A student sets up a simple electric bell. Inside the bell, an electromagnet is used to make a metal striker hit the bell repeatedly as long as the switch is pressed. Explain why the striker moves towards the electromagnet when the switch is pressed, and why it then moves away again, allowing the bell to ring continuously.

Eduqas GCSE Physics — 2.5 Magnetism and electromagnetism · Explain · 4 marks · View as Markdown

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

An electric bell contains an electromagnet connected in series with a springy metal strip and a contact screw. When the circuit is complete, the electromagnet pulls the metal strip towards it. As the strip moves, it breaks the circuit at the contact screw.

Model answer (4 marks)

When the switch is pressed, current flows through the coil, creating a magnetic field that makes the coil an electromagnet. The magnetic field attracts the iron striker, pulling it towards the coil. As the striker moves, it breaks the circuit at the contact screw, so the current stops. With no current the electromagnet loses its magnetism, and the springy striker returns to its original position, closing the circuit again. The cycle repeats, making the bell ring continuously.

Examiner tips

  • Use the word "current" and "magnetic field" to show understanding of electromagnetism.
  • Explain the role of the spring in returning the striker.
  • Show the sequence: switch pressed → current → magnetism → attraction → circuit broken → no current → loss of magnetism → spring action.
  • Use correct terminology: electromagnet, coil, magnetic field, current, circuit, contact screw, springy striker.

Common mistakes

  • Confusing the striker with the coil; the striker is the metal piece that is attracted.
  • Forgetting to mention the spring that returns the striker.
  • Using vague terms like "pulls it" without linking to current and magnetic field.

Mark scheme (4 marks)

  1. When the switch is pressed, current flows through the coil, creating a magnetic field / making the coil magnetic
  2. The electromagnet attracts the iron/metal striker/strip, pulling it towards the coil
  3. As the striker moves, the circuit is broken (contact is lost at the screw), so the current stops flowing
  4. With no current, the electromagnet loses its magnetism, so the springy strip returns to its original position, completing the circuit again / allowing the cycle to repeat

Key terms in this question

electromagnet

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