Thermodynamics of open
biological environments.
Heat and Thermodynamics
First Law of Thermodynamics
Second Law of Thermodynamics
Entropy
Adiabatic Process
Heat Engine Cycle
Enthalpy
First Law of Thermodynamics
The first law of thermodynamics is the
application of the conservation of
energy principle to heat and
thermodynamic processes:
The first law makes use of the key
concepts of internal energy, heat ,
and system work. It is used
extensively in the discussion of
heat engines .
System Work
When work is done by a thermodynamic
system, it is ususlly a gas that is doing
the work. The work done by a gas at
constant pressure is:
For non-constant pressure, the work can be
visualized as the area under the pressure-volume
curve which represents the process taking place.
The more general expression for work done is:
Work done by a system decreases the internal
energyof the system, as indicated in the First
Law of Thermodynamics. System work is a
major focus in the discussion of heat engines.
Second Law of
Thermodynamics
The second law of thermodynamics is
a general principle which places
constraints upon the direction of heat
transfer and the attainable efficiencies
of heat engines . In so doing, it goes
beyond the limitations imposed by the
first law of thermodynamics. It's
implications may be visualized in terms
of the waterfall analogy.
Second Law: Heat Engines
Second Law of Thermodynamics: It is
impossible to extract an amount of
heat QH from a hot reservoir and use it
all to do work W . Some amount of heat
QC must be exhausted to a cold
reservoir. This precludes a perfect heat
engine .
This is sometimes called the "first
form" of the second law, and is referred
to as the Kelvin-Planck statement of
the second law.
Second Law: Refrigerator
Second Law of Thermodynamics: It is not
possible for heat to flow from a colder
body to a warmer body without any work
having been done to accomplish this flow.
Energy will not flow spontaneously from a
low temperature object to a higher
temperature object. This precludes a
perfect refrigerator . The statements about
refrigerators apply to air conditioners and
heat pumps , which embody the same
principles.
Entropy
Second Law of Thermodynamics: In
any cyclic process the entropy will
either increase or remain the same.
Entropy : a state variable whose
change is defined for a reversible
process at T where Q is the heat
absorbed.
Entropy:a measure of the amount
of energy which is unavailable to
do work.
Entropy :a measure of the disorder
of a system. Entropy :a measure of
the multiplicity of a system.
Entropy in Terms of Heat and
Temperature
The macroscopic relationship
which was originally used to
define entropy S is
dS = Q/T
This is often a sufficient
definition of entropy if you
don't need to know about the
microscopic details.
Since entropy gives information
about the evolution of an isolated
system with time, it is said to give
us the direction of "time's arrow " .
If snapshots of a system at two
different times shows one state
which is more disordered, then it
could be implied that this state
came later in time. For an isolated
system, the natural course of
events takes the system to a more
disordered (higher entropy) state.
Alternative statements: Second
Law of Thermodynamics
Biological systems are highly
ordered; how does that square
with entropy?
Adiabatic Process
An adiabatic process is one in which
no heat is gained or lost by the system.
The first law of thermodynamics with
Q=0 shows that all the change in
internal energy is in the form of work
done. This puts a constraint on the
heat engine process leading to the
adiabatic condition shown below. This
condition can be used to derive the
expression for the work done during an
adiabatic process.
The ratio of the specific heats g =
CP/CV is a factor in determining the
speed of sound in a gas and other
adiabatic processes as well as this
application to heat engines. This
ratio g = 1.66 for an ideal
monoatomic gas and g = 1.4 for air,
which is predominantly a diatomic
gas.
Heat Transfer
The transfer of heat is normally from a
high temperature object to a lower
temperature object. Heat transfer
changes the internal energy of both
systems involved according to the First
Law of Thermodynamics.
Heat Conduction
Conduction is heat transfer by means of
molecular agitation within a material without
any motion of the material as a whole. If one
end of a metal rod is at a higher
temperature, then energy will be transferred
down the rod toward the colder end because
the higher speed particles will collide with
the slower ones with a net transfer of energy
to the slower ones. For heat transfer
between two plane surfaces, such as heat
loss through the wall of a house, the rate of
conduction heat transfer is:
Q = heat transferred
in time = t
T = thermal
conductivity of the
barrier
A = area
d = thickness of
barrier
Heat Convection
Convection is heat transfer by mass
motion of a fluid such as air or water
when the heated fluid is caused to
move away from the source of heat,
carrying energy with it. Convection
above a hot surface occurs because
hot air expands, becomes less
dense, and rises (see Ideal Gas Law).
Hot water is likewise less dense than
cold water and rises, causing
convection currents which transport
energy. Convection is thought to
play a major role in transporting
energy from the center of the Sun to
the surface, and in movements of
the hot magma beneath the surface
of the earth
It is difficult to quantify the effects of
convection since it inherently depends
upon small nonuniformities in an
otherwise fairly homogeneous medium.
In modeling things like the cooling of
the human body, we usually just lump
it in with conductio
Heat Engines
A heat engine typically uses energy
provided in the form of heat to do work
and then exhausts the heat which
cannot be used to do work.
Thermodynamics is the study of the
relationships between heat and work.
The first law and second law of
thermodynamics constrain the
operation of a heat engine.
The first law is the application of
conservation of energy to the
system, and the second sets limits
on the possible efficiency of the
machine and determines the
direction of energy flow.
Enthalpy
Four quantities called
"thermodynamic potentials" are useful
in the chemical thermodynamics of
reactions and non-cyclic processes.
They are internal energy, the enthalpy,
the Helmholtz free energy and the
Gibbs free energy. Enthalpy is defined
by
H = U + PV
where P and V are the pressure and
volume, and U is internal energy.
Enthalpy is then a precisely
measurable state variable, since it is
defined in terms of three other
precisely definable state variables. It is
somewhat parallel to the first law of
thermodynamics fora constant
pressure system
Q = DU + PDV
since in this case Q=DH
It is a useful quantity for tracking
chemical reactions. If as a result of an
exothermic reaction some energy is
released to a system, it has to show up
in some measurable form in terms of
the state variables. An increase in the
enthalpy H = U + PV might be
associated with an increase in internal
energy which could be measured by
calorimetry, or with work done by the
system, or a combination of the two.
The internal energy U might be thought of
as the energy required to create a system in
the absence of changes in temperature or
volume. But if the process changes the
volume, as in a chemical reaction which
produces a gaseous product, then work
must be done to produce the change in
volume. For a constant pressure process
the work you must do to produce a volume
change DV is PDV. Then the term PV can be
interpreted as the work you must do to
"create room" for the system if you presume
it started at zero volume.