Electric force between point charges
Cambridge A-Level Physics (9702) · Unit 18: Electric fields · 7 flashcards
Electric force between point charges is topic 18.3 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 field of a point charge. In one line: Coulomb's Law states that the electric force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them.
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 — 2 definitions, 3 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 2 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.
Coulomb's Law in words
Coulomb's Law states that the electric force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them.
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).
- Understand that, at any point outside a charged spherical conductor, the sphere's charge can be treated as a point charge located at its centre.
- Recall Coulomb's law, F = Q₁Q₂/(4πε₀r²), and use it to find the force that two point charges exert on each other 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 force between point charges
- › Draw separate vector arrows for the force from each charge and then determine the resultant direction using the parallelogram of forces.
For a uniformly charged spherical conductor, where can the entire charge be considered to be located when calculating the electric field or force outside the sphere?
For points outside the sphere, the entire charge can be considered to be located at the center of the sphere. This simplifies calculations by treating the sphere as a point charge.
State Coulomb's Law in words.
Coulomb's Law states that the electric force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them.
Write the formula for Coulomb's Law, defining all terms.
F = Q₁Q₂ / (4πε₀r²), where: F is the electric force, Q₁ and Q₂ are the magnitudes of the charges, r is the distance between the charges, and ε₀ is the permittivity of free space.
What is the value of the permittivity of free space (ε₀)? Include units.
The permittivity of free space (ε₀) is approximately 8.85 x 10⁻¹² C² N⁻¹ m⁻².
Two point charges, +2 μC and -4 μC, are separated by a distance of 0.5 m in a vacuum. Calculate the magnitude of the electric force between them.
Using Coulomb's Law: F = (2 x 10⁻⁶ C)(4 x 10⁻⁶ C) / (4π(8.85 x 10⁻¹² C² N⁻¹ m⁻²)(0.5 m)²) ≈ 0.288 N
How does the electric force change if the distance between two point charges is doubled?
If the distance is doubled, the electric force is reduced by a factor of four (inverse square law). F ∝ 1/r² so if r becomes 2r, F becomes F/4.
Two identical spheres carry equal charge. They are separated by a certain distance. If the charge on each sphere is doubled, how does the electric force between them change?
The electric force is quadrupled. Since the force is proportional to the product of the charges, doubling both charges results in a force that is 2*2 = 4 times larger.
Review the material
Read full revision notes on Electric force between point charges — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 18 — Electric fields
Electric force between point charges 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 force between point charges 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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