Pressure - Thermodynamics:
While working in the system, thermodynamic medium exerts a force on boundaries of vessel in which it is kept. The vessel may be a container, or an engine cylinder having a piston etc. The exerted force F per unit area (A) on a surface that is normal to the force is called as intensity of pressure or simply pressure p. Therefore
P = F/A= .g.h
It is expressed in Pascal (1 Pa = 1 Nm2), bar (1 bar = 105 Pa),
standard atmosphere (1 atm =1.0132 bar),or technical atmosphere (1 kg/cm2 or 1 atm).
1 atm means 1 atmospheric absolute.
The pressure is usually represented in following terms.
1. Atmospheric pressure
2. Gauge pressure
3. Vacuum pressure
4. Absolute pressure