KS3: Magnetism Questions
Q3. The diagram shows an electromagnet used in a door lock.
(a) The push switch is closed and the door unlocks. Explain in detail how this happens.
The switch completes the circuit and a current flows through the coil. 
The coil becomes an electromagnet and its iron core becomes magnetised.
The iron bolt is attracted to the electromagnet.
The force of attraction between the electromagnet's core and the iron bolt is greater than the force of the spring, so the iron bolt is pulled out of its housing in the door.
5 marks
(b) The switch is released and the door locks. Explain in detail how this happens.
The current stops flowing when the circuit breaks.
There is therefore no more magnetism around the coil and the core demagnetises. The bolt is therefore pushed back into the door housing by the spring.
3 marks
(Total 8 marks)
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