Every science has its own unique vocabulary associated with it. Precise definition of basic
concepts forms a sound foundation for development of a science and prevents possible
misunderstandings. Careful study of these concepts is essential for a good understanding
of topics in thermodynamics.

Thermodynamics and Energy

Thermodynamics can be defined as the study of energy, energy transformations and its
relation to matter. The analysis of thermal systems is achieved through the application of
the governing conservation equations, namely Conservation of Mass, Conservation of
Energy (1st law of thermodynamics), the 2nd law of thermodynamics and the property
relations. Energy can be viewed as the ability to cause changes.

First law of thermodynamics:
one of the most fundamental laws of nature is the
conservation of energy principle. It simply states that during an interaction, energy can
change from one form to another but the total amount of energy remains constant.

Second law of thermodynamics:
energy has quality as well as quantity, and actual
processes occur in the direction of decreasing quality of energy.
Whenever there is an interaction between energy and matter, thermodynamics is
involved. Some examples include heating and air‐conditioning systems, refrigerators,
water heaters, etc.

Dimensions and Units

Any physical quantity can be characterized by dimensions. The arbitrary magnitudes
assigned to the dimensions are called units. There are two types of dimensions, primary or
fundamental and secondary or derived dimensions.
Primary dimensions are: mass, m; length, L; time, t; temperature, T
Secondary dimensions are the ones that can be derived from primary dimensions such as:
velocity (m/s2), pressure (Pa = kg/m.s2).

There are two unit systems currently available SI (International System) and USCS (United
States Customary System) or English system. We, however, will use SI units exclusively in
this course. The SI units are based on decimal relationship between units.

Important note:
in engineering all equations must be dimensionally homogenous. This
means that every term in an equation must have the same units. It can be used as a sanity
check for your solution.

Any characteristic of a system is called a property. In classical thermodynamics, the
substance is assumed to be a continuum, homogenous matter with no microscopic holes.
This assumption holds as long as the volumes, and length scales are large with respect to
the intermolecular spacing.

Intensive properties:
Are those that are independent of the size (mass) of a system, such
as temperature, pressure, and density. They are not additive.

Extensive properties:
Values that are dependant on size of the system such as mass,
volume, and total energy U. They are additive.

Generally, uppercase letters are used to denote extensive properties (except mass
m), and lower case letters are used for intensive properties (except pressure P,
temperature T).

Extensive properties per unit mass are called specific properties, e.g. specific
volume (v=V/m).

State and Equilibrium

At a given state, all the properties of a system have fixed values. Thus, if the value of even
one property changes, the state will change to different one.

In an equilibrium state, there are no unbalanced potentials (or driving forces) within the
system. A system in equilibrium experiences no changes when it is isolated from its
surroundings.

Thermal equilibrium: when the temperature is the same throughout the entire
system.

Mechanical equilibrium: when there is no change in pressure at any point of the
system. However, the pressure may vary within the system due to gravitational
effects.

Phase equilibrium: in a two phase system, when the mass of each phase reaches
an equilibrium level.

Chemical equilibrium: when the chemical composition of a system does not
change with time, i.e., no chemical reactions occur.

Processes and Cycles

Any change a system undergoes from one equilibrium state to another is called a process,
and the series of states through which a system passes during a process is called a path.
Fig. 6: To specify a process, initial and final states and path must be specified.
Quasi‐equilibrium process: can be viewed as a sufficiently slow process that allows the
system to adjust itself internally and remains infinitesimally close to an equilibrium state
at all times. Quasi‐equilibrium process is an idealized process and is not a true
representation of the actual process. We model actual processes with quasi‐equilibrium
ones. Moreover, they serve as standards to which actual processes can be compared.
Process diagrams are used to visualize processes. Note that the process path indicates a
series of equilibrium states, and we are not able to specify the states for a non‐quasi‐
equilibrium process.

Prefix iso‐ is used to designate a process for which a particular property is constant.
Isothermal: is a process during which the temperature remains constant
Isobaric: is a process during which the pressure remains constant
Isometric: is process during which the specific volume remains constant.
A system is said to have undergone a cycle if it returns to its initial state at the end of the
process.