These concepts are dealt with by Norton's and Thévenin's theorems.

Much like Thévenin's theorem, the extra element theorem breaks down one complicated problem into several simpler ones.

Thévenin's theorem states that any two-terminal network can be reduced to an ideal voltage generator plus a series impedance.

Thévenin's theorem holds, to illustrate in DC circuit theory terms, that (see image):

Application of Thévenin's theorem and Norton's theorem gives the quantities associated with the equivalence.

This process is known as a source transformation, and is an application of Thévenin's theorem and Norton's theorem.

In accordance with Thévenin's theorem both the input and output circuits comprise a signal e.m.f. in series with an impedance.

Thévenin's theorem: Any network of voltage or current sources and resistors is electrically equivalent to a single voltage source in series with a single resistor.

Notice that here the input representation satisfies Thévenin's theorem while the output representation satisfies Norton's theorem.

In general, the concept of source transformation is an application of Thévenin's theorem to a current source, or Norton's theorem to a voltage source.

A practical electrical power source which is a linear electric circuit may, according to Thévenin's theorem, be represented as an ideal voltage source in series with an impedance.

Thévenin's theorem and its dual, Norton's theorem, are widely used for circuit analysis simplification and to study circuit's initial-condition and steady-state response.

It is an extension of Thévenin's theorem stating that any collection of voltage sources and resistors with two terminals is electrically equivalent to an ideal current source.

Voltage sources and current sources are sometimes said to be duals of each other and any non ideal source can be converted from one to the other by applying Norton's or Thévenin's theorems.

Thévenin's theorem can be used to convert any circuit's sources and impedances to a Thévenin equivalent; use of the theorem may in some cases be more convenient than use of Kirchhoff's circuit laws.

Filter-Order Filters: Shortcut via Thévenin Equivalent Source - showing on p. 4 complex circuit's Thévenin's theorem simplication to first-order low-pass filter and associated voltage divider, time constant and gain.

Depending on perspective, this impedance can be modeled as being in series with an ideal voltage source, or in parallel with an ideal current source (see: Thévenin's theorem, Norton's theorem, Series and parallel circuits).

In fact, Norton and his associates at AT&T in the early 1920s are recognized as some of the first to perform pioneering work applying Thevenin's equivalent circuit and who referred to this concept simply as Thévenin's theorem.

Just as impedance extends Ohm's law to cover AC circuits, other results from DC circuit analysis such as voltage division, current division, Thévenin's theorem, and Norton's theorem can also be extended to AC circuits by replacing resistance with impedance.

As a result of studying Kirchhoff's circuit laws and Ohm's law, he developed his famous theorem, Thévenin's theorem, which made it possible to calculate currents in more complex electrical circuits and allowing people to reduce complex circuits into simpler circuits called Thévenin's equivalent circuits.