Oftentimes we can switch out circuits with equivalent ones for simplification.
characteristic
An important idea is the characteristic for a circuit. Two circuits are equivalent (in their port behaviour, or response, but not in their internals) if they have identical characteristics at a specified pair of terminals. This motivates the idea of Thevenin’s theorem and Norton’s theorem.
Source transformation
Source transformation gives us a handy method of simplifying complex linear circuits.
Note that the reverse (where the source type is switched) still holds, hence the interchangeability of Thevenin and Norton’s theorems.
Elements
Resistors, capacitors, and inductors, when connected in series or in parallel, can be simplified into one equivalent value.