A copper wire and a longer copper wire of the same thickness are each connected separately to identical batteries. Explain why the longer wire has a greater resistance and describe the effect this greater resistance has on the current flowing through it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Copper is a conductor commonly used in electrical circuits. The resistance of a wire depends on several factors, including its length.
Model answer (5 marks)
The electrons that carry charge travel through the wire and collide with the atoms in the metal lattice.
1. A longer wire contains more atoms for the electrons to encounter.
2. Therefore there are more collisions, which makes it harder for the electrons to move.
3. This increase in collisions raises the resistance of the wire.
4. With the same battery voltage, a higher resistance reduces the current.
5. Hence the longer wire carries a lower current than the shorter one.
1. A longer wire contains more atoms for the electrons to encounter.
2. Therefore there are more collisions, which makes it harder for the electrons to move.
3. This increase in collisions raises the resistance of the wire.
4. With the same battery voltage, a higher resistance reduces the current.
5. Hence the longer wire carries a lower current than the shorter one.
Examiner tips
- Use the word "collide" to show knowledge of electron–atom interactions.
- Show the chain: length → more atoms → more collisions → higher resistance → lower current.
- Mention the battery voltage is unchanged to justify the current drop.
Common mistakes
- Saying the wire is longer so it has more resistance without linking to collisions.
- Confusing resistance with voltage or current; e.g., claiming the voltage changes.
- Using vague terms like "harder to flow" without explaining the collision mechanism.
Mark scheme (5 marks)
- Electrons (charge) flow through the wire and collide with atoms in the wire
- The longer wire has more atoms for electrons to collide with / electrons must travel through more atoms
- More collisions make it harder for electrons to flow, so resistance increases with length
- Greater resistance means the current is lower / the current decreases as resistance increases
- The same potential difference (voltage) now has to push the current through a greater resistance, which is why the current is reduced (owtte — linking p.d., resistance and current correctly)
Key terms in this question
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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