Part 08 · Circuits and magnetism · Card 5 of 12
Kirchhoff's junction rule
At a junction, the current going in equals the current going out, because charge is conserved.
Last updated
Analogy
Water at a pipe fork: what flows in must flow out.
Example
Junction
In: 5 AOut: 2 A + 3 A = 5 A
In practice
5 A arriving at a junction can split into 2 A and 3 A.
Practice
Flashcards
- What does Kirchhoff's junction rule say?Current in equals current out at a junction.
Questions people ask
Why does the junction rule always hold?
Charge cannot pile up at a junction in steady current, and it cannot be created or destroyed. So the charge arriving per second must equal the charge leaving per second. It is a direct statement of conservation of charge applied to wires.
How do I use it with unknown currents?
Pick a direction for each unknown current, even if you guess wrong. Write one equation for each junction, in equals out. If your answer comes out negative, the real current runs the opposite way. Combine with the loop rule to solve for all unknowns.