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Thermodynamic Systems and Intensive vs Extensive Properties

Terms you will need

  • thermodynamic systemA thermodynamic system is a specific amount of matter or a defined region in space that you study to understand how energy and matter move in or out of it.
  • surroundingsThe surroundings are everything outside the thermodynamic system that can interact with it, such as exchanging heat or work.
  • system boundaryThe system boundary is an imaginary line that separates the thermodynamic system from its surroundings, defining what is inside and what is outside.
  • open systemAn open system is a type of thermodynamic system where both matter and energy can move in and out across the system boundary.
  • closed systemA closed system is a type of thermodynamic system where matter cannot move in or out, but energy can still be exchanged with the surroundings.
  • isolated systemAn isolated system is a type of thermodynamic system where neither matter nor energy can move in or out, making it completely separated from its surroundings.

How this topic runs

  1. 1Start here
  2. 2How it works

Thermodynamics is the study of energy conversion between heat and work. It began with early philosophical ideas about the nature of heat and evolved into a rigorous scientific field during the 17th century. Initially, it focused on understanding temperature and heat transfer, which could not be explained by Newtonian mechanics. The development of thermal machines in the 19th century pushed thermodynamics to explain practical applications like engine efficiency. By 1865, Rudolf Clausius formalized the foundational principles of thermodynamics, including the first and second laws. Thermodynamics is essential for understanding how heat engines operate and how energy transformations shape modern technology, from locomotives to aircraft. This chapter introduces the fundamental concepts of thermodynamic systems and properties, including the distinction between intensive and extensive properties.

Figure (1)

Figure 3.1 A weak cold front of air pushes all the smog in northeastern China into a giant smog blanket over the Yellow Sea, as captured by NASA’s Terra satellite in 2012. To understand changes in weather and climate, such as the event shown here, you need a thorough knowledge of thermodynamics. (credit: modification of work by NASA)

Thermodynamics studies how energy changes between heat and work. A thermodynamic system is a defined part of the world you're analyzing. Everything else is the surroundings.

Systems can be closed (no mass enters or leaves) or open (mass can move in and out). Energy moves in two main ways: work, which is organized energy transfer, and heat, which is disorganized energy transfer. Intensive properties are characteristics that don’t depend on how much of a substance you have.

Temperature and pressure are examples. If you split a block of ice in half, each half still has the same temperature and pressure. Extensive properties depend on the amount of substance.

Mass and volume are examples. Split the ice block, and each half has less mass and volume. To picture this, imagine a full water bottle.

Its mass and volume are extensive—they change if you pour out some water. But its temperature and pressure are intensive—they stay the same no matter how much water is in the bottle. Understanding these properties helps you track how energy moves in and out of systems during processes like heating or compression.

Key Points

  • A thermodynamic system is an arbitrary study subject with boundaries impermeable to mass, containing a fixed set of particles with fixed mass whose properties can change but are not mixed with others.
  • A system boundary is an impermeable boundary that defines the limits of a thermodynamic system, separating it from its surroundings and determining what is included in the system's mass and energy accounting.
  • A closed system is a thermodynamic system that contains a fixed amount of mass with boundaries impermeable to mass transfer, allowing energy transfers in the form of heat and work but not mass.

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