Objectives of thermodynamics and its equations of balance

Mass density, velocity, and temperature

During a process the mass density, the velocity, and the temperature of a fluid are, in general, not homogeneous in space, nor are they constant in time. Therefore mass density, velocity, and temperature are called time-dependent

fields

.

Fluid

mechanics

proposes to calculate the fields of mass density

ρ

(

x

i

,

t

), and velocity

w

j

(

x

i

,

t

) in a fluid.

Thermodynamics

proposes to calculate the fields of mass density

ρ

(

x

i

,

t

), velocity

w

j

(

x

i

,

t

), and temperature

T

(

x

i

,

t

) in the fluid. Therefore thermodynamics is more accurate than fluid mechanics: In addition to the motion of the fluid and its inertia, it takes into consideration how “hot” the fluid is.

On the interface between two bodies - the fluid and the wall of the container (say) - the temperature is continuous. This property defines temperature, it is the basis of all measurements of temperature by contact thermometers, and it is often referred to as the

Zeroth Law of Thermodynamics

.

Most thermometers rely on the thermal expansion of the thermometric substance, often mercury. In this book we shall usually employ the Celsius scale - or centigrade scale - but often also the absolute or Kelvin scale. Both scales use the same degree of temperature such that melting ice and boiling water at normal pressure differ by 100 degrees. The values of temperature of these fix points are 0°C and 100°C, or 273.15K and 373.15K, respectively.