Thermodynamics deals with the relation of heat energy to mechanical work and other forms of energy. When we rub our palms they become warm. This is due to conversion of mechanical work performed in rubbing our hands together into heat. Thermodynamics deals with macroscopic properties and does not go into the microscopic level. For example the state of gas in thermodynamics is specified by macroscopic variables such as pressure, volume, temperature, mass and composition that are felt by our sense perceptions. The distinction between mechanics and thermodynamics is worth bearing in mind. In mechanics our interest is in the motion of particles or bodies under action of forces and torques. Thermodynamics is not concerned with motion of a system as a whole. It is concerned with the internal macroscopic state of a body.
e.g.: When a bullet is fired from a gun, it is embedded into the wooden plank and stops. Then kinetic energy of the bullet gets converted into heat, which causes change of temperature of bullet and surrounding layers of the plank. Temperature is related to the energy of internal motion (disorder) of the bullet but not the whole motion of bullet.

Thermal equilibrium: 
          The term "equilibrium" in thermodynamics appears in different context. According to this the state of a system is in equilibrium state if the macroscopic variables that characterise the system do not change in time. For example a gas inside a closed rigid container, completely insulated from its surroundings with fixed values of mass, pressure, volume, temperature and compositions that do not change with time, is in a state of "thermal equilibrium". A state of equilibrium depends on surroundings and nature of the wall that separates the system and surroundings. There are two types of walls 1) Adiabatic wall 2) Diathermic wall.
Adiabatic wall: It is an insulating wall that does not allow flow of energy from one to another system/ surroundings.
Diathermic wall: It is a conducting wall that allows energy flow from one to another until both the systems attain equilibrium states. After that there is no change in their states.   

 The concept of temperature in thermodynamics is explained by "Zeroth law of thermodynamics" which states that two systems in thermal equilibrium with a third system separately are in thermal equilibrium with each other".
            Let the systems A and B are separately in equilibrium with a system C. Then

TA   = TC and TB = TC. This implies TA = TB. i.e., A and B are said to be in thermal equilibrium.
Heat and temperature: Heat is a form of energy. When it is added to a body it increases the internal energy of the body and is responsible for the change in its thermal condition. It flows from a hot body to a cold body, till both attain same state of hotness or coldness.
Unit: Calorie in C.G.S., Joule in SI
D.F.: [ML2T-2]
First law of Thermodynamics:
When certain amount of heat is given to a system, a part of it is used to increase the internal energy and remaining part is used in doing external work. 

Where ∆Q is heat supplied to the system by the surroundings.
∆U is change in internal energy of the system.
∆W is workdone by the system on the surroundings.
This law is a particular form of law of conservation of energy.
* In case of an isolated system, there is no interaction with surroundings. No work is done by or on the system i.e., dW = 0, dQ = 0
  dU = 0  U = constant
i.e., internal energy of an isolated system is constant.
¤ When heat supplied is completely converted into work out changing the temperature of the system, the internal energy of the system remains constant.

 Sign convention:
*  If the heat energy dQ is added to the system, it is taken as positive.
* If heat is given out by the system, it is taken as negative.
*  If work is done by the system is taken as +ve.
* If work is done on the system is taken as -ve.
*  The increase in internal energy is taken as +ve.
*  The decrease in internal energy is taken as -ve.
Applications of  I law of  thermodynamics:
1) When a substance melts, the change in volume (dV) is very small, the temperature of a substance remains unchanged during melting process. Internal energy changes during the melting process. From I law of thermodynamics
dQ = dU + dW
dW = P(dV)  dQ = dU + PdV
*  If change in volume is neglected i.e., dV = 0 then dQ = dU

*  If volume increases, then dV is +ve then dQ = dU + P(+dV)

2) In case of boiling process:
    mL = (Us - Uw) + P(Vs - Vw)
    Us - Uw = mL - P(Vs - Vw)
    Where Us, Uw are internal energies of steam and water respectively.
    Vs, Vw are volumes of steam and water respectively.  

Thermodynamic Processess:
Quasi-Static Process: It is an infinitesimally slow process in which the system remains in thermal and mechanical equilibrium with surroundings at each and every intermediate stage.
              Consider a conducting cylinder filled with a given mass of gas and fitted with a frictionless piston. A few small weights are placed on the piston.
Due to the weight the piston moves down performing work on the gas. The volume decreases and pressure increases. After attaining the thermodynamic equilibrium with surroundings, the state of gas is defined by its Pressure P, Volume V and Temperature T. If small weights are removed one by one, the piston raised slowly. The gas expands performing external work on the piston. The volume increases and attains a new value for the volume and pressure in a short time and attains a new thermodynamical equilibrium with surroundings. If weights are removed infinitesimally small, the change in pressure, volume and temperature of the gas are infinitesimally small. These small changes make the differences ∆P, ∆V, ∆T between gas and surroundings. Thus any process taking place very slowly can be considered as a quasi-static process.

Isobaric Process: Let the gas be heated at constant pressure to the same increase in its temperature dT. It is utilised to increase the internal energy (dU) and to do external work (dW) in moving the piston through a small distance dx against constant pressure P. 
            Then (dQ)p = dU + dW
             Cp dT = dU + dW (for one mole)
             nCp dT = dU + dW (for 'n' moles)
P - V graph of this process is a straight line parallel to volume axis. 
            "The process in which a system undergoes a change in volume and temperature at constant pressure by the exchange of heat energy with the surroundings is called isobaric process".
Isochoric Process: Let the gas heated at constant volume, its temperature increases through 'dT'. At constant volume heat supplied is utilised only to increase the internal energy of the gas.
dQ)v = nCv (dT) = dU, dW = 0 

             P - V graph for this process is straight line parallel to pressure axis.
             "The process in which a system undergoes a change in pressure and temperature at constant volume by the exchange of heat energy with surroundings is called Isochoric process".
e.g.: The process of conversion of ice into water. Ice converts into water at 0°C by supplying heat. In the reverse process of conversion of water into ice at 0°C, heat is taken out of water at the same rate under normal pressure. (i.e., very slowly)

            "A process that cannot be retraced back in opposite direction is called an irreversible process". In this process, the system does not pass through the same intermediate states as in the direct process, even if the same initial state is reached.
e.g.: In the process of rubbing of our hands, heat is produced, increasing the temperature of the hands. The mechanical work done against friction is converted into heat. In the reverse process, we cannot make the hands to move in the same way on heating them.
Carnot Engine:
       Carnot engine is a reversible heat engine operating between two temperatures. The working substance in the Carnot's engine taken through a reversible cycle consisting of following steps.
(1) The cylinder containing an ideal gas is placed on the source and gas is allowed to expand slowly at constant temperature T1, absorbing heat Q1. This is ''Isothermal change" is represented by the curve AB in the indicator diagram.(2) The cylinder is then placed on the non conducting stand and gas is allowed to expand adiabatically till the temperature falls from T1 to T2. This is adiabatic expansion, is represented by BC in the indicator diagram.
(3) The cylinder is placed on the sink and the gas is compressed at constant temperature T2 and releases the heat Q2 to the sink, is represented by the curve CD in the diagram. This is isothermal compression.
(4) Finally the cylinder is placed on the non-conducting stand and compression is continued, so that gas return to its initial values (stage), is represented by the Curve DA in the diagram.

During isothermal compression of the gas from (P3, V3, T2) to (P4, V4, T2), heat released Q2 by the gas to the reservoir at temperature T2. Then workdone on the gas by the environment is