Kirchhoff's first law
Kirchhoff's current law (KCL) states that the sum of currents entering a junction equals the sum of currents leaving it. Mathematically, Σ I_in = Σ I_out. This law is a consequence of charge conservation.
Formula
ΣI_in = ΣI_out
Real World
At a motorway interchange, the number of cars entering a junction each minute must equal the number leaving — the same conservation principle that governs current at a circuit node.
Exam Focus
Label current directions explicitly on your diagram before applying KCL; sign errors from unlabelled diagrams are the most common source of lost marks.
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