Teacher to teacher

12th March 2004 at 00:00
Electric attraction I found my Year 8 class were struggling with the details of how an electric bell works. When the circuit has been switched on charge flows through the wire and magnetises an electromagnet. This attracts an iron arm attached to a hammer which clangs the bell. The attraction of the iron arm breaks the circuit and demagnetises the electromagnet so the arm flips back to its original position. This completes the circuit and the process happens again.

I got the class to draw two diagrams of the circuit: one in the electromagnetised position and one in the non-electromagnetised position.

Any clear circuit diagram of an electric bell can be used.

Pupils stuck one of the circuit diagrams fully into their books and stuck the other one face up on top, but only attached it by one edge. A pencil or pen can then be used to flap the top diagram back and forth so a simple animation effect is seen. These flicker books enabled pupils to see exactly what an electric bell looks like as it is clanging.

They enjoyed this activity and completed a similar flicker book for homework to show how a relay works. The relay also used two diagrams in the same flicker arrangement. A relay uses a small current to activate an electromagnet, which attracts an iron bar pivot. The other end of the pivot pushes two contacts together, which completes another circuit, usually one connected to the mains electricity. An example of this is a central heating timer. The difference with this is that the circuit is only broken if the switch is turned off again, so pupils have to be reminded to change the switch in the diagrams.

Sara Hartley, science PGCE student at Manchester University, currently at a placement at Bacup and Rawtenstall Grammar School, Lancashire

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