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Thermodynamics can be defined as the science of energy. Although everybody has a
feeling of what energy is, it is difficult to give a precise definition for it.
Energy can be viewed as the ability to cause changes.
The name thermodynamics stems from the Greek words therme (heat) and dynamis
(power), which is most descriptive of the early efforts to convert heat into power. Today
the same name is broadly interpreted to include all aspects of energy and energy
transformations, including power generation, refrigeration, and relationships among the
properties of matter.
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. That is, energy cannot be created or
destroyed.
The change in the energy content of a body or any other system is equal to the
difference between the energy input and the energy output, and the energy balance is
expressed as Ein - Eout = DE.
Thermodynamics is a science and, more importantly, an engineering tool used to
describe processes that involve changes in temperature, transformation of energy, and
the relationships between heat and work. It can be regarded as a generalization of an
enormous body of empirical evidence. It is extremely general: there are no hypotheses
made concerning the structure and type of matter that we deal with. Thermodynamics is
used to describe the performance of propulsion systems, power generation systems,
and refrigerators, and to describe fluid flow, combustion, and many other phenomena.
As with all sciences, thermodynamics is concerned with the mathematical modeling of
the real world. In order that the mathematical deductions are consistent, we need some
precise definitions of the basic concepts.
The Continuum Model
Matter may be described at a molecular (or microscopic) level using the techniques of
statistical mechanics and kinetic theory. For engineering purposes, however, we want
“averaged” information, i.e., a macroscopic, not a microscopic, description. There are
two reasons for this:
First, a microscopic description of an engineering device may produce too much
information to manage.
Second, and more importantly, microscopic information is not useful for determining
how macroscopic systems will act or react unless, for instance, their total effect is
integrated.
The information we have about a continuum represents the microscopic information
averaged over a volume. Classical thermodynamics is concerned only with continua.
The Concept of a “System”
A thermodynamic system is a quantity of matter of fixed identity, around which we
can draw a boundary. The boundaries may be fixed or moveable. Work or heat can
be transferred across the system boundary.
Everything outside the boundary is the surroundings.
The Concept of “Equilibrium”
The state of a system in which properties have definite, unchanged values as long as
external conditions are unchanged is called an equilibrium state.
A system in thermodynamic equilibrium satisfies:
1. mechanical equilibrium (no unbalanced forces)
2. thermal equilibrium (no temperature differences)
3. chemical equilibrium.
The figures below demonstrate the use of thermodynamics coordinates to plot isolines,
lines along which a property is constant. They include constant temperature lines, or
isotherms, on a P-v diagram, constant volume lines, or isochors on a T-p diagram, and
constant pressure lines, or isobars, on a T-v diagram for an ideal gas. Real substances
may have phase changes (water to water vapor, or water to ice, for
example), which we can also plot on thermodynamic coordinates.
Equations of state
It is an experimental fact that two properties are needed to define the state of any
pure substance in equilibrium or undergoing a steady or quasi-steady process.
Zeroth Law of Thermodynamics
The Zeroth Law is based on observation. We start with two such observations:
1. If two bodies are in contact through a thermally-conducting boundary for a
sufficiently long time, no further observable changes take place; thermal equilibrium is
said to prevail.
2. Two systems which are individually in thermal equilibrium with a third are in thermal
equilibrium with each other; all three systems have the same value of the property
called temperature.
These closely connected ideas of temperature and thermal equilibrium are expressed
formally in the “Zeroth Law of Thermodynamics:”
Zeroth Law: There exists for every thermodynamic system in equilibrium a
property called temperature. Equality of temperature is a necessary and sufficient
condition for thermal equilibrium.
Work vs. Heat – which is which?
We can have one, the other, or both: it depends on what crosses the system boundary
(and thus, on how we define our system). For example consider a resistor that is heating
a volume of water
1. If the water is the system, then the state of the system will be changed by heat
transferred from the resistor.
2. If the system is the water and the resistor combined, then the state of the system will
be changed by electrical work.