Kirchhoff's Laws
Current in equals current out; voltages around a loop sum to zero.
About this topic
Kirchhoff's current law says that the total current entering a junction equals the total leaving, which is conservation of charge. His voltage law says that the sum of voltage rises and drops around any closed loop is zero, which is conservation of energy.
Together with Ohm's law they let you analyse circuits too complex for series/parallel shortcuts, by writing simultaneous equations. For example, if 5 A flows into a junction and 2 A leaves along one wire, the other must carry 3 A. In a loop with a 12 V battery and two resistors, the voltage drops across them must total exactly 12 V.
They are the foundation of circuit analysis in electronics and are built into simulation software that designs computer chips.
Tips: choose a direction for each current and a direction around each loop, and stay consistent with signs. If you get a negative answer for a current, it simply flows opposite to your guess, so the method still works.
Key ideas
- Junction rule: ΣI = 0
- Loop rule: ΣV = 0
- Solve complex circuits
Quick check
Use cases: where Kirchhoff's Laws helps
- Homework: review the key idea behind Kirchhoff's Laws before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Kirchhoff's Laws without notes.
- Visual learning: play with the interactive lab to see how Kirchhoff's Laws changes when you adjust its inputs.
- Teaching a friend: explain Kirchhoff's Laws in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Kirchhoff's Laws?
Current in equals current out; voltages around a loop sum to zero.
What will I learn in Kirchhoff's Laws?
Junction rule: ΣI = 0. Loop rule: ΣV = 0. Solve complex circuits.
Is Kirchhoff's Laws part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Kirchhoff's Laws lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Kirchhoff's Laws?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.