A student investigates how the resistance of a component changes as it heats up during use. At room temperature, the component allows a large current to pass through it. After several minutes of use, the student notices the current has decreased even though the voltage has not changed. Explain, in terms of particles, why the resistance of the component increases as its temperature rises.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A component is connected to a fixed voltage supply. When first switched on, the current through the component is large. After several minutes, the current falls noticeably.
Model answer (5 marks)
The metal component contains free (delocalised) electrons that carry charge through it.
As the component heats up, the ions/atoms in the metal vibrate more vigorously with greater amplitude.
The more vigorous vibration of the ions causes them to obstruct the free electrons more frequently, increasing the number of collisions.
These frequent collisions make it harder for the electrons to move through the material; the electrons are slowed down more, so the resistance of the component increases.
Because the applied voltage is unchanged and the resistance has increased, the current through the component decreases, which explains the observed drop in current after several minutes of use.
As the component heats up, the ions/atoms in the metal vibrate more vigorously with greater amplitude.
The more vigorous vibration of the ions causes them to obstruct the free electrons more frequently, increasing the number of collisions.
These frequent collisions make it harder for the electrons to move through the material; the electrons are slowed down more, so the resistance of the component increases.
Because the applied voltage is unchanged and the resistance has increased, the current through the component decreases, which explains the observed drop in current after several minutes of use.
Examiner tips
- Use the exact phrase "free (delocalised) electrons" and "ions/atoms vibrate more" to match the mark scheme. Show the logical chain: temperature ↑ → ion vibration ↑ → electron‑ion collisions ↑ → resistance ↑ → current ↓ (Ohm’s law).
- Mention that the voltage is unchanged to link the increase in resistance to the decrease in current, satisfying the last point of the scheme.
Common mistakes
- Failing to mention the free electrons or the ion vibrations. Confusing the direction of the effect (e.g., saying resistance decreases with temperature). Omitting the link that the unchanged voltage leads to a lower current.
Mark scheme (5 marks)
- The metal component contains free (delocalised) electrons that carry charge through it
- As temperature increases, the ions / atoms in the metal vibrate more / with greater amplitude
- The more vigorously vibrating ions obstruct / collide with the free electrons more frequently
- This makes it harder for the electrons to move through the material / the electrons are slowed down more, so resistance increases
- Because voltage is unchanged and resistance has increased, the current decreases (linking back to the observation)
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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