The entropy of a system depends on its internal energy and the external variables, such as the volume.

Larger river systems tend to be wide so the influence of external variables is minimized, and the sun reaches the surface.

In the C programming language, an external variable is a variable defined outside any function block.

To understand how external variables relate to the keyword, it is necessary to know the difference between defining and declaring a variable.

An external variable may also be declared inside a function.

An external variable can be accessed by all the functions in all the modules of a program.

The heat capacity depends on how the external variables of the system are changed when the heat is supplied.

If the only external variable of the system is the volume, then we can write:

Here the are the external variables and the the corresponding generalized forces.