The branch of physics where relation between heat and work done is discussed called thermodynamics. In thermodynamic process firstly we have to know about thermodynamic system. What is thermodynamic system? The system is a closed area where thermodynamic substance observed. Two types of system are i) close system and ii) open system.

photo by spectrose

For example if we consider a cylinder where some gases are closed in the cylinder. Inside the body of the cylinder is closed thermodynamic system. A candle light is the example of open thermodynamic system.

In thermodynamic system has a boundary called system boundary. For open thermodynamic system in candle light we feel heat within a certain circle from fire. This circle is system boundary for open thermodynamic system. For close system if we consider a cylinder then the body of the cylinder is the boundary of close thermodynamic system. The shape of system of thermodynamic can be any.

Pressure (P), temperature (T) and volume (V) are three important terms expresses the condition of a thermodynamic system. These three terms are known as thermodynamic co-ordinates.

**Internal energy:** every system has an energy which can convert to other type energy. This energy depends on the internal structure of the matter. It is called internal energy.

photo by http://chemed.chem.purdue.edu

Internal energy = kinetic energy of atom + potential energy

If we heat water for few minutes, it takes energy from heat and the internal energy of the atom increases. Increasing or decreasing of internal energy relates with temperature. A system temperature increases with increasing of internal energy and temperature decreases with decreasing of internal energy.

Internal energy = kinetic energy of atom + potential energy

If we heat water for few minutes, it takes energy from heat and the internal energy of the atom increases. Increasing or decreasing of internal energy relates with temperature. A system temperature increases with increasing of internal energy and temperature decreases with decreasing of internal energy.

Consider a cylinder contains with gas enclosed by its piston. If we heat the gas from external source, the particle of gas will gain energy from heat and its kinetic energy will high finally also achieve potential energy from displacement of gas particle. It can be said the internal energy will be high. For increasing internal energy work done will held and the temperature of the system will decrease because the internal energy will be spent for work done. Similarly if we compress the gas by pressure with the help of piston of the cylinder, the internal energy will be high and the system temperature will also high.

There are four thermodynamic processes viz. 1) Isothermal process

2) Adiabatic process

3) Isochoric process

4) Isobaric process

Isothermal and adiabatic processes are most important for thermodynamics.

**1) Isothermal process:** The process where volume changes due to change of pressure but temperature is fixed called isothermal process.

T = temperature fixed

P = pressure increases or decreases

V = volume increases or decreases

Mathematically, V ∝ 1/P

∆T=0,

∆Q≠0

Removing proportional sign we get, PV = K

where K is proportional constant.

The main fact is here if volume rise the internal energy will be spent for rising volume then temperature will fall in this system. How the temperature will same. We have to keep same temperature while rise or compression of volume. In that case an external source of heat is used to keep remain the temperature same. When the volume compresses from high volume to low then internal energy increases and temperature also. For keeping fixed temperature in that case the body of the cylinder must be good conductor and the pressure on the piston of the cylinder must be very low and it should be applied gradually as the increasing temperature can go easily through the conductor or cylinder body.

In this way temperature remains constant in isothermal process. Above picture shows the two states of volume but temperature is constant. The graph of isothermal process is curve obtained.

**2) Adiabatic process:** The process where volume, pressure and temperature changes but heat of system does not change is called adiabatic process. In this process system does not take any heat from outside and does not leave any heat from inside that means heat of the system remains constant.

Mathematical expression of adiabatic process is where

Now question is what specific heat at constant pressure and volume is.

To increase the temperature of 1 mole gas at constant pressure given heat is called specific heat at constant pressure.

To increase the temperature of 1 mole gas at constant volume given heat is called specific heat at constant volume.

The fact is how the heat of the system remains same? But volume of the system increases – decreases so that heat can be not be same, it is general logic. For keeping

Heat constant the cylinder body is constructed by insulator as any heat cannot go inside

Or outside. The image showing that volume, temperature, pressure changes but heat does not change for using insulator.

If we draw a graph for volume versus pressure for adiabatic process we get a curve shown in picture.

**3) Isochoric process:** The process where volume remains constant but pressure and temperature changes called isochoric process.

In this process temperature increases for heat and pressure rising.

But volume remains same or constant. The graph of volume vs pressure is a straight line parallel to Y axis.

**4) Isobaric process:** The process where pressure remains same but volume, temperature changes known as isobaric process.

If we consider a cylinder fill up with gas, for keeping same pressure we put some weight on piston as increasing – decreasing of volume can balance pressure.

For isobaric process volume versus pressure graph is a straight line parallel to X axis.