Kirchhoff's First Law
Kirchhoff's First Law
Current in equals current out
- At any junction, the total current flowing in equals the total current flowing out. That is .
- The reason is conservation of charge: charge cannot pile up at a point in a wire, and it cannot vanish. If a billion electrons enter a junction each second, a billion must leave it each second.
Your turn— tap to reveal the worked answer (9702-style)
In the figure, 3.0 A and a current I flow into junction P, and 7.5 A flows out. Find I.
Answer: , so A into the junction. (9702-style)
How the exam words it
- You may also meet: “the algebraic sum of the currents at a junction is zero”. Same statement, with currents in counted positive and currents out counted negative.
- Multiple-choice questions like to pair each law with the wrong conservation law. The first law is about charge, always.
Your turn— tap to reveal the worked answer (9702/12/M/J/25 Q37)
A multiple-choice row pairs a statement of Kirchhoff's first law with the quantity it conserves. Which pairing is correct?
Answer: “the algebraic sum of currents at a junction is zero” paired with charge. The loop statement about e.m.f.s belongs to the second law. (9702/12/M/J/25 Q37)
First law = junctions = charge. If the statement talks about a loop or about e.m.f.s, it is the second law, not the first.