Unit Overview

Electric fields (A2)

CIE A-Level Physics· 5 min read 📊 n/a

1. 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

Introduces core definitions, properties and field line representations of electric fields.

★★⏱ 4 min

Coulomb's law

Covers the inverse-square law for electrostatic force between two point charges.

★★★⏱ 5 min

Electric field strength

Calculates field strength for point charges, parallel plates and uniform electric fields.

★★★⏱ 6 min

Electric potential

Defines electric potential, potential difference, and relates it to potential energy.

★★★★⏱ 7 min

Charged particle motion in E-fields

Applies kinematics to analyze projectile-like motion of charged particles in uniform fields.

★★★★⏱ 6 min

2. Common Pitfalls

Wrong move:

Confusing electric field strength with electric potential, especially for negative charges.

Why:

Field strength is a vector, while potential is a scalar with zero defined at infinity.

Correct move:

Remember $E$ points from positive to negative, and potential decreases in the direction of $E$ .

Wrong move:

Applying Coulomb's inverse-square law to large irregular charged objects.

Why:

Coulomb's law only holds for point charges or uniformly charged spheres.

Correct move:

Use the point charge approximation only when distance is much larger than object size.

Wrong move:

Overcomplicating energy calculations for charges moving through non-uniform fields.

Why:

Students often think the relationship only works for uniform fields.

Correct move:

Kinetic energy change is always $q Δ V$ , regardless of field shape.

3. Quick Reference Cheatsheet

Concept	Formula
Coulomb's Law (force between point charges)	$F = \frac{1}{4 π ϵ _{0}} \frac{Q _{1} Q _{2}}{r ^{2}}$
Electric field strength (point charge)	$E = \frac{1}{4 π ϵ _{0}} \frac{Q}{r ^{2}}$
Electric potential (point charge)	$V = \frac{1}{4 π ϵ _{0}} \frac{Q}{r}$
Electric field strength (uniform parallel plates)	$E = \frac{V}{d}$
Kinetic energy gained by charge through potential difference	$Δ E_{k} = q V$

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.