Definition

Kirchhoff's Laws for current and voltage

Part of the Electronics glossary:

Also see Ohm's Law .

Kirchhoff's Laws for current and voltage are two principles that apply to DC circuits and networks.

The total current flowing into any DC circuit node, also called a branch point, is always the same as the total current flowing out of the node. An example is shown in the top illustration. There are four current-carrying conductors ( a , b , c , and d ) leading into the node (black dot), and two conductors ( e and f ) leading out. Direct currents in parallel add together arithmetically. Therefore, the total current flowing into the node is a + b + c + d , and the total current flowing out is e + f . These total currents, according to Kirchhoff's First Law , must be equal.

kirchhof.gif (6800 bytes)

Kirchhoff's Second Law deals with voltage . An example is shown in the bottom illustration. A source having voltage equal to a is connected in a circuit with five passive components having voltage differences b , c , d , e , and f across them. The voltages across the passive components add together arithmetically because they are connected in series. According to the Second Law, the total voltage across the set of passive components is always equal and opposite to the source voltage. Therefore, the sum of the voltage differences across all the circuit elements (including the source) is always zero.

This was last updated in April 2005
Posted by: Margaret Rouse

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