LECTURE 2
2.1 Intensive Properties

Three most important intensive properties in engineering thermodynamics
- Specific Volume
- Pressure
- Temperature

2.2 Continuum

Continuum Hypothesis - A substance is viewed to be a continuous, homogeneous matter. This idealization is valid as long as the length scale one deals with is large relative to the intermolecular spacings.

2.3 Density


V' is the smallest volume for which the matter can be considered a continuum and is normally small enough that it can be consider a point.

V' contains enough particles for statistical averages to be significant.

2.4 Mass



2.5 Specific Volume

Specific Volume - reciprocal of the density, (volume per unit mass)

2.6 Pressure


Pressure - the normal force exerted by a fluid per unit area

For a fluid at rest, pressure at a given point is the same in all directions. The pressure in a fluid increases with depth as a result of the weight of the fluid. The pressure varies in the vertical direction as a result of gravitational effects, but there is no variation in the horizontal direction. The pressure in a tank containing a gas may be considered to be uniform since the weight of the gas is too small to make a significant difference.

2.6.1 Absolute Pressure


- the actual pressure at a given position.
- it is measured relative to absolute vacuum; i.e., absolute zero pressure.

* Absolute pressure must be used in thermodynamic relations.

2.6.2 Gage Pressure

Readings from a pressure-measuring device

the pressure in the system > the local atmospheric pressure




2.6.3 Vacuum Pressure

the pressure in the system < the local atmospheric pressure




2.6.4 Manometer


Manometer - U-tube containing a fluid such as mercury, water, alcohol or oil. Since the gravitational effects of gases are negligible, the pressure anywhere in the tank and at position 1 has the same value. Furthermore, since pressure in a fluid does not vary in the horizontal direction within a fluid, the pressure at 2 is the same as the pressure at 1.

Force Balance:


2.6.5 Barometer

- a device used to measure the atmospheric pressure

- discovered by Torricelli (1608 - 1647)




2.6.6 Standard Atmosphere


- the pressure produced by a column of mercury 760 mm (29.92 in) in height at 0 under standard gravitational acceleration (g = 9.807 m/s2)

if a water column is used instead, it would require 10.3 m.