Explain that the sum of the currents into a junction is the same as the sum of the currents out of the junction

4.3.2 Series and Parallel Circuits

Kirchhoff’s Current Law (KCL)

At any junction (node) in an electric circuit, the sum of currents flowing into the junction is equal to the sum of currents flowing out of the junction.

Mathematically: \$I{\text{in}} = I{\text{out}}\$

Think of it like a traffic intersection – the number of cars entering the intersection equals the number of cars leaving it (assuming no cars stop or appear out of nowhere).

Why It Matters

When you split a circuit into branches, you can use KCL to find unknown currents without measuring each one directly.

Example: A Simple Junction

Imagine a junction where three wires meet:

  • Wire A brings in a current of \$3\,\text{A}\$

  • Wire B brings in a current of \$2\,\text{A}\$

  • Wire C carries the current out of the junction

Using KCL:

  1. Sum of currents in: \$3\,\text{A} + 2\,\text{A} = 5\,\text{A}\$

  2. Therefore, current out through Wire C: \$I_{\text{C}} = 5\,\text{A}\$

Series vs. Parallel

In a series circuit, all components share the same current. In a parallel circuit, the total current splits among branches, but the sum of branch currents equals the source current.

BranchCurrent (A)
Branch 1\$I_1\$
Branch 2\$I_2\$
Branch 3\$I_3\$
Total\$I1 + I2 + I_3\$

Exam Tip 🚀

When solving for unknown currents, always:

  1. Identify all junctions.

  2. Apply KCL: \$I{\text{in}} = I{\text{out}}\$.

  3. Set up equations and solve simultaneously.

Remember: Currents are vectors – direction matters. Use arrows or signs to keep track.