# Kirchhoff's loop rule

Around any closed loop, the voltage rises equal the voltage drops, because energy is conserved.

Physics (university) · Part 08: Circuits and magnetism · Card 6 of 12 · Last updated 2026-10-19

## Analogy

Going round a hilly walk and ending at the same height.

## Example

**Loop**

9 V - 4 V - 5 V = 0

## In practice

A 9 V battery with two resistors drops 4 V and 5 V in total 9 V.

## Questions people ask

### What sign do I give a voltage drop?

Choose a direction around the loop. Crossing a battery from negative to positive is a rise, so use plus. Crossing a resistor in the direction of the current is a drop, so use minus. The sum around the whole loop must be zero.

### Why does the loop rule work?

The electric field of a steady circuit is conservative, so moving a charge once around a closed loop gives zero net energy change. Any energy a charge gains in a battery is lost in resistors. It is energy conservation in loop form.

## Related concepts

- [Electric current](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-1)
- [Ohm's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-2)
- [Series and parallel resistors](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-3)
- [Electric power](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-4)

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