Introduction to Electric Circuits

42 DC circuit analysis

3.3 KIRCHHOFF'S CURRENT LAW

Kirchhoff's current law may be stated as follows. The sum of the currents

entering a node is equal to the sum of the currents leaving that node. This

means that the algebraic sum of the currents meeting at a node is equal to zero.

Applying the law to the node shown in Fig. 3.3, we see that

I2j

I1

Figure 3.3

I~+I2+I3=I4+15

Rearranging,

11 + 12 + 13 -- 14 -- Is - 0 (3.1)

Figure 3.4

For the node shown in Fig. 3.4

I1+Iz+I3+I4--0

_...r I2

(3.2)

3.4 KIRCHHOFF'S VOLTAGE LAW

Kirchhoff's voltage law may be stated as follows. The sum of the voltage

sources around any closed path is equal to the sum of the potential drops

around that path. This means that the algebraic sum of all the potential

differences around any closed path is equal to zero. The important points to

remember are (1) the path must be closed and (2) it is an algebraic sum. Always

decide upon a positive direction (say clockwise) for a trip around the path: