Another household item, a doorbell uses electromagnetism to convert electrical energy, stored in a battery or power grid, into mechanical and then sound energy. Two electromagnets (coils of copper wire wound around metal pole pieces) produce magnetic fields which are rapidly switched on and off by an electrical contact point that moves with the quick vibration of the bell striker. The magnetic fields produced, in turn, cause the striker to move and ring the bell, which, in turn, switches on (and off) the electromagnets, initiating a cycle, which continues until the electricity is turned off.
The electromagnetic coils inside the doorbell assembly can produce painful electrical shocks if touched directly. A plastic shield is provided to prevent this, but in the event of its loss or removal, please do not allow students to touch the exposed coils. Beyond this, the doorbell is a safe, hands-on demonstration.
Questions for Students / Items of Interest:
- The contact point is the point at which all electricity in the doorbell is controlled. As the striker moves, the circuit is repeatedly closed and opened. The sparks are electrons jumping across the small air gap between the contact point and rest of the circuit.
Q2) Why is it important for the electromagnets to be turned on and off quickly?
- For the bell to ring repeatedly, the striker must move in a back and forth motion. The rapid activation and deactivation of the electromagnets attracts the striker one moment, and releases it the next, creating a vibration motion, which causes the continued ring of the bell.
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The University of Kentucky Physics Petting Zoo - 2004