======Model Electric Bell======


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**Materials: **{{$demo.materials_description}}\\
**Difficulty: **{{$demo.difficulty_description}}\\
**Safety: **{{$demo.safety_description}}\\
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**Categories:** {{$demo.categories}}
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**Alternative titles:** Vibrating Electromagnet
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====Summary====                   
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====Procedure====  
  - Wind about twenty turns of PVC-covered copper wire around one arm of an iron C-core.
  - Connect one end of the coil to a low-voltage DC supply (about 1 V).
  - Fix a hacksaw blade or a strip of hard steel under another supply terminal so that it projects outward. Attach a small mass (e.g., Plasticine) to the free end to slow vibrations.
  - Position the C-core just beneath the projecting blade, close enough that the blade can be pulled downward but not touching.
  - Connect the other end of the coil to the remaining terminal of the supply using a short wire. Tape the bare end of this short wire along the blade so that the coil’s free end touches it lightly.
  - Switch on the supply. The coil becomes an electromagnet, pulling the blade downward. This breaks contact, switching off the coil. The blade springs back up, restoring contact and re-energizing the coil. The cycle repeats, causing vibrations.
  - Observe the vibrating blade as an analogue of how an electric bell repeatedly strikes.




====Links==== 
Large model electric bell demonstration - Physics with Simon Poliakoff: 
{{youtube>YBKdY3APPVI?}}\\


📄 A model electric bell - IOP: [[https://spark.iop.org/model-electric-bell]]\\


====Variations====
  * Purchase an electric bell demonstration kit.
  * Add a small metal striker to the end of the vibrating blade to hit a bell and make the demonstration more realistic.
  * Vary the number of coil turns to show how electromagnet strength affects the vibration speed.
  * Try different blade materials (steel vs aluminum) to show that only ferromagnetic materials work effectively.
  * Adjust the added mass to change the vibration frequency.



====Safety Precautions====
  * Use a low-voltage DC supply (1–2 V) to keep the current safe.
  * Ensure the hacksaw blade has no sharp teeth exposed, or replace with a smooth steel strip.
  * Do not allow wires to short-circuit across supply terminals.
  * Keep the electromagnet connections secure to avoid sparking.




====Questions to Consider====
  * Why does the blade repeatedly move up and down? (The coil magnetizes only when current flows; motion breaks and remakes the circuit, producing vibration.)
  * How does this relate to the ringing of a real electric bell? (In a bell, the vibrating arm repeatedly strikes a bell dome while current cycles on and off.)
  * What role does the added Plasticine mass play? (It slows down vibrations, making the effect easier to observe.)
  * What would happen if the blade were made of copper or plastic instead of steel? (They would not be attracted to the electromagnet, so the device would not work.)
  * How would increasing the number of turns on the coil affect the electromagnet’s pull? (It would strengthen the field and increase the attraction to the blade.)