# THERMODYNAMICS:

Heat flow is thermodynamics, it is characterized as the analysis of the energy change or transformation that follows the physical and chemical change in matter.

#### Thermal Equilibrium:

Two physical systems are in thermal equilibrium if, when bound by path of permeable membrane, there is no heat flow between them.

### Laws of Thermodynamics:

#### Zeroth Law of thermodynamics:

It may be succinctly stated as ”if two thermodynamics system A and B are in thermal equilibrium and B and C are also in thermal equilibrium then A and C are also in thermal equilibrium.

A system in thermal equilibrium is a system whose properties(like pressure,temperature,volume etc)are not changing with time.
In other words, this law can be stated as”when two objects are separately in thermodynamic equilibrium with a third object they are also in equilibrium with each other. Objects in the equilibrium have same temperature.
##### Example:
A hot cup of coffee in your kitchen is not in thermal equilibrium with its surroundings, because it is cooling down,its temperature is dropping. Once its temperature stops decreasing,it will be at the room temperature, and it will in thermal equilibrium with its surroundings.

#### The First Law of Thermodynamics:

The first law of thermodynamics is in fact law of conservation of energy. This law stated that energy cannot be created or destroyed, although it may change from one form to another.
It can also be stated as the total energy of the system and its surroundings is conserved.
##### Example:
Suppose we have a ball and we throw its upward in straight line. Due to gravity of earth its velocity will be slow and a point occur when its velocity will be zero. Then the ball falls on ground with same velocity that we use to throwing upward.
Movement of energy is KE that use to move ball upward at the point when KE is zero its convert into potential energy.The Potential energy is converted into Kinetic energy as the ball moves down on earth.
Some kinetic energy are used in air molecule resistance during moving ball upward and downward. So its clear that the energy transfer from one form to other its all according to first law of thermodynamics.

Q =ΔE +W
heat=internal                      energy+work
ΔE=Q  –  W
(-ve work done by the system  on the surrounding
+ve means that work done on the system by the surrounding) .

We have two system with a gas. When  we heat  them gas will be expand and piston move upward its mean change occur in the system. Its a work done by the system.

W  =  -PΔV
ΔE=Q  –  W
ΔE=q-(-pΔV )
=q  +  w
if the volume is constant then Work done will be zero.   (w=0).
ΔE=  Q