Explain Norton's theorem and its relationship to Thevenin's theorem.

Norton's theorem states that any linear circuit can be replaced by an equivalent current source (In) in parallel with a resistance (Rn). It is the dual of Thevenin's theorem - Norton current equals Thevenin voltage divided by Thevenin resistance (In = Vth/Rth), and Norton resistance equals Thevenin resistance. Source transformation allows conversion between the two equivalent circuits.