# Ohm's law

V = I x R. Potential difference (V) equals current (A) times resistance (ohms). For a fixed resistor at steady temperature, current is proportional to voltage.

Physics (school) · Part 06: Electric circuits · Card 4 of 8 · Last updated 2026-10-19

## Analogy

Pushing harder (voltage) on the same pipe gives a bigger flow (current).

## Example

**Resistor**

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

## In practice

Electricians use it to choose the right resistor, or to predict the current in a circuit.

## Practice

### Flashcards

- **State Ohm's law.** V = I x R.

### Interview questions

**A 230 V kettle draws a current of 8 A. Calculate its power and its resistance.**

- Power = V x I = 230 x 8 = 1,840 W
- Resistance = V / I
- R = 230 / 8 = 28.75 ohms

## Questions people ask

### How do I use V = IR?

Cover the quantity you want in the V-I-R triangle. For current, divide voltage by resistance. For resistance, divide voltage by current. For voltage, multiply. A 12 V supply across a 6 ohm resistor gives a current of 12 / 6 = 2 A. Always write the answer with the correct unit.

### What does a straight line through the origin on a V-I graph mean?

It shows the component obeys Ohm's law: the current is directly proportional to the voltage, so resistance is constant. A fixed resistor at steady temperature gives such a line. A bulb filament gives a curve, because its resistance rises as it gets hotter.

## Related concepts

- [Electrical power](https://studyvisually.com/learn/science/physics-school/physics-school-part-6/physics-school-part-6-concept-7)
- [Electric current](https://studyvisually.com/learn/science/physics-school/physics-school-part-6/physics-school-part-6-concept-1)
- [Potential difference](https://studyvisually.com/learn/science/physics-school/physics-school-part-6/physics-school-part-6-concept-2)
- [Resistance](https://studyvisually.com/learn/science/physics-school/physics-school-part-6/physics-school-part-6-concept-3)

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