A student is investigating how the resistance of a wire changes with its length. The student connects a wire to a circuit with a fixed potential difference and measures the current through the wire. The student then repeats the investigation using wires of increasing length, keeping all other variables the same. Explain why a longer wire has a greater resistance than a shorter wire of the same material and thickness.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student notices that when a longer wire is used in a circuit, the current measured by the ammeter is lower than when a shorter wire is used, even though the potential difference across the wire stays the same.
Model answer (5 marks)
A longer wire contains more atoms for the electrons to travel through.
Electrons moving through a conductor collide with the atoms in the wire.
In a longer wire there are more collisions between electrons and atoms.
These collisions make it more difficult for the electrons to flow, effectively slowing them down.
Therefore the resistance of a longer wire is greater, so for the same potential difference the current is lower.
Electrons moving through a conductor collide with the atoms in the wire.
In a longer wire there are more collisions between electrons and atoms.
These collisions make it more difficult for the electrons to flow, effectively slowing them down.
Therefore the resistance of a longer wire is greater, so for the same potential difference the current is lower.
Examiner tips
- Use the exact wording from the mark scheme (e.g. "more atoms", "collide", "more collisions", "slower electrons", "greater resistance").
- Show the logical sequence: length → more atoms → more collisions → slower electrons → higher resistance → lower current.
Common mistakes
- Confusing resistance with voltage or current; not linking length to resistance.
- Using vague terms like "harder" instead of "more collisions" or "slower electrons".
- Omitting the final consequence that current is lower for the same potential difference.
Mark scheme (5 marks)
- A longer wire contains more atoms (for electrons to interact with / travel through)
- Electrons collide with the atoms in the wire
- In a longer wire there are more collisions (between electrons and atoms)
- These collisions make it more difficult for electrons to flow / slow the electrons down
- Therefore resistance is greater (in a longer wire) / current is lower for the same potential difference
Key terms in this question
resistance · potential difference
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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