The Attractive Science
Magnets are much more than fun toys. We use them in all kinds of ways, like telling direction with a compass or keeping refrigerators closed. In these activities, students will use simple experiments and observations to explore magnetism from determining what is a magnet to creating their own electromagnet.
Students will be able to:
- Tell if something is magnetic.
- Know which parts of magnets are the most attractive.
- Explain the relationship between electricity and magnetism.
A magnet is an object that creates a magnetic field. This field is invisible but is responsible for the most obvious property of a magnet, which is the ability to attract some materials like iron and attract or repel other magnets.
The ancient Greeks and Chinese discovered that some rare stones were naturally magnetized. These stones seemed to attract small pieces of iron in a magical way and when the stones were freely suspended, they always pointed in the same direction. The first written mention of a magnet is from the fourth century BC in China. The Chinese learned how to make artificial magnets by heating pieces of ore to red hot and then cooling the pieces in a north/south position. The magnet was then placed on a piece of reed and floated in a bowl of water. These needle compasses and uses in navigation are first mentioned in a Chinese text in 1088 AD and the compass was widely used on Chinese ships by the eleventh century AD.
We now make magnets in various shapes and sizes for different uses. One of the most common magnets — the bar magnet — is a long, rectangular bar that attracts pieces of ferrous objects.
Every magnet has one north pole and one south pole. If you break a bar magnet in half, each half will have a north and a south pole, even if you break it in half a bunch of times. The north poles of two magnets will repel each other, as will their south poles. A north pole and a south pole attract each other.
By convention, we say that magnetic field lines leave the north end of a magnet and enter the south end of a magnet. The forces of a magnet are strongest at the poles. This is because the magnetic field tends to be concentrated at the poles (and spread out and bulging between them).
Magnetic field lines don’t exist physically — they’re a mathematical construct to help us model how magnets work. However, iron filings around a magnet will line themselves up along the field lines, so we can see how the magnetic field “looks”.
What makes a magnet?
The atoms of a magnetic material are themselves tiny magnets. When groups of magnetic atoms are lined up in the same direction they’re called a magnetic domain. If many magnetic domains are themselves aligned, the material’s magnetic field is strong enough to affect other materials some distance away.
A permanent magnet carries its own persistent magnetic field. A good example is a fridge magnet. Permanent magnets can be made of iron, nickel, and cobalt; materials whose atomic “magnets” can be aligned. These materials are called ferromagnetic materials, and they’re also attracted to magnets.
Moving electrons also create a magnetic field. For example, if one end of a battery is connected to the other with a wire, electrons move along the wire. If you bring a compass near the wire, it will move in response to the magnetic field around the wire.
The whole Earth is also a magnet but the mechanism is unknown. It may be that the heat-generated movement of molten iron in the Earth’s core is responsible but no one is sure how. One interesting point is that the magnetic field created by the molten core of the earth must have a magnetic south pole near the geographic north pole in order to attract the "N" end of our bar magnet and compass needles.
Some things to remember:
- North poles point north, south poles point south.
- Like poles repel, unlike poles attract.
- Magnetic forces attract only magnetic materials.
- Magnetic forces act at a distance.
- While in contact with a magnet, a magnetic material acts as a magnet itself.
- A coil of wire with an electric current flowing through it becomes an electromagnet.
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