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Foundations of Thermodynamics

There are four Laws of Thermodynamics:

  1. The Zeroth Law of Thermodynamics says that if two bodies are in thermal equilibrium with a third body, the two bodies are also in thermal equilibrium with one another

  2. The First Law of Thermodynamics states that energy can neither be created nor destroyed: it always exists, and can only be converted from one form into another

  3. The Second Law of Thermodynamics

  4. The Third Law of Thermodynamics


The Macroscopic Viewpoint

We know that substances are made up of particles and molecules. For example, a gas exerts a pressure on its container due to the individual molecule collisions with each other and the walls. This is called classical thermodynamics, but this microscopic viewpoint is not particularly helpful for engineering problems as it overcomplicates things massively.


Instead, we model the particles grouped together as a substance. This is called the macroscopic viewpoint and it applies the continuum assumption that properties within a substance are equally and evenly distributed.




Systems and Control Volumes

Systems are defined as certain amounts of matter within a specified space. Everything outside the system is known as the surroundings, where the boundary is the zero-thickness line that separates the two. Boundaries can be fixed (such as a pressure vessel) or movable (a piston).


If no matter can cross the boundary, the system is said to be closed. Energy in both heat and work forms can cross the boundary, and the volume is not fixed.


If no matter nor energy can cross the boundary, the system is said to be isolated.


If both mass and energy can cross the boundary, the system is said to be open – or more commonly, it is described as a control volume. Examples include turbines or compressors: the volume is arbitrarily defined in space and is fixed, though the boundaries can move.


Properties of Systems

All systems have properties that are true anywhere in the system. These could be intensive of extensive:

  • Intensive properties are independent of a system’s size (e.g. temperature, pressure, and any constants such as viscosity)

  • Extensive properties are dependent on a system’s size (e.g. total mass and volume)

Specific properties are extensive properties per unit mass, and are noted using the lower-case version of their assigned letter:

  • V is the total (extensive) volume of the system

  • v is the specific volume, the volume per unit mass, V/m

The properties of a system in a given state do not depend on the circumstances by which the system came to be in that state.

Simple Compressible Systems

A ‘simple compressible system’ is one which can be fully defined by two independent intensive properties. This is when the impact of gravity, motion, and many other properties such as magnetic fields can be neglected.