Activity: You can ring my bell - make an electromagnetic doorbell
This door bell will alert you when unwanted guests arrive. If the bell circuit is too fiddly, just making the electromagnet component is easy and highly satisfying. Please refer to the diagram and pictures, as the positioning of the components can be tricky.
You will need
3 m of insulated electrical wire
Alligator clip electrical wire connectors (optional but recommended)
An iron or steel bolt
Plasticine
A six volt battery
A small steel stand; I used a small clamp but another bolt would work.
A cotton reel
A nail or screw
Scissors
A light, fine metal strip of some sort; I used a metal file fastener. A nail file could also work.
A piece of firm cardboard
A bell or other item that will ring when tapped
What to do
Cut the wire into two pieces, one around 10 cm and the other piece as long as possible. Strip the plastic off the ends of the wires.
Now make your electromagnet! Wrap the long wire firmly around the bolt, winding it around tightly, lots of times, but leaving the end of the bolt exposed. Once a current is running through this wire, this becomes your electromagnet. Fix this electromagnet horizontally to the cardboard base with plasticine or string.
If you only want to make the electromagnet, stop here and connect each end of the wire to the battery. What kinds of items are attracted to the bolt-electromagnet?.
With your bolt-electromagnet, attach one end of the long wire to the battery and the other end to the fine metal strip. Alligator clip connector wires are helpful here.
Attach the 10 cm piece of wire to the metal stand/other bolt.
Secure the fine metal strip to the curved side of the cotton reel (a rubber band is an easy way).
Arrange so the fine metal strip is touching the metal stand/other bolt, but only 3 - 4 mm away from the exposed end of the electromagnet-bolt. See the diagram, as this is tricky to explain.
Place the reel base-down on the cardboard and secure using a nail pushed up through the cardboard into the reel's inner tube.
Secure the metal stand/other bolt to the cardboard.
Now connect the loose end of the small piece of wire (connected to the metal stand) to the free battery terminal.
This should have completed the circuit and you'll see what happens! The fine metal strip should vibrate as it moves between the electromagnet and the metal stand.
Lastly, place a bell for the metal strip to strike and you have the sweet chimes of a homemade doorbell.
You may need to fiddle (quite a lot) with the positions of the components in the circuit to get it to work. If you are not getting any vibration, check all your electrical contacts.
What's happening?
By running electrical current through a wire, you create a magnetic field. When the electrical current runs around metal, the metal becomes an electromagnet.
In this circuit, when the bolt becomes magnetic it attracts the metal strip. The strip moves away from the circuit to touch the magnetic bolt. When this happens, the circuit is broken; electricity is no longer running through the wire and the electromagnet stops working. The nail file then springs back and hits the metal stand and this completes the circuit, electricity then flows through the wire and re-activates the electromagnet. This cycle repeats over and over, making the metal file vibrate which then hits a bell and makes a noise. It's quite a nifty little circuit.
An electromagnet starts with a battery (or some other source of power) and a wire. What a battery supplies is electrons, or electricity. An electron is a part of an atom that makes a path around the nucleus (or centre) of an atom. Electrons carry a negative charge and are capable of moving from one atom to another.
If you look at a battery, say from a torch, you can see that there are two ends: one marked plus and the other marked minus. Electrons collect at the negative end of the battery. If you attach a wire directly between the positive and negative terminals, three things will happen:
Electrons will flow from the negative side of the battery to the positive side as fast as they can.
The battery will drain fairly quickly. For this reason, it is not a good idea to directly connect the two terminals of a battery to one another. Normally, you connect some kind of load in the middle of the wire so the electrons can do useful work. The load might be a motor, a light bulb, a radio or whatever.
A small magnetic field is generated in the wire. It is this small magnetic field that is the basis of an electromagnet.
There is a magnetic field around all wires carrying an electrical current. Straight wires you see in the street have a weak magnetic field around them.
When you wind a wire around a steel (iron) object and attach a battery, the strength of the magnetic field around the wire is increased, and the steel bolt becomes magnetised.
Applications
You'll find electromagnets in many vital everyday items - from electrical motors in your washing machine or toy car, to speakers in your hi-fi or headphones and even in your computer.
