# Capacitance and RC circuits

> Charge storage, energy in a capacitor and the exponential charge/discharge of an RC circuit, including how to read the time constant from data.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.capacitance
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 4.1
- Difficulty: 4/5
- Assessment objectives: AO1, AO2, AO3
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 4.1(a), (c)-(e) | define capacitance and explain factors affecting a parallel-plate capacitor | AO1, AO2 |
| Unit 4.1(b), (g) | calculate energy stored in a capacitor | AO2 |
| Unit 4.1(h) | calculate equivalent capacitance in series and parallel combinations | AO2 |
| Unit 4.1(i)-(j) | interpret charging and discharging curves using the time constant RC | AO2, AO3 |

## Summary

Charge storage, energy in a capacitor and the exponential charge/discharge of an RC circuit, including how to read the time constant from data.

## What earns marks

- Capacitance is charge stored per unit potential difference, C = Q/V; a larger plate area or smaller separation increases capacitance for a parallel-plate capacitor.
- The stored energy is E = ½QV = ½CV² = Q²/(2C), so raising voltage has a squared effect on energy.
- In a series circuit capacitors share charge and reciprocal capacitances add; in parallel they share potential difference and capacitances add.
- With constant R and C, capacitor charge and voltage rise towards their final values during charging, reaching about 63% after τ = RC; charging current falls to about 37%. During discharge, charge, voltage and current magnitude all fall to about 37% after τ.

## Where marks are lost

- Using the resistor-current equation alone and forgetting that capacitor current changes during the transient.
- Adding series capacitances as if they were resistors in parallel.
- Calling τ the time to reach the final value; it is the characteristic scale, not an instant finish.

## Related

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