A filament lamp is connected in series with a diode and a battery. Explain what happens to the resistance of the filament lamp as the current through it increases, and describe what effect the diode has on the current in the circuit when the battery is connected in reverse.

OCR A-Level Physics A (H556) — 3.4 Materials · Explain · 5 marks · View as Markdown

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

Filament lamps contain a thin metal filament that heats up when current flows through it. Diodes are components that only allow current to flow in one direction.

Model answer (5 marks)

As the current through the filament increases, the filament heats up. The electrons that carry the current collide more frequently with the vibrating atoms of the metal, transferring energy to them. The increased atomic vibration makes it harder for electrons to move, so the filament’s resistance rises.

When the battery is connected in reverse, the diode presents a very high resistance to current flow. Consequently, almost no current flows through the circuit in the reverse‑bias condition.

Examiner tips

  • Use the word ‘increase’ for temperature and resistance; explain the electron‑atom collision mechanism; mention the diode’s high reverse resistance; keep the answer concise and use correct terminology.
  • Show the logical sequence: current → heating → electron‑atom collisions → increased resistance; then battery reverse → diode blocks → negligible current.

Common mistakes

  • Saying the resistance decreases with temperature; confusing the direction of current flow; omitting the diode’s reverse‑bias effect or describing it as a short circuit instead of high resistance.

Mark scheme (5 marks)

  1. As current increases, the filament's temperature increases
  2. The electrons collide with the vibrating atoms in the filament, transferring energy to them
  3. The increased vibrations of the atoms make it harder for electrons to flow, so resistance increases
  4. When the battery is reversed, the diode has a very high resistance in that direction
  5. Therefore no current (or negligible current) flows in the circuit when the battery is reversed

Key terms in this question

filament lamp · diode · resistance · current

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