Thermodynamics
ENERGY, WORK, AND HEAT
Work is defined for mechanical systems as the action of a force on an object through a distance.
It equals the product of the force (F) times the displacement (d).
W = Fd
(1-15)
where:
W
=
work (ft-lbf)
F
=
force (lbf)
d
=
displacement (ft)
Example:
Determine the amount of work done if a force of 150 lbf is applied to an object until it
has moved a distance of 30 feet.
Solution:
Using Equation 1-15
W = Fd
W = (150 lbf)(30 ft)
W = 4500 ft-lbf
In dealing with work in relation to energy transfer systems, it is important to distinguish between
work done by the system on its surroundings and work done on the system by its surroundings.
Work is done by the system when it is used to turn a turbine and thereby generate electricity in
a turbine-generator. Work is done on the system when a pump is used to move the working fluid
from one location to another. A positive value for work indicates that work is done by the
system on its surroundings; a negative value indicates that work is done on the system by its
surroundings.
Heat
Heat, like work, is energy in transit. The transfer of energy as heat, however, occurs at the
molecular level as a result of a temperature difference. The symbol Q is used to denote heat.
In engineering applications, the unit of heat is the British thermal unit (Btu). Specifically, this
is called the 60 degree Btu because it is measured by a one degree temperature change from 59.5
to 60.5°F.
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