First law — current (junction) law

Based on conservation of electric charge (the free electrons flow steadily through the conductor and do not accumulate at any point).

At any junction (node), the sum of the currents entering equals the sum of the currents leaving. Equivalently, the algebraic sum of the currents at a node is zero: ΣI = 0.

If you count currents entering as positive and leaving as negative, then I₁ + I₂ − I₃ − I₄ = 0, which is the same statement.

Example 5. Currents of 7 A and 2 A enter a junction and a current of 5 A leaves along one wire. The unknown current I in the third wire: 7 + 2 = 5 + I, so I = 4 A, leaving the junction.

Example 5a. At a node, 6 A and 3 A enter, and 4 A and an unknown current I leave. 6 + 3 = 4 + I so I = 5 A, leaving.