Fundamentals of Fluid Mechanics - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 13

minus the change in the amount of energy lost by the system due to the work ( w)

done by the system on the surroundings ( Figure 2.5 ). Another relationship from this
law states that an isolated system can hold or lose energy, but the total energy of the
entire system remains constant. This law is dependent on the particular pathway in
which energy is added to the system and not just the difference between the beginning
and end points (i.e., for some energy changes, such as potential energy, it is typically
assumed that the objects path does not play a role in the energy change; only the
starting point and ending point matter). This law should be familiar from a beginning
thermodynamics course.

δ

δ U 5δ Q 2δ w ð 2 : 3 Þ

4. The Second Law of Thermodynamics: The entropy (S, disorder) of an isolated system will
increase over time, until the system reaches equilibrium ( Figure 2.6 ). In physics, this
law is used to determine the direction in which time moves (is time travel possible?)
and the total energy or efficiency of a system. The classic example of this law is that a
mug that falls off a table and breaks will not mend, unless external energy is put into
the system. The same would be true in biological systems; if a bone breaks it does not

FIGURE 2.5 Representation of

the first law of thermodynamics
where the internal energy of a
system is equal to the heat added
to the system from the surround-
ings minus the work that the sys-
tem conducts on its surroundings.
In this example, a piston is moved
up by the material within the sys-
tem as heat

Work to move piston (w)

Heat to (or from)
surroundings (Q)

U 1 U 2

is being lost to the

surroundings.

State 1 State 2

FIGURE 2.6

The second law of
thermodynamics states that the
disorder of a system will increase
in time. In this case, there is a
boundary between circular and
square objects. Once this boundary
has been removed, the system will
tend to move from an orderly sys-
tem to a disorderly system.

State 1 State 2