A fire door is held open by an electrical holder containing a coil wound around an iron core. An electrically isolated iron plate is attached to the door. During an alarm, a switch opens the circuit containing the coil. A door closer can then shut the door.
Which explains why the iron plate is released when the switch opens?
The iron plate stops carrying current, so the motor-effect force that was holding it against the core disappears.
The magnetic forces on the core and plate become equal and opposite, so they cancel and release the plate.
The fields made by different turns cancel when the circuit opens, and this cancellation pushes the iron plate away.
The coil's magnetic effect becomes greatly reduced when the current stops, so the holder can no longer attract the iron plate.
90 exam-style questions on AQA GCSE Physics 7.2 The motor effect, covering 7.2.1 Electromagnetism, 7.2.2 Fleming's left-hand rule (HT only), 7.2.3 Electric motors (HT only), and 7.2.4 Loudspeakers (HT only). Each one has a worked solution and a mark scheme showing where the marks go.