NEET Physics Notes Mechanics Thermodynamics Second Law of Thermodynamics
Second Law of Thermodynamics
Second Law of Thermodynamics
Two most common statements of second law of thermodynamics are given below
- Clausius Statement
It is impossible for a self-acting machine, working in a cyclic process to transfer heat from a colder body to a hotter body without ,the aid of an external agency. - Kelvin-Planck’s Statement
It is impossible to design an engine which extracts heat from a reservoir and fully converts it into work without producing any other effect.
Reversible and Irreversible Processes
A reversible process means, if a P process takes up the path AB, then on reversing the condition it comes back by BA. A thermal process however cannot be reversible. It could be reversible, if the change is extremely small.
In irreversible process one will not reach back to A, if the process AB has occurred.
Irreversibility is the rule of nature. No process in nature is itself irreversible.
Cyclic Process
In cyclic process, if the process takes the path AxB it returns via ByA, the initial and final points are same.
Carnot Engine and Its Efficiency
Carnot engine is a theoretical, ideal heat engine working in a reversible cyclic process operating between two temperatures T1 (heat source) and T2(heat sink). The Carnot’s cycle consists of two isothermal processes connected by two adiabatic processes as shown in the figure. The efficiency of a Carnot’s cycle is given by
The efficiency does not depend on the nature or quantity of the working’substance
Refrigerator
Refrigerator is a device which takes heat from a cold body, work is done on it and the work done together with the heat absorbed is rejected to the source. An ideal refrigerator can be regarded as Carnot’s ideal heat engine working in the reverse direction.
Coefficient of Performance of a Refrigerator
It is defined as the ratio of quantity of heat removed per cycle (Q2) to the work done on the working substance per cycle to remove this heat.
