A filament lamp and a diode are components used in electrical circuits. Explain why the current through a filament lamp does not increase proportionally as the potential difference across it is increased, and state one way in which the behaviour of a diode differs from that of a filament lamp.

OCR A-Level Physics B: Advancing Physics (H557) — 3.4 Materials · Explain · 5 marks · View as Markdown

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

A student is comparing how different components behave when the potential difference across them is changed. She notices that a filament lamp behaves differently from an ohmic conductor, and that a diode behaves differently again.

Model answer (5 marks)

A filament lamp is a non‑ohmic resistor. As the potential difference is increased the current rises, so electrons collide more frequently with the atoms in the filament. These collisions transfer energy to the atoms, raising the filament’s temperature. The higher temperature increases the amplitude of the atomic vibrations, which makes it harder for electrons to move through the material – the resistance rises. Because the resistance increases with temperature, the current does not increase proportionally with the applied potential difference.

A diode differs in that it only conducts in one direction. In the forward direction it has a low resistance and current flows, whereas in the reverse direction it has a very high resistance and essentially no current flows.

Examiner tips

  • Use the term ‘non‑ohmic’ and explain the temperature‑dependent resistance mechanism; mention electron–atom collisions and increased atomic vibration. State the diode’s one‑way conduction explicitly. Keep the answer concise and use correct terminology.

Common mistakes

  • Saying the filament’s resistance decreases with voltage; confusing temperature rise with resistance drop. Mixing up the direction of diode conduction or omitting that it blocks reverse current.

Mark scheme (5 marks)

  1. As current increases (through the filament lamp), electrons collide with atoms in the filament / resistor
  2. These collisions transfer energy to the atoms, causing the filament's temperature to increase
  3. The increased temperature causes the atoms to vibrate more, making it harder for electrons to flow
  4. Therefore the resistance of the filament lamp increases (as potential difference / temperature increases), so the current does not increase proportionally
  5. A diode only allows current to flow in one (forward) direction / has a very high resistance in the reverse direction (so no current flows in reverse), whereas a filament lamp allows current in both directions

Key terms in this question

filament lamp · diode · potential difference · current

Related

More Materials questions

▶ Try answering this question with AI marking (free) →