A Practical View of Work in Closed and Open Thermodynamic Systems
Thermodynamics contains a large number of equations, but an engineer does not always need to memorize all of them independently. For many practical problems, especially in chemical and mechanical engineering, it is more useful to identify what kind of system we are dealing with and how energy is transferred. A simple way to organize the subject is to separate thermodynamic systems into closed systems and open systems . An even more useful way to think about the first law is to treat it like an energy accounting system. The system is like an account: energy can enter, leave, or remain stored inside the system. The first law simply keeps the account balanced. 1. The First Law as Energy Accounting The first law of thermodynamics is fundamentally an accounting equation. Using the common engineering convention in which heat entering the system and work done on the system are positive: \[ \boxed{\Delta U=Q+W} \] The interpretation is straightforward: \(Q...