2nd Law of Thermodynamics

Thermal Energy Reservoir:

A hypothetical body which can absorb infinite amount of heat without undergoing any change in temperature having relatively large thermal heat capacity which pass into specific heat. 

For example: ocean, rIver,  atmospheric air, two phase system, furnace, etc.

Source:

It can supply heat energy to the system.

Sink:

It can absorb heat energy from the system.

Heat Engine

Device which concerts heat energy into mechanical work.

The work net is given by:

Net work = work input - work output 

Or 

Net work = Heat absorbed - Heat rejected 

The thermal efficiency of heat engine is given by:

The example of heat engine is Rankine cycle or steam power plant which we will discuss later.

Can we save Heat Rejected?

We cannot save heat rejected due to the following reasons:

1. Pump does not work on Gaseous phase.

2. If we replace sink by a source, then source will further heat the water and convert it into superheated Vapour.  

The 2nd Law of Thermodynamics:

There are two statements of 2nd low of thermodynamics which are as follows:

1. Kelvin- Planck Statement:

It states that;

It is impossible for any device that operates on a cycle that receives heat from a source and convert it completely into useful mechanical work.

2. Clausius Statement:

It states that;

It is impossible to cause heat to flow from a low temperature body to a higher one without the expenditure  of energy.

 Refrigerator:

Device which transfer heat from a low temperature medium to a high temperature one is called Refrigerator.

The Coefficient of performance ( COP ) is the ratio of Desired output to the required input which is given by:

Heat Pump:

Device which transfer heat from a low temperature medium to a high temperature one is called Heat Pump.

The Coefficient of performance ( COP ) is the ratio of Desired output to the required input which is given by:

Perpetual Motion Machine:

A device which violates the laws of thermodynamics is called Perpetual Motion Machine.

There are two types of Perpetual Motion Machine in thermodynamics which are:

1. Perpetual Motion Machine type-1:

Device which violates 1st law of thermodynamics is called Perpetual Motion Machine type-1 ( PMM 1 ).

2. Perpetual Motion Machine type-2:

Device which violates 2nd law of thermodynamics is called Perpetual Motion Machine type-2 ( PMM 2 ).

Reversible or Ideal Process:

Process that can be reversed without leaving any traces on the surrounding.

For example: frictionless pendulum, quasi-equilibrium process, etc.

Irreversible or Real Process:

Process that cannot be reversed.

For example: frictional losses, heat losses, leakages, etc.

Internally Reversible:

If no irreversibilities occur within the boundaries of a system.

For example: Quasi-equilibrium process ( no change in equilibrium ), Isothermal Process ( for a Phase Change Process ).

Externally Reversible:

If no irreversibilities occur outside the boundaries of a system.

Totally Reversible:

If no irreversibilities occur within and outside the boundaries of a system.

The Carnot Cycle:

The best known Reversible cycle having highest efficiency is called Carnot Cycle.

Carnot cycle consist of four Reversible process, two are Isothermal Process and two are adiabatic.

The Reversed Carnot Cycle:

SInce,  carnot heat engine cycle is a total Reversible cycle, so all the processes that it comprises can be reversed. In this case, it becomes Carnot Refrigeration Cycle. The cycle remains the same, however the direction of any hest and work interactions see reversed.

The Carnot Principle:

It states that:

1. The efficiencye of an irreversible heat engine is always less than  the efficiency of a revisible.

2. The efficiency of all Reversible heat engines operating between the two resources is always the same.

The Thermodynamics Temperature Scale:

A temperature scale that is independent of the properties of the substances that are used to measure temperature.

The thermal efficiency of reversible heat engine is independent of the working fluid and it's properties, rather it's the function of the reservoir's temperature only.

Similarly, all three engine's efficiency is given by:

So after canceling the similar ones, we get:

The Carnot Heat Engine:

The hypothetical heat engine that operates on a Reversible carnot cycle is called The Carnot Heat engine.

The thermal efficiency of any heat engine Reversible or irreversible is given by:

The efficiency of a carnot heat engine or any Reversible heat engine is given by:

Important relations to be noted:

The Carnot Refrigerator:

A refrigerator that operates on a reversed carnot cycle is called Carnot Refrigerator.

The COP of any refrigerator Reversible or irreversible is given by:

The COP for the Reversible refrigerator is:

Important relations to be noted:

The Carnot Heat Pump:

A heat pump that operates on a reversed carnot cycle is called Carnot Heat Pump.

The COP of any Heat pump Reversible or irreversible is given by:

The COP for the Reversible heat engine is:

Important relations to be noted:

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