Energy is a fundamental concept of thermodynamics and one of the most significant aspects of engineering
analysis. Energy can be stored within systems in various macroscopic forms: kinetic energy,
gravitational potential energy, and internal energy. Energy can also be transformed from one form to
another and transferred between systems. For closed systems, energy can be transferred by work and
heat transfer. The total amount of energy is conserved in all transformations and transfers.
Work
In thermodynamics, the term work denotes a means for transferring energy. Work is an effect of one
system on another that is identified and measured as follows: work is done by a system on its surroundings
if the sole effect on everything external to the system could have been the raising of a weight. The test
of whether a work interaction has taken place is not that the elevation of a weight is actually changed,
nor that a force actually acted through a distance, but that the sole effect could be the change in elevation
of a mass. The magnitude of the work is measured by the number of standard weights that could have
been raised. Since the raising of a weight is in effect a force acting through a distance, the work concept
of mechanics is preserved. This definition includes work effects such as is associated with rotating shafts,
displacement of the boundary, and the flow of electricity.
Work done by a system is considered positive: W > 0. Work done on a system is considered negative:
W < 0. The time rate of doing work, or power, is symbolized by and adheres to the same sign
convention.
Energy
A closed system undergoing a process that involves only work interactions with its surroundings
experiences an adiabatic process. On the basis of experimental evidence, it can be postulated that when a closed system is altered adiabatically, the amount of work is fixed by the end states of the system and
is independent of the details of the process. This postulate, which is one way the first law of thermodynamics
can be stated, can be made regardless of the type of work interaction involved, the type of
process, or the nature of the system.
As the work in an adiabatic process of a closed system is fixed by the end states, an extensive property
called energy can be defined for the system such that its change between two states is the work in an
adiabatic process that has these as the end states. In engineering thermodynamics the change in the
energy of a system is considered to be made up of three macroscopic contributions: the change in kinetic
energy, KE, associated with the motion of the system as a whole relative to an external coordinate frame,
the change in gravitational potential energy, PE, associated with the position of the system as a whole
in the Earth’s gravitational field, and the change in internal energy, U, which accounts for all other
energy associated with the system. Like kinetic energy and gravitational potential energy, internal energy
is an extensive property.
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