A kettle element is made from a long, thin wire. When the kettle is switched on, the current through the element causes it to heat up. Explain why the resistance of the kettle element increases as it heats up, and describe how this affects the current flowing through the element.

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 kettle element is a resistor made from a long, thin wire that heats water when a current passes through it. As the element warms up, its electrical properties change.

Model answer (5 marks)

As current flows through the wire, electrons collide with the atoms of the metal. The kinetic energy of the electrons is transferred to the atoms, causing them to vibrate more strongly. The increased atomic vibration makes it harder for the remaining electrons to move through the lattice, so the resistance of the wire rises.
Because the resistance increases while the applied voltage remains the same, Ohm’s law (I=V/R) shows that the current through the element decreases.

Examiner tips

  • Use the word "collide" and "vibrate" to show the mechanism of heating.
  • Mention Ohm’s law to link resistance and current.
  • Keep the answer concise and use the exact terms from the mark scheme.

Common mistakes

  • Confusing resistance with voltage or current; not stating that the voltage is constant.
  • Failing to explain that increased vibration makes electron flow more difficult.
  • Using vague terms like "gets hotter" instead of describing atomic vibration.

Mark scheme (5 marks)

  1. As current flows, electrons collide with atoms in the wire / resistor
  2. Energy is transferred to the atoms, causing them to vibrate more
  3. Increased vibration of atoms makes it more difficult for electrons to flow through the wire
  4. This causes the resistance of the element to increase
  5. As resistance increases, the current through the element decreases (for the same potential difference)

Key terms in this question

resistance · current

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