# Thermodynamics: Cycles and systems

1. May 23, 2014

### influx

In the lecture notes my university provided I am told that according to the first law of thermodynamics:

ΣQ = ΣW (for a cycle)
ΣQ = ΣW + ΔU (for a closed system)

Does this mean that there is always a change in internal energy in a closed system? Or can ΔU=0? In other words, can ΣQ = ΣW, and if yes, this means that closed systems can be cyclic? Can open systems be cyclic?

Thanks

2. May 23, 2014

### Staff: Mentor

Yes, you can have ΔU=0 for a closed system, but that doesn't mean that the process was cyclic. In a cyclic process, the system ends up in the same state that it started from, and therefore must have ΔU=0. Most often, cyclic processes are assumed to happen in closed systems. In principle, you could have an open system that is run on a cycle, which would mean that any exchange of matter with the environment would balance out over one cycle.

3. May 23, 2014

### influx

Thanks for the great answer. One more question if you don't mind, does ''control volume'' refer to the same thing as an ''open system''?

Thanks

4. May 24, 2014

### Staff: Mentor

No. A control volume is a subset of a system, and is usually taken to be representative of the greater system. It is an open system, but open systems are generally not control volumes. An open system is any system that can exchange matter with the outside world.