why do the clapper of a bell move back though the switch is on?
The simplest kind of door bell mechanism, a buzzer, works with the help of an electromagnet whose two ends of the winding are connected to the electrical circuit. One end of the winding is connected directly to the electrical circuit while the other end of the wire is connected to a metal contact placed adjacent to a moving contact arm. The contact arm is a light thin metal arm, whose one end rests against the contact point, while the other end is anchored and connected to the electrical circuit.
When the door bell button is pressed, the electrical circuits closes and results in the generation of an electromagnetic field. The magnetic field attracts the iron bar, which pulls the contact arm away from the metal contact. The moving away of the contact arm breaks the circuit and thus turns off the electromagnet. The contact arm snaps back to its original position. This closes the electrical circuit and reestablishes the flow of electrical current through the circuit. Once again the magnetic field is generated and the process repeats itself.
The sound is generated due to the contact arm hitting rapidly to the magnet and the stationary contact, several times in a second.