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State properties and processes

A state property such as pressure, volume, temperature or internal energy depends only on the system's current state, not on the path it took to get there. Heat and work, on the other hand, are path properties that depend on the process itself. Processes are named after what is held constant along the way.

\text{Isothermal: } T = \text{const.}

constant temperature

\text{Isobaric: } p = \text{const.}

constant pressure

\text{Isochoric: } V = \text{const.}

constant volume

Symbols

$U$	internal energy (state property)	J
$Q,\ W$	heat and work (path properties)	J

Example

A gas is heated from $350$ K to $500$ K while its volume is held constant at $0.01$ m³ – this is an isochoric process, since only $V$ is constant.

U

is always a state property, while

Q

and

W

depend on which path the process took between two states.

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