# Kirchhoff’s Laws

Using *Kirchhoff’s Laws* we can analyze one electric circuit and determinate the values of some physics units current , voltages ,resistance .*There are two Kirchhoff’s Laws* : for one circuit loop and for one circuit node .*The first** Kirchhoff’s law* is for one circuit node and says : *The algebraic sum of all currents entering and exiting a node must equal zero .*

I1+I2-I3=0

*Example of the first Law .*

We have I1 = 2A I2 = 3A and I3 = 6A and we need to find the value of I4

We choose the way for I4 current ( arbitrary ) like in the figure bellow and we have :

I1 + I2 - I3 + I4 = 0

=> I4 = I3 - I2 - I1 = 6 - 3 - 2 = 1A =>

The way for I4 is correct . If the value obtained for I4 is with minus the way chosed for I4 is reverse .

A circuit node is one point from the electric circuit where is connected more than two sides of the circuit ( minim three ) .*Kirchhoff’s second law** says* :* The algebraic sum of all voltages in a loop must equal zero .*

A circuit loop is a part from the circuit with at least two sides which makes a closed polygonal line.

Example for the second Law .

U1 + E1 = R1I1

U2 + E2 = R2I2

U3 + E3 = R3I3

U4 + E4 = R4I4

U1 - U2 + U3 + U4 = 0

E1 - E2 - E3 - E4 = R1I1 - R2I2 + R3I3 + R4I4

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