Also known as the Junction rule · Kirchhoff's first law.
The algebraic sum of currents entering a node equals the sum of currents leaving it — the net current at any junction is zero.
Currents in (blue) balance currents out at every node.
ΣI(in) = ΣI(out)
I
each branch current meeting at the node (amperes)
In plain English: Charge doesn't pile up at a junction. Whatever flows in must flow out — like water at a pipe tee. If 3 A comes in and one branch takes 1 A, the other carries 2 A, no exceptions.
Where you'll meet it: Splitting current between parallel branches, checking that measured branch currents add up, and the basis of nodal analysis in circuit theory courses.