Environmental Engineering Reference
In-Depth Information
These are but a few specific examples of the many possible kinds of work interaction between a
thermodynamic system and its environment.
3.3.2
Heat Interaction
We are very familiar with the processes whereby substances are warmed or cooled. Cooking or
refrigerating food requires increasing or decreasing its temperature by bringing it into contact with
a warmer or cooler environment. A temperature difference between a system and its environment
is required for a heat interaction to transpire. If the environment undergoes a temperature increase
after a system warmer than the environment is brought into contact with it, then a heat interaction
has taken place. The incremental amount of the heat interaction
d
, which in this case equals the
energy transfer from the system to the environment, is equal to the product of the heat capacity
C
en
of the environment times its temperature increase
dT
en
. But by convention the energy transferred
to
a system in a heat interaction is regarded as a positive quantity so that in this case the energy
transfer is negative. Consequently,
Q
d
Q
=−
C
en
dT
en
(3.6)
We usually describe this interaction as
heat transfer,
although it is energy which is being exchanged
in a process solely involving a heat interaction.
Both heat and work quantities involved in an interaction of a system and its environment
are recognized by their effects in the environment, as described in equations (3.2)-(3.6) above.
Furthermore, both heat and work interactions may occur simultaneously, being distinguishable by
their different physical effects in the environment (e.g.,
F
en
dr
en
vs.
C
en
dT
en
).
3.4
THE FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics is an energy conservation principle. It relates the incremental
change in energy
dE
of a system with the increments of work
d
recognizable in the
environment during an interaction of the system with its environment. In words, it states that the
increment in system energy
dE
equals the increment in heat
d
W
and heat
d
Q
Q
transferred to the system minus
the work
d
W
done by the system on the environment,
dE
=
d
Q
−
d
W
(3.7)
It is an energy conservation principle in the sense that the sum of the system energy change
dE
,
the work
d
added to the environment is zero; that is, this sum is a conserved
quantity in any interaction with the environment.
Equation (3.7) expresses the first law in differential form. If many successive incremental
changes are added to accomplish a finite change in the system energy
E
from an initial state
i
to a
final state
f
, the first law may be expressed in integral form as
W
, and the heat
−
d
Q
f
f
E
f
−
E
i
=
d
Q
−
d
W
(3.8)
i
i
