Thermodynamics studies the interaction of heat and work, and defines a number of energy-related properties and their relationships. In contrast, classical mechanics deals only with forces, momentum, and mechanical energy, treating losses (from dissipation of mechanical energy due to friction or air resistance) as empirical corrections. On the other hand, heat transfer deals only with thermal effects, treating energy inputs from frictional dissipation as sources. By putting the mechanics and heat together, thermodynamics can analyze the processes of converting heating-fuel to useful work in engines, or of using electricity to cool the inside of a refrigerator. The properties it defines can be used for analyzing the humidity in the air and the heat from chemical processes like combustion.
The problems solved by engineers frequently involve using and conserving energy, whether in combustion, energy conversion, conservation, or environmental protection. Just as statics is fundamental to designing structures and mechanisms and analysing stress and strain, thermodynamics is fundamental to the study of thermal processes and systems, and is a prerequisite for many upper-division engineering courses.
Classical mechanics seemed hard when you first started High School physics; but now, looking back, F=ma has become routine. Similarly, thermodynamics will seem hard when we first introduce new concepts; but once you have grasped them, will become obvious.
In engineering courses, we do not use the theoretical model of "knowledge transfer," but the educational paradigm called the "problem-solving method." You will learn concepts by using them; lectures, discussion sessions, and textbook will help prepare you, but the homework is the key to your learning experience. You will do homework problems every week. The most important learning takes place when you first think about how to approach the problem, before writing anything down! You may make a number of false starts: do not try to avoid this by asking someone else how to start, but get help only if you get stuck after several attempts. If you figure the problems out yourself, you will be well prepared for the tests.
The engineering method of analysis is to identify one of more governing equations which model the physical problem, and then to decide what system to apply them to. In classical mechanics, a useful governing equation might be Newton's law F=ma, or else the conservation of mechanical energy PE+KE=constant, where one or the other can be much easier for a particular problem, so one might try both. A typical system is the "free body," also called a closed system in thermodynamics, or a closed control volume in fluid mechanics.
In thermodynamics there is a whole tool-chest full of useful governing equations: conservation of total (mechanical plus thermal) energy, property relations such as pressure-volume-temperature (p-v-T) tables or equations of state, the second law of thermodynamics, etc. Picking the right tools, in the right order, is the secret to solving problems, and might require several tries.
There is also a whole range of possible systems. To study a steam engine, the old-time engineers made the whole engine room an open system, keeping track of the fuel and air that went into the burner, the flue gas that went out the stack, the heat that was carried off by the cooling system, and the power generated for the shaft. This allowed them to define specific fuel efficiency in lb/BHP-hr, but was too complicated to learn how to improve the engine.
We can narrow down the problem by making only the steam itself the system, and taking the combustion outside the domain within our "dotted line." We can now focus on the heat added in the boiler, the work extracted by the piston, the cooling required to condense the steam, and the work required to pressurize the feedwater. This system is closed (no mass enters or leaves by crossing the "dotted line"). If the operation is also steady, not storing energy, it is also steady-state, remaining at the original state.
Alternatively, we could have identified a pound of steam somehow, calling it a closed control volume or closed system, and followed it around through the processes in the boiler, expander, condenser, and feedwater pump. A closed system which returns to the original state is called a cycle; applying conservation laws to it is particularly easy, because only heat and work enter or leave.
If we wanted to study a process in the expander, we might make the contents of the of the cylinder during the piston motion the closed system or closed control volume. Since we do not return to the original state, we now have to keep track of heat transfer (if any) through the cylinder wall, work done on the piston, and also change in the energy stored in the steam; when we do that, we will have to define "internal energy" somehow.
Alternatively, if the the expander is a turbine operating steadily, at constant conditions and constant mass flow, we might make the contents of the turbine an open steady-state steady-flow (SSSF) system or control volume. We have to keep track of the rates of mass flow, heat transfer (if any), work output, and change in state from input to output. As we go to more flexible systems, we have to obtain more data, but might be able to get more information. We might have to try several "dotted lines" to find the solution we want.
For the record, there is a fourth, more general system we will touch on: open but not steady. If the contents of the control volume are uniform throughout, and the state of the in- or outflow is constant, it is called uniform-state uniform-flow (USUF).
Presenting the solution process so that someone else can follow it requires several elements: