recall Kirchhoff’s first law and understand that it is a consequence of conservation of charge

Kirchhoff’s Laws

Kirchhoff’s First Law (KCL)

🔌 What it says: At any junction (node) in an electrical circuit, the algebraic sum of all currents flowing into the node equals the algebraic sum of all currents flowing out of the node.



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

or equivalently, \$\sum I_{\text{node}} = 0\$

⚡ Why it works: It’s a direct consequence of the conservation of electric charge. Charge cannot magically appear or disappear at a junction; it must flow in and out in balance.

Analogy: Water in Pipes

Imagine a junction where several water pipes meet. The amount of water that flows into the junction must equal the amount that flows out, otherwise water would either pile up or run out.



In a circuit, current (I) is like the flow rate of water, and a node is like the junction where pipes meet. The KCL rule ensures that the “water” (charge) is conserved.

Simple Example

Consider a node where three branches meet:

  • Branch A: 3 mA flows into the node.

  • Branch B: 2 mA flows out of the node.

  • Branch C: 1 mA flows out of the node.

Check KCL:

\$3\,\text{mA} \;(\text{in}) = 2\,\text{mA} + 1\,\text{mA} \;(\text{out})\$

✔️ The currents balance, so KCL holds.

Key Points to Remember

  1. At every node, the sum of currents entering equals the sum leaving.

  2. Currents entering can be considered positive, leaving negative (or vice versa).

  3. Use KCL to write equations for complex circuits.

  4. It follows directly from the conservation of charge.

Exam Tip Box

📌 Tip: When you see a node, write an equation that sets the sum of all currents (with appropriate signs) to zero.


📝 Check your signs: If you’re unsure, assume all currents are entering (positive). If the sum isn’t zero, change the sign of the current that’s actually leaving.


💡 Remember: KCL is always true, no matter how complex the circuit. It’s a powerful tool for solving unknown currents.