The First Law of Thermodynamics
A fundamental law describes changes of the energy of a system by interaction with its surroundings. The First Law of Thermodynamics is often written in differential form in the notation of thermodynamics as:
dE = 5Q – Ш, (3.7)
where dE is the change in system energy and 5Q and 5W represent the differential flow of heat energy to and differential work done by the system. In some textbooks, the work is defined as that done on the system by the surroundings. Then, the sign on 5W is reversed. We use the more common engineering form expressed in Eq. 3.7. In rate form, the First Law of Thermodynamics may be written as:
where e is the internal energy due to random molecular motion at any point within the system and V 2/2 is the kinetic energy per unit mass. An additional term is often added to this expression to account for the potential energy due to position in a gravity field. This normally is not needed in the gas flows of interest in this book. It could be important in flows of liquid involving large elevation changes.