Thevenin's theorem states, that the equivalent circuit will simply be an e.m.f. in series with an internal resistance , (exactly like the power supply models introduced in the previous section).
The values of the e.m.f and internal resistance are calculated as follows.
The formal statement of Thevenin's theorem is: "The linear network behind a pair of output terminals, can be replaced with an e.m.f. in series with an internal resistance." (The term linear network ,just refers to a circuit containing components like resistors to which Ohm's law applies).
Thevenin's theorem is best illustrated with the example on the following page. This example only uses a very simple circuit to illustrate how to apply the theorem. In practice the circuits we use Thevenin's theorem with, can be much more complicated.