# Ohm's law

For many conductors, voltage equals current times resistance, V = IR. Resistance is in ohms.

Physics (university) · Part 08: Circuits and magnetism · Card 2 of 12 · Last updated 2026-10-19

## Analogy

A narrow pipe resists water flow more than a wide pipe.

## Example

**Resistor**

I = V / R
I = 12 / 4
I = 3 A

## In practice

A 12 V battery across a 4 ohm resistor drives 3 A.

## Practice

### Flashcards

- **State Ohm's law.** V = IR.
- **Write the power formulas for a resistor.** P = IV = I^2 R = V^2/R.

### Interview questions

**A 12 V battery is connected to a 6 ohm resistor in series with a parallel pair of 6 ohm and 3 ohm resistors. Find the current from the battery and the power used.**

- Parallel pair is 2 ohm
- Total is 8 ohm
- I = 1.5 A, P = 18 W

## Questions people ask

### Do all materials obey Ohm's law?

No. Many metals do at constant temperature, so current is proportional to voltage. Bulb filaments, diodes and LEDs do not, because their resistance changes with temperature or voltage. Ohm's law is a description of certain materials rather than a law of nature.

### How do length and thickness change a wire's resistance?

Resistance equals resistivity times length divided by cross-sectional area. Longer wires resist more, and thicker wires resist less. Copper has low resistivity, about 1.7e-8 ohm metres, so it is used for most cables. Temperature also matters, because metals resist more when hot.

## Related concepts

- [Electric power](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-4)
- [Series and parallel resistors](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-3)
- [Electric current](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-1)
- [Kirchhoff's junction rule](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-5)

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