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GCSE Level Questions: Magnets and Electromagnets

Q7. Here is a diagram of an electromagnetic lock used to secure a door. The door unlocks when the switch is closed.

(a) Tick one box to indicate which of the following materials the bolt should be made from

Aluminium  
Brass  
Copper  
Iron

[1 mark]

(b) Explain why the door unlocks when the switch is closed.

When the switch is closed a current flows in the solenoid/circuit. This creates a magnetic field and attracting the bolt towards the solenoid - out of the door.

[3 marks]

(c) When the door unlocks, a force of 2.88 N is applied to the spring.

The spring extends by 1.50 cm.

Calculate the spring constant of the spring.

1.50 cm = 0.015 m

F = kΔx

2.88 = k × 0.015

k = 2.88 / 0.015

k = 192 N/m

[4 marks]

(d) Give two ways the resultant force on the bolt could be increased.

Any two from:

increase the current (in the solenoid / circuit)

add more turns to the solenoid. If you said 'add more coils' you got no marks - the coil has 'turns' - you do not need more coils (solenoids) - just more turns on the one you have got!

use a spring with a lower spring constant (weaker spring)

[2 marks]

(TOTAL 10 marks)