Kcl refers to Kirchhoff's current law, and kvl refers to Kirchhoff's voltage law. Kirchhoff's current law, also known as node current law, was put forward by German physicist G.R. Kirchhoff (1824 ~ 1887) in 1845. The content is that the sum of the current flowing into the node at any time in the circuit is equal to the sum of the current flowing out of the node (also abbreviated as Kirchhoff's law), which is the basic law of voltage in the circuit and the basis for analyzing and calculating more complex circuits. The content is that in any closed loop, the algebraic sum of voltage drop on each element is equal to the algebraic sum of electromotive force, that is, when starting from one point and going all the way back to that point, the algebraic sum of voltages in each section is always equal to zero, that is, ∑U=0. Can't just use the effective value to calculate, it must be (effective value ∠ initial phase angle). Because the AC voltage (current) in sinusoidal steady-state circuit needs to be described by two physical quantities, effective value and initial phase angle, which are expressed as (u phasor) = u ∠ φ, where phasor is a sine function expressed by complex numbers, and KCL and KVL are expressed by complex numbers. The biggest difference from DC circuit is that AC circuit has a phase angle problem, and solving complex KCL and KVL equations is much more complicated than real numbers. Sinusoidal functions are transformed into complex numbers by mathematical transformation. If we give up mathematical transformation and directly use KCL and Kⅴ L equations of sine function, we need to solve differential equations. Although the transient and steady state are solved at the same time, the solution process is more complicated.