Environmental Engineering Reference
In-Depth Information
Note as well that based on the devaluation reaction, the so-called standard entropy
σ
n
of the devaluation reaction can be determined. For example, again for the photosyn-
thesis reaction, based on Equation (2.2.19), the standard entropy of the devaluation
reaction,
σ
n
,
SU
, is:
σ
n
,
SU
=
6(
s
H
2
O
+
s
CO
2
)
n
−
6(
s
O
2
)
n
−
s
n
,
SU
(2.2.22)
where
s
H
2
O
,
s
CO
2
, and
s
O
2
are the absolute standard entropies of the respective gases.
The stoichiometric factor 6 results from Equation (2.2.19).
2.2.4 Exergy of photon gas
Radiation has two meanings: it could be the process of a radiating body or it could
be the product of this process. This process can be considered twofold - either as
the propagating magnetic field or as the population of photons traveling in space.
Then the photon populations can be imagined as the photons trapped in a system
of limited space, or as the freely traveling flux of photons. The states of trapped or
traveling photons are similar to the substance concepts of thermodynamic functions of
state which are respectively the internal energy or enthalpy. The traveling photons in a
form, e.g. of emitted radiation from a substance body (emission), or as a bundle of rays
from many bodies of different temperatures (arbitrary radiation flux), are considered
respectively in the next paragraph 2.2.5 and 2.2.6. In the present paragraph the large
population of photons trapped in a space is considered.
The
temperature
concept in radiation problems can be applied only to a photon
batch and the temperature
T
of thermal radiation can only be determined indirectly,
i.e. by measuring the temperature of the substance with which the radiation is in
equilibrium.
It can be derived that the internal energy
U
, in J, of the photon gas within a system
of volume
V
, is:
aVT
4
U
=
(2.2.23)
10
−
16
J/(m
3
K
4
)). The rest mass of photon
gas is zero, therefore, energy
U
cannot be related to the mass of this gas but rather to
its volume. Thus the photon gas energy density
u
, J/m
3
, is:
where
a
is the universal constant (
a
=
7
.
564
·
U
V
=
aT
4
u
=
(2.2.24)
The entropy density
s
S
, J/(K m
3
), of black radiation in the system is derived as:
4
3
aT
3
=
s
S
(2.2.25)
Radiation transports a linear momentum and may exert radiation
pressure
on an
object by irradiating it. Radiation pressure can be considered either as the effect of
interaction between radiation and substance, or as the effect only within the internal
structure of the radiation. In the first case, radiation pressure exerted on a substance
Search WWH ::
Custom Search