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
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- recall and use E = Q / (4πε0 r 2) for the electric field strength due to a point charge in free space
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
- › Divide the total charge by 1.6 x 10^-19 to express the charge value in terms of e.
- › Draw separate vector arrows for the force from each charge and then determine the resultant direction using the parallelogram of forces.
- › Always double-check the syllabus formulae for point charges: Force is (Q₁Q₂)/(4πε₀r²) whereas Field Strength is Q/(4πε₀r²).
- › Apply the rule that a negative charge experiences a force in the opposite direction to the electric field to balance gravitational weight acting downwards.
- › Draw and describe field lines for a point charge or isolated sphere as radial lines that appear to originate from the center.
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
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