# Thermodynamics

## First law of thermodynamics

Heat energy given to a system is used in the following two ways:
1. In increasing the temperature and hence internal energy of the system.
2. In doing work by the system.
3. The mathematical statement to first law of thermodynamics is :
AQ = AU + AW

Where,
AQ - heat energy given to the system
AU - Increase in the internal energy of the system
AW - work done by the system.

» First law of thermodynamics is equivalent to principle of conservation of energy.

## Isothermal Process

If the changes are taking place in a system in such a way that temperature of the system remains constant throughout the change, then the process is said to be isothermal.

If the changes are taking place in a system in such a way that there is no exchange of heat energy between the system and the surrounding, then the process is said to be an adiabatic process.
If carbon dioxide is suddenly expanded, it is changed into dry ice. This is an example of adiabatic process.

## Second Law of Thermodynamics

The first law of thermodynamics guarantees that in a thermodynamic process, energy will be conserved. But this law does not tell whether a given process in which energy is conserved will take place or not. The second law of thermodynamics gives the answer.
Through this law can be stated in many forms, the following two forms are worth mentioning:
Kelvin's statement : Whole of the heat can never be converted into work.

Clausius statement: Heat by itself can not flow from a body at lower temperature to a body at higher temperature.

## Heat Engine

Heat engine is a device which converts heat energy into mechanical work continuously through a cyclic process.
Every heat engine basically consists of the three parts :
1. source (a hot body)
2. sink (a cold body)
3. a working substance

## Heat engine may be divided into two types :

1. Internal Combustion Engine : In this engine, heat is produced in the engine itself. Example : Otto engine or petrol engine (efficiency p. 52%), Diesel engine (efficiency = 64%)

2. External Combustion Engine : In this engine heat is produced outside the engine. Steam engine is an example of external combustion engine, (efficiency = 20%).

## Refrigerator or Heat Pump

A refrigerator is an apparatus which transfers heat energy from cold to a hot body at the expanse of energy supplied by an external agent. The working substance here is called refrigerant.
In actual refrigerator, vapours of freon (CCl2F2) acts as refrigerant.