Modern Engineering Thermodynamics

4.12 Heat Modes of Energy Transport 127
4.11 THE STATE POSTULATE
To carry out a reversible work mode calculation using the formulae given earlier, we must know the exact behavior
of both the generalized force (an intensive property) and the generalized displacement (an extensive property)
for each work mode. Systems with multiple work modes have a variety of property values that must be
monitored during the work process to utilize the proper work mode formulae. Therefore, it seems reasonable to
expect that a simple relation exists between the number of work modes present in any given system and the
number of independent property values required to fix the state of that system. This is the purpose of the following
state postulate:
The number of independent intensive thermodynamic property values required to fix the state of a closed system
that is
1. Subject to the conditions of local equilibrium,
2. Exposed to n (nonchemical) work modes of energy transport, and
3. Composed of m pure substances is n + m.
Therefore, a pure substance (m = 1) subjected to only one work mode (n = 1) requires two (n + m = 2) independent
property values to fix its state. Such systems are called simple systems, and any two independent intensive
properties determine (or “fix”) its state.
The compression or expansion of a pure gas or vapor is a simple system. The work mode is moving system
boundary work, and any two independent intensive property values (p, v; p, T; v, T, etc.) fix its state. In fact, a
simple system occurs when each of the nonchemical reversible work modes just discussed is individually applied
to a pure substance. On the other hand, if two of them are simultaneously applied to a pure substance, then
n + m = 3 and three independent intensive property values are required to fix the state of the system.
4.12 HEAT MODES OF ENERGY TRANSPORT
We now introduce the three basic modes of heat transport of energy. Since a good heat mode analysis is somewhat
more complex than a work mode analysis and since its understanding is very important to a good engineering
education, most mechanical engineering curricula include a separate heat transfer course on this subject.
Consequently, this section is meant to be only an elementary introduction to this subject.
A system with no heat transfer is said to be adiabatic, and all well-insulated systems are considered to be adiabatic.
A process that occurs with no heat transport of energy is called an adiabatic process.
In the late 18th century, heat was thought to be a colorless, odorless, and weightless fluid, then called caloric. By
the middle of the 19th century, it had been determined that heat was in fact not a fluid but rather it represented
energy in transit. Unfortunately, many of the early heat-fluid technical terms survived and are still in use today.
This is why we speak of heat transfer and heat flow, as though heat were something physical, but it is not.
Because these conventions are so deeply ingrained in our technical culture, we use the phrases heat transfer, heat
transport, and the heat transport of energy interchangeably.
After it was determined that heat was not a fluid, late 19th century physicists defined heat transfer simply as
energy transport due to a temperature difference. In this framework, temperature was the only intensive property
driving force for the heat transport of energy.
Today, the simplest way to define heat transport of energy is as any energy transport that is neither a work mode
nor a mass flow energy transport mode. More precisely, modern nonequilibrium thermodynamics defines heat
transfer as just the transport of internal energy into or out of a system. With this definition, all other energy
transport modes are automatically either work or mass flow modes.
The basic heat transfer formulae were developed empirically and, unlike the previous work mode formulae, give
actual rather than reversible heat transport values. In fact, since heat transfer always occurs as a result of energy
WHAT DOES THE WORD ADIABATIC MEAN?
The term adiabatic was coined in 1859 by the Scottish engineer William John Macquorn Rankine (1820−1872). It comes
from the Greek word, αδιαβατοσ, meaning “not to pass through.” In thermodynamics, it means heat does not pass through
the system boundary, or simply that there is no heat transfer. Adiabatic is the analog of the word aergonic (meaning “no
work”) introduced earlier in this chapter.