21. Kirchhoff’s law
It is the fundamental law of the electrical circuit. There are first law for the current and second law for the voltage.
（1）Kirchhoff’s first law
At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node, or: The algebraic sum of currents in a network of conductors meeting at a point is zero.
The general formula is as follows.
（2）Kirchhoff’s second law
The sum of the emfs in any closed loop is equivalent to the sum of the potential drops in that loop, or: The algebraic sum of the products of the resistances of the conductors and the currents in them in a closed loop is equal to the total emf available in that loop.
About closed circuit (a)
About closed circuit (b)
The general formula is as follows.