Current source density analysis

Current sources and sinks are local electrical currents that flow from a location where they can't be detected into a location where they can be detected (current source) or vice versa (current sink). By analogy to the flow of water, a current source would be like a mountain spring. A current of water flows from a hidden location underground to the surface where it can be easily seen. A sink on the other hand would be like water flowing down a drain. It flow from where you can see it to where you can't. Electrical current can have the same basic properties. Current flowing from the "live" wires of an electrical outlet would be a source, while current flowing back along the common would be a sink (although with AC circuits this distinction gets a little murkey).

There is no actual physical difference between a current source and a current sink. Whether you call the flow of current a source or a sink depends entirely on your point of reference. If you were a troll living by an underground stream, the aforementioned spring would be a sink from your point of view, and a source from the point of view of an observer on the surface, even though it is the same current of water.

Current sources and sinks have particular relevance in [[electrophysiology| current flow across biological membranes.