Physics with Calculus/Thermodynamics/Intrinsic and Extrinsic Properties

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Thermodynamic properties can be divided into two categories. Properties that are proportional to the size of the sample are extrinsic properties. Examples of extrinsic properties are number of molecules N, or moles n, sample mass ms, volume V, internal energy U, and entropy S, among others. Extrinsic variables have definite values regardless of whether or not a sample is in a state of equilibrium.

Properties that are not proportional to the sample size are called intrinsic properties. Examples of intrinsic properties are pressure P, temperature T, density ρ, heat capacities Cv, Cp, and rms velocity vrms. When a sample is in a state of equilibrium the values of all intrinsic properties will be uniform throughout the sample. However, when a sample undergoes a rapid transition from one equilibrium state to another, the value of intrinsic parameters can vary wildly from location to location and from instant to instant.

A quasi static transition from one state to another is carried out gradually with intrinsic properties not varying from location to location. As the transition proceeds the intrinsic properties will change from their initial to their final values but the incremental changes are simultaneous at all locations in the sample. One can say that at each step along a quasi static process, the sample is in equilibrium with itself.

Transitions are driven when the intrinsic parameters of the sample are not equal to those of the surroundings. An object at a low temperature exposed to a reservoir at a high temperature attempts to come to equilibrium by absorbing heat. A high pressure gas expands a piston against atmospheric pressure. A quasi static process that is driven by only infinitesimal differences between the intrinsic parameters of the sample and surroundings is called a reversible process.