12.3 Direct Current and Alternating Current 351
demonstrated in Figure 12.5. As explained earlier, physical laws are based on observations, and
the Kirchhoff ’s current law is no exception. This law represents the physical fact that charge is
always conserved in an electrical circuit. The charge cannot accumulate or deplete at an elec-
trical node; consequently, the sum of the currents entering a node must equal the sum of the
currents leaving the node. For example, Kirchhoff ’s current law applied to the circuit shown in
Figure 12.5 leads to:i 1 i 2 i 3 i 4.Example 12.1 Determine the value of currenti 3 in the circuit shown in Figure 12.6.
i 4
i 2
i 1 i 3
■Figure 12.5
The sum of currents entering a
node must equal the sum of
the current leaving the node:
i 1 i 2 i 3 i 4.i 2 = 1 A
i 1 = 3 A i 3 =?
N
■Figure 12.6 The circuit for Example 12.1.
We can solve this simple problem by applying Kirchhoff ’s current law to nodeN. This
leads to:i 3 2 A
i 1 i 2 i 3
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