Transcript Second Law of Thermodynamics

Second Law of
Thermodynamics
nd
2
The
law of thermoD. is a
general principle which places
constraints upon :
 The direction of heat transfer
 The attainable efficiencies of
heat engines.

implications may be visualized in terms of the
waterfall analogy.
Qualitative Statements: Second
Law of Thermodynamics

The 2nd law of thermoD is a profound
principle of nature which affects the way
energy can be used. There are several
approaches to state this principle
qualitatively. Here are some approaches to
giving the basic sense of the principle.



1. Heat will not flow spontaneously
from a cold object to a hot object.
2. Any system which is free of
external influences becomes more
disordered with time. This disorder
can be expressed in terms of the
quantity called entropy.
3. You cannot create a heat engine
which extracts heat and converts it
all to useful work.
4. There is a thermal bottleneck:

The 2nd Law says that no heat engine
can use all the heat produced by a fuel
to do work. The Carnot cycle sets the
ideal efficiency which can be obtained
if there is no friction, mechanical
losses, leakage, etc., but real machine
efficiencies are much less.
Second Law: Heat Engines

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
It is not possible for heat to flow from a
colder body to a warmer body without any
work having been done
 Energy will not flow spontaneously from a
low temperature object to a higher
temperature object. This precludes a perfect
refrigerator
Also true for air conditioners and heat pumps
 This is the "second form" or Clausius
statement of the second law.

Second Law: Entropy
 2nd
Law:
In any cyclic process the
entropy will either increase
or remain the same.




Entropy: S= Q/T, 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 gives 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.
Energy and Order in Biological
Systems

The concept of entropy and the 2nd law
suggests that systems naturally progress
from order to disorder. If so, how do
biological systems develop and maintain
such a high degree of order? Is this a
violation of the 2nd law?

Order can be produced with an
expenditure of energy, and the order
associated with life on the earth is
produced with the aid of energy from
the sun

For example, plants use energy from the
sun in tiny energy factories called
chloroplasts. Using chlorophyll in the
process called photosynthesis, they convert
the sun's energy into storable form in
ordered sugar molecules. In this way,
carbon and water in a more disordered state
are combined to form the more ordered
sugar molecules.