Work

Now that we have used the term "work" it is necessary to emphasize that work, like
heat, must also be regarded only as a type of energy in transition across a well defined, zero
thickness, boundary of a system. Consequently work, like heat, is never a property or any
quantity contained within a system. Whereas heat is energy driven across this boundary by
a difference in temperature, work is energy driven across by differences in other driving
forces on either side of it. Various kinds of work are identified by the kind of driving force
involved and the characteristic extensive property change which accompanied it.
Work is measured quantitatively in much the same manner as heat. Any driving
force other than temperature, located outside the system on its external boundary, is
multiplied by a transported extensive property change within the system which was
transferred across the system boundary in response to this force. The result is the numerical
value of the work associated with this system and driving force. It is important to
emphasize that the extensive property change within the system which is used in this
computation must be a transported quantity whose transfer across the system boundary
depends on a particular driving force with different values inside and outside the system.
This transported extensive property change within the system always occurs with the same
magnitude but with opposite sign in the surroundings.
Neither work nor heat results from any part of a change in an extensive property of a
system which has not been transported in this manner without alteration in magnitude
across the system boundary. A non-transported extensive property change within a system
when multiplied by an appropriate driving force property located within the system measures a form of internal energy change in the system but not work or heat.
Conventionally the quantity of work calculated by this procedure is given a positive
sign when work is done by the system on the surroundings and energy crosses the boundary
in a direction from the system to the surroundings. An energy transport in the opposite
direction, when work is done by the surroundings on the system, is given a negative sign.
It is awkward that the sign given to energy transferred as work is opposite to that given to
energy transferred as heat in the same direction, but tradition has established the convention
and it is important that it be followed consistently. Like heat, both the absolute value and the
sign of what is called work depend entirely on how the system is specified.
Several thermodynamic driving forces and their characteristic displacements are
listed in Table I. Any of these properties, other than temperature and entropy, can measure
various types of work when the driving force is located on the outer side of the system
boundary and the displacement is a transported quantity whose change is located within the
system. The product, when given the proper sign, is a type of work transfer for this system.