Question

Figure shows the essential features of a battery-operated bell. The hammer strikes the gong when the switch is closed. State and explain the effect of using the following material successively to form the core.

1. Plastic
2. Steel
3. Copper

Answer 1

1. Plastic: No effect. Plastic is non-magnetic, so it does not concentrate magnetic flux. The electromagnet becomes very weak and the bell does not ring.
2. Steel: Steel is magnetic but it retains magnetism after current is switched off. It becomes a hard magnet, so the armature may stay attracted and the make-and-break action stops. The bell gives at most one strike or continuous attraction, not repeated ringing.
3. Copper: Copper is non-magnetic. It does not increase magnetic strength, so the electromagnet remains weak and the bell does not work properly.