Intro to 2nd Law Flashcards
Problems associated with First Law (These processes cannot occur even though they are not in violation of the first law.)
- A cup of hot coffee does not get hotter in a cooler room!!
- Transferring heat to a paddle wheel will not cause it to rotate!!
- Transferring heat to a wire will not generate electricity!!
MAJOR USES OF THE SECOND LAW
- The second law may be used to identify the direction of
processes. - The second law also asserts that energy has quality as well
as quantity. - The second law of thermodynamics is also used in
determining the theoretical limits for the performance of
commonly used engineering systems, such as heat engines
and refrigerators, as well as predicting the degree of
completion of chemical reactions.
Statements of THE SECOND LAW
- Kelvin-Planck Statement
- it is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. - Clausius Statement
- it is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.
The devices that convert heat to work.
- They receive heat from a high-
temperature source (solar energy, oil furnace, nuclear
reactor, etc.). - They convert part of this heat
to work (usually in the form of a
rotating shaft.) - They reject the remaining waste heat to a low-temperature sink (the atmosphere, rivers, etc.).
- They operate on a cycle.
A hypothetical body with a relatively large thermal energy capacity (mass x specific heat) that can supply or absorb finite amounts of heat without
undergoing any change in temperature is called a
Thermal Energy Reservoir
In practice, large bodies of water such as “blank” can be modeled accurately as
thermal energy reservoirs because of their large
thermal energy storage capabilities or thermal
masses.
oceans, lakes, and rivers as well as the atmospheric air
A “blank” supplies energy in
the form of heat,
and a “blank” absorbs it.
- Source
- Sink
Review the Steam Power Plant Figure
May bayad picture huhu
Why is it impossible to have 100 thermal efficiency?
The impossibility of having a 100% efficient heat engine is not due to friction or other dissipative effects. It is a limitation that applies to both the idealized and the actual heat engines.
Statement in thermal efficiency
- Even the most efficient
heat engines reject almost
one-half of the energy they
receive as waste heat. - Some heat engines perform
better than others (convert
more of the heat they
receive to work). - Every heat engine must waste some energy by transferring it to a low-temperature reservoir in order to complete the cycle, even under idealized conditions.
The transfer of heat from a low-
temperature medium to a high-
temperature one requires special devices called
refrigerators
The working fluid used in the
refrigeration cycle is called a
refrigerant
The most frequently used
refrigeration cycle is the
vapor-compression refrigeration cycle.
The efficiency of a refrigerator is expressed in terms of the
coefficient of performance
(COP).
Objective of refrigerator
to remove QL from the
cooled space.
