Kirchhoff's First Law
Kirchhoff's First Law
- Current is the rate of flow of charge. At a junction, charge cannot continuously collect or disappear.
- That conservation idea gives a rule for every junction in a circuit.
Current into a junction equals current out
Definition
1 markWhat is meant by Kirchhoff's first law?
Model answer: The sum of the currents entering a junction equals the sum of the currents leaving it.
This law is a statement of conservation of charge. If 5.0 A reaches a junction each second, a total of 5.0 A must leave it.
Use the shared p.d. before the junction law
- Parallel branches connect between the same two junctions, so they have the same p.d.
- Find a branch current with , then use the junction law to find the remaining current.
- A negative answer means the real current is opposite to the direction you assumed. It is not a failed calculation.
Your turn9702/22/M/J/25 Q6(b)(ii–iii)
A 430 Ω resistor and a 240 Ω resistor are in series in one parallel branch across an 8.0 V supply. The total current is 0.030 A. Calculate the current in the other branch. [3] (9702/22/M/J/25 Q6)
Show worked answer
The first branch current is . Kirchhoff's first law gives .