Thermodynamics' basics.

• Main definitions
• Ideal Gas
• The three laws of thermodynamic
• Heat Engines

Thermodynamic :

1. Describes processes that involve changes in temperature, transformation of energy, relationships between heat and work.
2. It is a science, and more importantly an engineering tool, that is necessary for describing the performance of propulsion systems, power generation systems, refrigerators, fluid flow, combustion, ....

To talk about thermodynamics you need to know some basic values of this science.
The following values are called state variables:

Temperature defines the direction of spontaneous transfer of the heat between material bodies.
In the North America Farenheit scale is used for measure of the temperature. For thermodynamics calculations Kelvin scale is used. For zero in this scale the temperature of absolute zero is accepted. In Europe the Celsius scale is used widely.
Absolute temperature is the temperature measured in K.
You can recalculate temperature from one scale to another using following formula.
T [K] = t [°C] + 273.15

Density is ratio between mass and volume of some material.

Pressure p (lower case) is a measure of force per unit area.

The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m^-2 or kg·s^-2·m^-1). A convenient unit for atmospheric air pressure is the hectopascal (hPa), which gives the same number as the older millibar (mbar). Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (PSI) and the bar. In the United States air pressure is still measured in inches of mercury (as in the mercury barometer).

Pressure is sometimes measured not as an absolute pressure, but as the excess of that pressure above atmospheric pressure, sometimes called gauge pressure. An example of this is the air pressure in a tire of a car, which might be said to be "thirty PSI", but is actually thirty PSI above atmospheric pressure. In technical work, this would be written as "30 PSIG" or, more commonly, "30 psig".

The standard atmosphere (atm) is a curious unit of pressure, defined to six figures of precision to approximate a reality that varies constantly from place to place and moment to moment. It is approximately equal to typical air pressures at sea level and defined to be

1 atm = 101 325 Pa = 101.325 kPa = 1013.25 hPa. k = kilo and h = hecto.

Obsolete manometric units of pressure such as inches of water or millimeters of mercury are based on the pressure exerted by the weight of some "standard" fluid under some "standard" gravity. They are effectively attempts to define a unit for expressing the readings of a manometer.

Manometric pressure units should no longer be used for scientific or engineering purposes, due to the lack of repeatability inherent in their definitions.

Static pressure is the pressure due to the density and depth of a fluid. Stagnation pressure is the pressure due to the velocity of a fluid, and is defined to include static pressure. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid.