# Alternating currents and AC circuits

> Sinusoidal voltage, rms values, phase and impedance: why capacitors, inductors and transformers behave differently at different frequencies.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.alternating-currents
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 4 Option A
- Difficulty: 5/5
- Assessment objectives: AO1, AO2, AO3
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 4 Option A(a) | use rms and peak values for sinusoidal voltage and current | AO1, AO2 |
| Unit 4 Option A(b) | explain frequency-dependent reactance and phase in AC circuits | AO1, AO2 |
| Unit 4 Option A(c) | apply transformer ratios and power transfer to transmission | AO2, AO3 |
| Unit 4 Option A(d) | explain resonance bandwidth damping and quality factor | AO2, AO3 |

## Summary

Sinusoidal voltage, rms values, phase and impedance: why capacitors, inductors and transformers behave differently at different frequencies.

## What earns marks

- For a sinusoidal supply, Vᵣₘₛ = V₀/√2 and Iᵣₘₛ = I₀/√2; rms values give the equivalent DC heating effect.
- Capacitive reactance decreases as frequency rises, while inductive reactance increases; phase relationships determine whether current leads or lags voltage.
- A transformer changes voltage through the turns ratio; power is approximately conserved, so a step-up voltage reduces current and lowers transmission losses for a fixed power.
- Resonance occurs when reactances balance; the response curve and quality factor describe selectivity, bandwidth and the effect of damping.

## Where marks are lost

- Using peak voltage in a mains-power calculation that requires rms voltage.
- Saying a transformer changes frequency as well as voltage; the frequency is set by the supply.
- Calling resonance the point of minimum current in a series RLC circuit.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Alternating currents and AC circuits lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.alternating-currents)