Sources represent the interaction of a systems with its environment. There are two types of source elements: effort sources (Se) and flow sources (Sf).

In the table below the effort and flow description of the source elements is shown. Examples of a flow source are an electrical current source and a hydraulic pump that generates a constant flow of liquid. Examples of an effort source are an electrical voltage source and a constant mechanical force.

The direction of effort and flow for sources are restricted. In other words: the Se-element (effort-out) and Sf-element (effort-in) have a fixed causality.