# Transformer

Two coils on one iron core change AC voltage. The ratio of voltages equals the ratio of turns.

Physics (university) · Part 09: Induction, light and optics · Card 5 of 11 · Last updated 2026-10-19

## Analogy

A gearbox for electricity.

## Example

**Step-down**

Vs = Vp x Ns / Np
= 230 x 50 / 1000
= 11.5 V

## In practice

A 230 V supply with 1000 primary turns and 50 secondary turns gives 11.5 V.

## Practice

### Flashcards

- **How does a transformer change voltage?** In proportion to the number of turns: Vs/Vp = Ns/Np.

## Questions people ask

### Why do transformers only work with AC?

A transformer needs changing flux to induce a voltage in the second coil. Alternating current keeps changing, so the flux keeps changing. Steady DC gives constant flux and no induced voltage after the first instant. That is why power grids use AC.

### Is a transformer 100 percent efficient?

Not quite, but large ones reach over 98 percent. Losses come from resistance in the coils, eddy currents in the core and the energy used to magnetise the core. A step-up transformer raises voltage but lowers current, so power in is close to power out.

## Related concepts

- [Magnetic flux](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-1)
- [Faraday's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-2)
- [Lenz's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-3)
- [Inductance](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-4)

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