Second Law of Thermodynamics

The second law of thermodynamics, also called entropy, abbreviated as S, is a fundamental principle in physics and thermodynamics that describes the behavior of energy in a system.  It can be stated in multiple ways, but one common formulation is, "The total entropy of an isolated system always tends to increase over time."  Entropy can be thought of as a measure of the disorder or randomness in a system.  The law implies that in any natural process, the overall level of disorder in the universe, as measured by entropy, will tend to increase or remain the same.

Another way to state the second law is that heat flows spontaneously from a hotter body to a colder body, but not in the reverse direction without external intervention.  This is known as the principle of the thermal gradient or the Clausius statement of the Second Law.  This law also leads to the concept of energy degradation or dissipation.  In any energy transformation or conversion, such as the conversion of heat energy into mechanical work, some energy will inevitably be lost or dissipated as waste heat, which increases the entropy of the system and the surroundings. 

It is important to note that while the second law of thermodynamics indicates a tendency towards increasing entropy and energy degradation, it does not imply that isolated systems cannot experience temporary decreases in entropy or localized decreases in entropy at the expense of greater increases elsewhere.  However, the net change in entropy for the entire system and its surroundings will always be positive or zero.

 

Second Law of Thermodynamics formula
\( \Delta S =  \Delta Q \;/\; T \)     (Second Law of Thermodynamics) \( \Delta Q = \Delta S \; T   \) \( T =  \Delta Q \;/\; \Delta S \)
Symbol	English	Metric
\( \Delta S \) = Entropy Differential	\(Btu\;/\;lbm-R\)	\(kJ\;/\;kg-K\)
\( \Delta Q \) = Heat Differential (Energy)	\(Btu\;/\;lbm\)	\(J\;/\;kg\)
\( T \) = Temperature	\(R\)	\(K\)