A thermodynamic system is the part of the physical world selected for an energy and matter balance. Draw its boundary before classifying an interaction as heat, work or transported matter. The boundary may move: a sealed piston encloses a closed system even while its volume changes. An isolated system additionally excludes all external energy transfers.[ref]
Thermodynamics and Propulsion. Thermodynamics and Propulsion, §1.2.2: system boundaries and closed systems
Boundary choice changes the ledger. If an electrical heater warms water, choosing only the water counts incoming heat; including the heater counts electrical work entering through its leads. The combined internal energy records storage, without separate stored categories for the two transfer mechanisms.[ref]
Thermodynamics and Propulsion. Thermodynamics and Propulsion, §1.3.4, Figure 1.14: resistor and water with alternative system boundaries
A thermodynamic system is a specific portion of the universe that is being studied or analyzed, separated by its boundaries from the surrounding environment. It can consist of matter and energy exchanges and is characterized by its properties, such as temperature, pressure, volume, and internal energy. Thermodynamic systems are typically classified into three main types: 1. **Isolated System**: Neither matter nor energy can be exchanged with the surroundings. An example is an insulated thermos bottle.
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