Explain what happens to the electric field strength between two parallel plates when the plates are moved closer together, and why this affects the force experienced by a charged particle placed between them.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two parallel plates are connected to a battery. One plate is positively charged and one is negatively charged. A small charged particle is placed between the plates.
Model answer (5 marks)
Electric field strength increases when the plates are moved closer together because the same potential difference is now spread over a smaller distance, giving a larger voltage per metre. The field between parallel plates is uniform, so the increase is the same everywhere between the plates. A charged particle placed in this field experiences a force F = qE. Since E has increased, the force on the particle also increases proportionally for a given charge.
Examiner tips
- State that E increases as d decreases (E=V/d).
- Mention the field is uniform between the plates.
- Show the force equation F=qE and link the increase in E to the increase in F.
Common mistakes
- Confusing electric field with potential difference; not recognising E=V/d.
- Assuming the field becomes non‑uniform when plates are moved.
Mark scheme (5 marks)
- Electric field strength increases when the plates are moved closer together
- The electric field between parallel plates is uniform
- A larger (greater) potential difference per unit distance / voltage is spread over a smaller gap
- The force on the charged particle increases (as electric field strength increases)
- Force on a charged particle depends on the electric field strength (F = EQ / force is proportional to field strength for a given charge)
Key terms in this question
electric field strength · parallel plates · charged particle
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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