  The concept of electromotive force (emf) may be introduced in various ways in
an undergraduate course of theoretical electromagnetism. The multitude of
alternate expressions for the emf is often the source of confusion to the
student. We summarize the main ideas, adopting a pedagogical logic that
proceeds from the general to the specific. The emf of a "circuit" is first
defined in the most general terms. The expressions for the emf of some familiar
electrodynamical systems are then derived in a rather straightforward manner. A
diversity of physical situations is thus unified within a common theoretical
framework.
