Second Law of Thermodynamics(refrigerator): 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
We can imagine thermodynamic processes which conserve energy but which never occur
in nature. For example, if we bring a hot object into contact with a cold object,
we observe that the hot object cools down and the cold object heats up until an
equilibrium is reached. The transfer of heat goes from the hot object to the cold
object. We can imagine a system, however, in which the heat is instead transferred
from the cold object to the hot object, and such a system does not violate the
first law of thermodynamics. The cold object gets colder and the hot object gets
hotter, but energy is conserved. Obviously we don't encounter such a system in
nature and to explain this and similar observations, thermodynamicists proposed
a second law of thermodynamics. Clasius, Kelvin, and Carnot proposed various
forms of the second law to describe the particular physics problem that each
was studying. The description of the second law stated on this slide was taken
from Halliday and Resnick's textbook, "Physics". It begins with the definition
of a new state variable called entropy. Entropy has a variety of physical
interpretations, including the statistical disorder of the system, but
for our purposes, let us consider entropy to be just another property of
the system, like enthalpy or temperature.
An example of a reversible process is ideally forcing a flow through a constricted pipe. Ideal means no boundary layer losses. As the flow moves through the constriction, the pressure, temperature and velocity change, but these variables return to their original values downstream of the constriction. The state of the gas returns to its original conditions and the change of entropy of the system is zero. Aerodynamicists call such a process an isentropic process. Isentropic means constant entropy.
The second law states that if the physical process is irreversible, the combined entropy of the system and the environment must increase. The final entropy must be greater than the initial entropy for an irreversible process:
Sf > Si (irreversible process)
An example of an irreversible process is the problem discussed in the second paragraph. A hot object is put in contact with a cold object. Eventually, they both achieve the same equilibrium temperature. If we then separate the objects they remain at the equilibrium temperature and do not naturally return to their original temperatures. The process of bringing them to the same temperature is irreversible.