For any region with a boundary which is penetrated by mass, a thermodynamic
analysis always requires a distinction between mass carried across the boundary by bulk
stream flow and mass carried across by diffusion processes resulting from molecular action.
In the case of bulk stream flow with no diffusion mass transport, energy is carried
into the region in two distinct ways. Part of the energy added to a region receiving mass by
stream flow is the work of a pressure which displaces a quantity of flowing fluid into the
region. The remaining part of the energy added is the internal energy content of this
quantity of fluid which enters. In a bulk stream flow process these two parts of the total
energy transport can be separated and evaluated. This is done most conveniently by
defining the system in this case as a fixed mass enclosed by moveable boundaries which are
not penetrated by mass at all. In this manner a small contiguous quantity of fluid in an
entering conduit becomes a homogeneous sub-region within the system and its energy thus
becomes a part of the total internal energy of the entire system. The boundary of this subregion
is acted upon by an external pressure which performs work on the entire system in
moving the boundary of the sub-region. When the system is defined in this way, no energy
is carried into the system in the form of the internal energy of mass crossing its boundaries.
In a region receiving mass transported by a diffusion process, part of the energy content of all molecules outside the region is used to propel some of them into the region. In
contrast to the situation in a purely bulk stream flow process, there is no way in this case to
define a system which excludes the internal energy of transported molecules from the energy
crossing the system boundary. There is no way to define a system in which the propelling
forces which induce the mass transport are a driving force for all of the energy which crosses
the system boundary in the transport process. The diffusion processes these propelling
forces result from the behavior of individual molecules and are not scalar thermodynamic
properties at all so that we cannot define an intensive thermodynamic driving force property
to represent them.
analysis always requires a distinction between mass carried across the boundary by bulk
stream flow and mass carried across by diffusion processes resulting from molecular action.
In the case of bulk stream flow with no diffusion mass transport, energy is carried
into the region in two distinct ways. Part of the energy added to a region receiving mass by
stream flow is the work of a pressure which displaces a quantity of flowing fluid into the
region. The remaining part of the energy added is the internal energy content of this
quantity of fluid which enters. In a bulk stream flow process these two parts of the total
energy transport can be separated and evaluated. This is done most conveniently by
defining the system in this case as a fixed mass enclosed by moveable boundaries which are
not penetrated by mass at all. In this manner a small contiguous quantity of fluid in an
entering conduit becomes a homogeneous sub-region within the system and its energy thus
becomes a part of the total internal energy of the entire system. The boundary of this subregion
is acted upon by an external pressure which performs work on the entire system in
moving the boundary of the sub-region. When the system is defined in this way, no energy
is carried into the system in the form of the internal energy of mass crossing its boundaries.
In a region receiving mass transported by a diffusion process, part of the energy content of all molecules outside the region is used to propel some of them into the region. In
contrast to the situation in a purely bulk stream flow process, there is no way in this case to
define a system which excludes the internal energy of transported molecules from the energy
crossing the system boundary. There is no way to define a system in which the propelling
forces which induce the mass transport are a driving force for all of the energy which crosses
the system boundary in the transport process. The diffusion processes these propelling
forces result from the behavior of individual molecules and are not scalar thermodynamic
properties at all so that we cannot define an intensive thermodynamic driving force property
to represent them.
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