**Thermodynamics :** The branch of science which deals with conversion of heat into other forms of energy or vice-versa .

**Thermodynamic Process :** If the state of a system changes in such a way that any of P , V , T etc . changes , the process is called Thermodynamic Process .

### Zeroth Law of Thermodynamics:

If two bodies A and B are in thermal equilibrium and A and C are also in thermal equilibrium, then B and C are also in thermal equilibrium.

### Prevost Theory of Exchanges:

According to this theory, every body is continuously emitting radiant energy in all directions at a rate depending only on the nature of its surface and its temperature and it is absorbing radiant energy from all surrounding bodies at a rate depending on its surface and the temperature of the surrounding bodies.

### Mechanical Equivalent of Heat:

Whenever mechanical work is transformed into heat or heat into mechanical work, there is a constant ratio between the work and the amount of heat. This ratio is called “mechanical equivalent of heat” and is denoted by J .

Thus, if W be the amount of work done and Q the amount of heat produced, we have

W/Q = J ,

⇒ W = JQ

If Q = 1 unit then J = W . Therefore , J is numerically equal to the mechanical work required to produce one unit of heat.

**Work done by the Gas : **

Small amount of work done dW in moving a piston by small distance dx is

dW = F dx

dW = P. A . dx

dW = P. dV

$\displaystyle W = \int_{V_1}^{V_2}P .dV $

#### First law of Thermodynamics :

It is the consequence of conservation of energy for gaseous system.

Heat supplied to the gas = Increase in internal energy + work done by the gas.

Q = ΔU + W

In differential form dQ = dU + dW

Q = + ve ⇒ Heat is supplied to the gas

Q = – ve ⇒ Heat is taken out from the gas

dQ = n C dT ; Where C = molar specific heat

C = C_{p} (constant pressure); C = C_{v} =(constant volume)