A student connects a length of nichrome wire to a battery. The student then replaces the nichrome wire with a piece of rubber of the same dimensions. Explain why the nichrome wire conducts electricity but the rubber does not.

WJEC A-Level Physics (Wales) — 2.1 Conduction and resistance · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Nichrome is a metal alloy. Rubber is a non-metallic insulator.

Model answer (5 marks)

Nichrome is a metal alloy, so its atoms have free or delocalised electrons in the outer shell. These electrons can move through the lattice when a potential difference is applied, carrying charge and forming an electric current. Rubber, on the other hand, is a non‑metallic insulator; its atoms are covalently bonded and there are no free or delocalised electrons available. Without charge carriers, no charge can move through the material, so no current is established. Consequently, rubber has a very high resistance and behaves as an insulator.

Examiner tips

  • Use the terms "free/delocalised electrons" and "charge carriers" to show understanding of conduction mechanisms. Explain the consequence (high resistance/insulator) to link cause and effect. Keep the answer concise – 5 marks can be earned with 4–5 short points.
  • common_mistakes
  • :
  • Confusing the role of ions with electrons; mentioning ions as carriers in metals. Saying the material is simply “non‑conductive” without explaining the lack of free electrons. Using vague terms like "poor conductor" instead of specifying high resistance and lack of charge carriers.

Mark scheme (5 marks)

  1. Nichrome (metal) contains free/delocalised electrons
  2. These free electrons can move through the metal, carrying charge / forming a current
  3. In rubber there are no free/delocalised electrons
  4. Because there are no charge carriers in rubber, charge cannot flow / no current can be established
  5. Resistance of rubber is (very) high / rubber is an insulator because it has no free charge carriers to allow current to flow

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