A filament lamp is connected to a battery in a simple circuit. Explain why the resistance of the filament lamp increases as the current through it increases.

AQA A-Level Physics (7408) — 3.5.1 Current Electricity · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

As the current increases, the electrons gain more kinetic energy and move faster through the filament.
The faster electrons collide more frequently with the atoms (ions) of the filament.
Each collision transfers energy to the atoms, increasing their vibrational motion.
The greater atomic vibration raises the lattice temperature, making it harder for electrons to move.
Thus the resistance of the filament increases with current.

Examiner tips

Use the word ‘increase’ to show the causal chain; include temperature as the key variable.
Mention electron–atom collisions and energy transfer explicitly; this shows understanding of the microscopic mechanism.
End with a clear statement that resistance rises because temperature rises.
Use correct terminology: ‘vibrational motion’, ‘lattice temperature’, ‘electron drift’ if appropriate.

Common mistakes

Failing to mention that the temperature rises; writing only ‘more collisions’ without linking to resistance.
Using vague phrases like ‘more energy’ without specifying electron motion or atomic vibration.
Confusing current with voltage or resistance; not explaining the microscopic cause of increased resistance.

Mark scheme (5 marks)

As current increases, electrons (charge) gain more energy / move faster
Electrons collide with atoms (ions) in the filament
Energy is transferred to the atoms, causing them to vibrate more
Greater atomic vibration makes it more difficult for electrons to flow through the filament
Therefore resistance increases (as temperature increases)

Key terms in this question

filament lamp · resistance · current