Magnetism

Kosmoi.com Science Physics Magnetism :

Magnets attract some metals, such as iron, nickel, cobalt, and some alloys. You can magnetise a piece of one of these metals by stroking it with a magnet.

Magnetism is a phenomenon by which materials exert an attractive or repulsive force on other materials. Some well known materials that exhibit easily detectable magnetic properties are iron, some steels, and the mineral lodestone; however, all materials are influenced to some extent by the presence of a magnetic field, although in most cases the influence is too small to detect without special equipment.

All magnetic effects ultimately stem from moving electric charges, and all materials have magnetic properties. Natural magnets (lodestones) of magnetic iron oxide have been known for many centuries. Some well known materials that exhibit easily detectable magnetic properties are iron, some steels, and the mineral lodestone; however, all materials are influenced to some extent by the presence of a magnetic field, although in most cases the influence is too small to detect without special equipment.

Permanent Magnets

A permanent magnet is usually made from a ferromagnetic material which at some time has been exposed to a magnetic field. A ferromagnetic material (one that can be magnetized) can be made into a magnet by placing it in the centre of an electric coil or solenoid and passing a large current through the coil. If the material is magnetically `hard', it will retain its magnetism once the current has been switched off. Permanent magnets are made from such hard materials as steel, nickel, and cobalt. Such magnetic alloys are used in electrical equipment and electronic devices.

Magnetic Poles

When a magnet is suspended freely, one side points North. This is called the North pole of the magnet. The other side points South, and is called the South pole of the magnet.

A compass is a magnet used for finding bearings from North.

The Earth acts like a giant magnet. The Earth's magnetic poles are close to, but not exactly at, the actual North and South poles of the Earth (the surface points of the Earth's rotation axis).

Like poles repel each other, and unlike poles attract.

Cut one bar magnet into halves, and each half becomes a magnet with two different poles. You can never create one with only one pole (some scientists think there might be a few magnetic monopoles left over from the Big Bang that created our Universe of space and time).

Magnetic Fields

A magnet is surrounded by an invisible force field. Electric coils, currents in wires, and permanent magnets are all sources of magnetic field. Magnetic fields are produced by moving charged particles: in electromagnets, electrons flow through a coil of wire connected to a battery; in permanent magnets, spinning electrons within the atoms generate the field. Lines of magnetic force can be seen around a magnet by sprinkling iron filings on to a sheet above it and tapping the sheet. The strength of the magnetic force is strongest close to the poles and gets weaker as you move away from the poles.

Magnetic Domains

In the atoms of magnetic metals, the electrons spinning around the nucleus create a small magnetic field. Normally the electrons are paired and their fields cancel; but in iron, some of the electrons are unpaired. Their spins tend to line up together, creating tiny 'pockets' of magnetism called magnetic domains.

In an unmagnetised piece of iron, these domains fields point in different directions. If you magnetise the iron, the domains line up in the same direction, creating a larger magnetic field.

Electromagnetism

Magnetic force can also be created by electricity flowing through a wire. The force can be increased by coiling the wire, and can be further increased by coiling it around some iron.

This is an extremely useful property that has many applications. Much of modern technology is based on electromagnetism, such as door bells, telephones, loudspeakers, cassette recorders, television monitors, electric clocks, electric motors, etc etc.

How many devices can you find in and around your house that depend on magnets? Some depend on electricity, and some may use 'permanent magnets'.

There is a fundamental connection between electrity - moving electrons - and magnetism. Moving electrons have magnetic fields, and magnetic fields make electrons travel curved paths. This is how television and computer monitors 'scan' a picture onto the screen.

Magnetic forces are fundamental forces that arise due to the movement of electrical charge. Maxwell's equations describe the origin and behavior of the fields that govern these forces (see also Biot-Savart's Law). Thus, magnetism is seen whenever electrically charged particles are in motion. This can arise either from movement of electrons in an electric current, resulting in 'electromagnetism', or from the constant subatomic movement of electrons, resulting in what are known as 'permanent magnets'.

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