Notes

The second law of thermodynamics asserts that processes occur in a certain direction and
that the energy has quality as well as quantity.
The first law places no restriction on the direction of a process, and satisfying the first law
does not guarantee that the process will occur. Thus, we need another general principle
(second law) to identify whether a process can occur or not.
Fig. 1: Heat transfer from a hot container to the cold surroundings is possible; however,
the reveres process (although satisfying the first law) is impossible.
A process can occur when and only when it satisfies both the first and the second laws of
thermodynamics.
The second law also asserts that energy has a quality. Preserving the quality of energy is a
major concern of engineers. In the above example, the energy stored in a hot container
(higher temperature) has higher quality (ability to work) in comparison with the energy
contained (at lower temperature) in the surroundings.
The second law is also used in determining the theoretical limits for the performance of
commonly used engineering systems, such as heat engines and refrigerators etc.
Thermal Energy Reservoirs
Thermal energy reservoirs are hypothetical bodies 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. Lakes, rivers, atmosphere, oceans are example of
thermal reservoirs.
A two‐phase system can be modeled as a reservoir since it can absorb and release large
quantities of heat while remaining at constant temperature.
A reservoir that supplies energy in the form of heat is called a source and one that absorbs
energy in the form of heat is called a sink.
Hot
container
Possible
Impossible
Q (Heat transfer)
Cold
surroundings