A student connects a length of nichrome wire to a battery and measures the current through it. The student then replaces the nichrome wire with a piece of rubber tubing of the same length and cross-sectional area, and finds that almost no current flows. Explain why the nichrome wire conducts electricity but the rubber does not.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Nichrome is a metal alloy used in heating elements. Rubber is commonly used as an electrical insulator.
Model answer (5 marks)
Nichrome is a metal alloy, so its atoms have free or delocalised electrons that can move through the lattice when a voltage is applied. These mobile electrons act as charge carriers, allowing a current to flow.
Rubber is an insulator; its electrons are tightly bound to the atoms and there are no free charge carriers. Without mobile electrons, a voltage cannot drive a current, so almost no current flows.
Thus the difference in conductivity is due to the availability of free electrons: metals conduct, insulators do not.
Rubber is an insulator; its electrons are tightly bound to the atoms and there are no free charge carriers. Without mobile electrons, a voltage cannot drive a current, so almost no current flows.
Thus the difference in conductivity is due to the availability of free electrons: metals conduct, insulators do not.
Examiner tips
- Use the term "free/delocalised electrons" for metals and "no free electrons" for insulators. Show the link between free electrons and current. Mention that resistance is higher when charge carriers are absent.
Common mistakes
- Confusing conductivity with resistance; stating that rubber has high resistance without explaining lack of charge carriers. Using vague phrases like "rubber is a poor conductor" without mentioning electrons. Forgetting to mention that metals have delocalised electrons.
Mark scheme (5 marks)
- Nichrome (as a metal) contains free/delocalised electrons
- These free electrons can move through the metal when a voltage/potential difference is applied, creating a current
- In rubber, there are no free/delocalised electrons (electrons are tightly bound to their atoms)
- Because there are no charge carriers free to move in rubber, no current can flow
- This makes rubber an electrical insulator (and nichrome/metals conductors) because resistance depends on the availability of free charge carriers
Related
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- Decode the mark scheme abbreviations →
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