# Electric fields (A2)

> CIE A-Level Physics · CIE 9702 A2
> Source: https://www.owlsprep.com/study/cie-9702-u21-overview/
> Weight: n/a

This A2 unit builds on your AS electricity knowledge to explore electric fields, covering core concepts, key laws, calculations, and charged particle motion, a foundational exam topic.

**Prerequisites:** AS-level CIE 9702 basic electricity and charge concepts

## Learning objectives

- Describe core electric field concepts and apply fundamental laws to charged systems
- Calculate electric field strength and electric potential for point charges and uniform fields
- Relate electric potential to work done and potential energy for charged systems
- Analyze and describe the motion of charged particles in uniform electric fields

## Unit at a glance

This unit follows a logical learning arc that builds from basic definitions to applied problem solving. We start by establishing what an electric field is, then move to the fundamental inverse-square law of electrostatic force (Coulomb's law). Next we extend this to calculate field strength and electric potential for both point charges and uniform fields, before finishing by applying these concepts to analyze how charged particles move in electric fields — a common CIE exam question topic.

This unit is split into 5 sequential sub-topics for learning:
- [Electric field concepts](https://www.owlsprep.com/study/cie-9702-u21-electric-field-concepts/) — Introduces core definitions, properties and field line representations of electric fields.
- [Coulomb's law](https://www.owlsprep.com/study/cie-9702-u21-coulomb-s-law/) — Covers the inverse-square law for electrostatic force between two point charges.
- [Electric field strength](https://www.owlsprep.com/study/cie-9702-u21-electric-field-strength/) — Calculates field strength for point charges, parallel plates and uniform electric fields.
- [Electric potential](https://www.owlsprep.com/study/cie-9702-u21-electric-potential/) — Defines electric potential, potential difference, and relates it to potential energy.
- [Charged particle motion in E-fields](https://www.owlsprep.com/study/cie-9702-u21-charged-particle-motion-in-e/) — Applies kinematics to analyze projectile-like motion of charged particles in uniform fields.

## Common pitfalls

- **Wrong:** Confusing electric field strength with electric potential, especially for negative charges.
  - Why it fails: Field strength is a vector, while potential is a scalar with zero defined at infinity.
  - Correct: Remember $E$ points from positive to negative, and potential decreases in the direction of $E$.
- **Wrong:** Applying Coulomb's inverse-square law to large irregular charged objects.
  - Why it fails: Coulomb's law only holds for point charges or uniformly charged spheres.
  - Correct: Use the point charge approximation only when distance is much larger than object size.
- **Wrong:** Overcomplicating energy calculations for charges moving through non-uniform fields.
  - Why it fails: Students often think the relationship only works for uniform fields.
  - Correct: Kinetic energy change is always $q\Delta V$, regardless of field shape.

## Cheatsheet

| Concept | Formula |
| --- | --- |
| Coulomb's Law (force between point charges) | $F = \frac{1}{4\pi\epsilon_0} \frac{Q_1 Q_2}{r^2}$ |
| Electric field strength (point charge) | $E = \frac{1}{4\pi\epsilon_0} \frac{Q}{r^2}$ |
| Electric potential (point charge) | $V = \frac{1}{4\pi\epsilon_0} \frac{Q}{r}$ |
| Electric field strength (uniform parallel plates) | $E = \frac{V}{d}$ |
| Kinetic energy gained by charge through potential difference | $\Delta E_k = qV$ |

## What's next

Start with the first sub-topic of this unit to build your foundational understanding of electric field concepts. Once you complete all sub-topics in this unit, you can progress to the next A2 unit on magnetic fields in the CIE 9702 syllabus.

- [Electric field concepts](https://www.owlsprep.com/study/cie-9702-u21-electric-field-concepts/)
- [Magnetic fields (A2)](https://www.owlsprep.com/study/cie-9702-u22-overview/)
- [Coulomb's Law](https://www.owlsprep.com/study/cie-9702-u21-coulomb-s-law/)

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