A copper connecting wire in a circuit has a low resistance. A student replaces the copper wire with a piece of plastic tubing of the same length and diameter. The plastic tubing has an extremely high resistance. Explain, in terms of the movement of charged particles, why copper has a low resistance and why the plastic tubing has an extremely high resistance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Copper contains free (delocalised) electrons that can move through the metal lattice and carry charge.
These free electrons act as charge carriers, so when a potential difference is applied the electrons drift easily and current flows.
Because there are many free electrons, the current can flow with little opposition – the resistance is low.
Plastic, in contrast, has no free electrons or other charged particles that are free to move.
Without free charge carriers, charge cannot flow through the plastic, so the resistance is extremely high.
These free electrons act as charge carriers, so when a potential difference is applied the electrons drift easily and current flows.
Because there are many free electrons, the current can flow with little opposition – the resistance is low.
Plastic, in contrast, has no free electrons or other charged particles that are free to move.
Without free charge carriers, charge cannot flow through the plastic, so the resistance is extremely high.
Examiner tips
- Use the word "free electrons" for copper and "no free electrons" for plastic. Include the idea that current is carried by electrons. Show the link between number of carriers and resistance. Mention that resistance is inversely related to carrier density.
Common mistakes
- Saying copper has "more electrons" but not emphasising they are free. Claiming plastic has electrons but are just bound. Forgetting to connect the lack of carriers to high resistance.
Mark scheme (5 marks)
- Copper contains free (delocalised) electrons
- These free electrons can move through the copper and carry charge / act as charge carriers
- Because there are many free electrons, current flows easily, so resistance is low
- Plastic does not have free electrons / charged particles that are free to move
- Because there are no free charge carriers in plastic, charge cannot flow through it, so resistance is extremely high
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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