Kirchhoff's junction rule
Also called Junction rule, Kirchhoff's current law
Kirchhoff's junction rule states that the total current entering any junction in a circuit equals the total current leaving it. It is conservation of electric charge: charge does not pile up at a point, so whatever arrives each second must depart each second.
A junction is a point where three or more conductors meet. The rule says the total amount of charge entering a junction per unit time equals the total amount of charge exiting that junction per unit time:
The justification is conservation of electric charge. Charge is not created at the node and does not accumulate there in a steady circuit.
What it buys you:
- At a parallel split, the branch currents add up to the current in the trunk, and the larger share goes down the branch with less resistance.
- Elements in series carry identical currents, because there is no junction between them for current to leave through.
Two traps. This rule is about current, not about potential difference, and applying it to a loop instead of to a node is the mix-up with its companion, the loop rule. And a bend in a wire, or two components joined end to end, is not a junction: nothing branches there.
Like the loop rule, it is not printed on the AP Physics 2 equation sheet. The parallel-resistor rule is what you get by applying the two rules together, and the series and parallel guide does that reduction with numbers.