Entropy

As indicated in Table I, entropy is the name given to the extensive property whose
change when multiplied by temperature gives a quantity of thermal energy. In classical
thermodynamics there is no need to give any physical description of this property in terms
of molecular behavior. A change in entropy is defined simply as a quantity of thermal
energy divided by the temperature driving force which propels it so that it always produces
the thermal energy identically when multiplied by the temperature. Because temperature is
an intensive property and this product is energy, we know that the entropy must be an
extensive property. Furthermore, thermal energy is a part of the total internal energy within
a system so that the entropy change computed this way is a change in a property of the
system Thermal energy crossing a system boundary is defined as heat so that the entropy
change transported by it is simply the quantity of heat transported divided by the
temperature which transports it. This transporting temperature is the temperature of the
external or surroundings side of the system boundary. It is important to realize that this
transported entropy change may be only apart of the total entropy change within the
system. Because thermal energy can be produced within a system by other means than
adding heat to it, a thermal energy increase in the system can be greater than the heat
transported into it. In this case the entropy change within the system accompanying its
thermal energy increase will be greater than the entropy change transported into the system
with the heat flow.