Electric field of a point charge
Cambridge A-Level Physics (9702) · Unit 18: Electric fields · 7 flashcards
Electric field of a point charge is topic 18.4 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 18 — Electric fields , alongside Electric fields and field lines, Uniform electric fields and Electric force between point charges. In one line: E = Q / (4πε₀r²), where E is the electric field strength, Q is the magnitude of the point charge, ε₀ is the permittivity of free space, and r is the distance from the charge.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 7 flashcards — 3 definitions, 2 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Formula for the electric field strength (E) due to a point charge (Q) in free space
E = Q / (4πε₀r²), where E is the electric field strength, Q is the magnitude of the point charge, ε₀ is the permittivity of free space, and r is the distance from the charge.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- For a point charge Q in free space, recall the electric field strength E = Q/(4πε₀r²) at distance r and use it in calculations.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Electric field of a point charge
- › Always double-check the syllabus formulae for point charges: Force is (Q₁Q₂)/(4πε₀r²) whereas Field Strength is Q/(4πε₀r²).
State the formula for the electric field strength (E) due to a point charge (Q) in free space.
E = Q / (4πε₀r²), where E is the electric field strength, Q is the magnitude of the point charge, ε₀ is the permittivity of free space, and r is the distance from the charge.
Describe how the electric field strength (E) due to a point charge changes with increasing distance (r) from the charge.
The electric field strength (E) is inversely proportional to the square of the distance (r) from the point charge. Therefore, as distance increases, the electric field strength decreases rapidly (E ∝ 1/r²).
A +5.0 nC point charge is placed in a vacuum. Calculate the electric field strength at a distance of 2.0 cm from the charge. (ε₀ = 8.85 × 10⁻¹² F m⁻¹)
E = Q / (4πε₀r²) = (5.0 × 10⁻⁹) / (4π × 8.85 × 10⁻¹² × (0.02)² ) ≈ 1.12 × 10⁵ N C⁻¹
What are the units for electric field strength (E) when calculated using E = Q / (4πε₀r²)?
The units for electric field strength (E) are Newtons per Coulomb (N C⁻¹).
How does the sign of the point charge (Q) affect the direction of the electric field it produces?
A positive point charge produces an electric field that points radially outward from the charge. A negative point charge produces an electric field that points radially inward towards the charge.
Two identical +10 nC point charges are separated by 4 cm. Determine the electric field strength at the midpoint between the charges.
The electric fields due to each charge are equal in magnitude but opposite in direction at the midpoint. Therefore, the net electric field strength at the midpoint is zero.
Define permittivity of free space (ε₀).
Permittivity of free space (ε₀) is a physical constant that quantifies the ability of a vacuum to permit electric fields. It appears in many equations relating to electromagnetism and has an approximate value of 8.85 × 10⁻¹² F m⁻¹.
Review the material
Read full revision notes on Electric field of a point charge — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 18 — Electric fields
Electric field of a point charge sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Electric field of a point charge deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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