About Magnets
The discovery of magnets was very important as they are used to make electric motors and generators. Things that would disappear if we had no electricity are telephones, lights, electric heat, computers, televisions.Some uses of electromagnets: Maglev trains, car crushers, scrap metal sorters, telephones, computers, doorbells, tape recorders etc. Maglev trains operate without wheels as they 'float' above the track due to magnetic repulsion between electromagnets in the track and underside of the train. Maglev trains can travel very fast, up to 480 km/h (300 mph).
Stories
of magnetism date back to the first century B.C in the writings of Lucretius,
and the magical powers of magnetite are mentioned in the writings of Pliny the
Elder. For many years following its discovery, magnetite was surrounded in
superstition and was considered to possess magical powers, such as the ability
to heal the sick, frighten away evil spirits and attract and dissolve ships made
of iron! Unlike amber (fossilized tree resin), magnetite was able to attract
objects without first being rubbed. This made magnetite far more magical. People
soon realized that magnetite not only attracted objects made of iron, but when
made into the shape of a needle and floated on water, magnetite always pointed
in a north-south direction creating a primitive compass. This led to an
alternative name for magnetite, that of lodestone or "leading stone".
The first attempt to separate fact from superstition came in 1269, when a soldier named Peter Peregrinus wrote a letter describing everything that was known, at that time, about magnetite. It is said that he did this while standing guard outside the walls of Lucera which was under siege. While people were starving to death inside the walls, Peter Peregrinus was outside writing one of the first 'scientific' reports and one that was to have a vast impact on the world. It wasn't until the experiments of William Gilbert in 1600 that significant progress was made in the understanding of magnetism and it was another century or so before other scientists began, by experimentation, to understand the phenomenon.
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Magnets can be made by placing a magnetic material such as iron or steel, in a strong magnetic field. Permanent, temporary and electromagnets can be made in this manner. |
Soft iron and certain iron alloys, such as permalloy (a mixture of iron and nickel) can be very easily magnetized, even in a weak field. As soon as the field is removed, however, the magnetism is lost. These materials make excellent temporary magnets that are used in telephones and electric motors for example. |
Other kinds of alloys such as alnico (an alloy of aluminum, nickel, iron, cobalt), make excellent permanent magnets. Ferrites (ceramic like materials made of iron oxides with nickel and cobalt) also make excellent permanent magnets. In these materials the domains are more difficult to dislodge, once they are aligned.
Electromagnets
are used when really strong magnets are required. Electromagnets are produced by
placing a metal core (usually an iron alloy) inside a coil of wire carrying an
electric current. The electricity in the coil produces a magnetic field. Its
strength depends on the strength of the electric current and the number of coils
of wire. Its polarity depends on the direction of the current flow. While the
current flows, the core behaves like a magnet, but as soon as the current stops,
the magnetic properties are lost. Electric motors, televisions, maglev trains,
telephones, computers and many other modern devices use electromagnets.
It was William Gilbert who first realized that the Earth was a giant magnet and that magnets could be made by beating wrought iron. He also discovered that the induced magnetism was lost if the iron was heated. In 1820, Hans Christian Řersted, demonstrated for the first time (at a public lecture), that there was a relationship between electricity and magnetism.
The area of force (magnetic field) surrounding a piece of magnetite or a bar magnet can be represented (visualized) by the lines of force as shown below, although these lines are no more real than the lines of latitude and longitude on a map or globe.
Rules of magnetism
| Like poles repel and unlike poles attract. | Lines of force are three-dimensional,
surrounding a bar magnet on all sides.
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| When opposite poles of a magnet are brought
together, the lines of force join up and the magnets pull together.
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When like poles of a magnet are brought together, the lines of force push away from each other and the magnets repel each other. |
How does a compass work?
The north and south ends of the Earth are called the north and south poles. The compass needle is a magnet, with north and south poles. The south pole of the needle is north-seeking, that is, it always points to the Earth's magnetic north pole, which is close to the geographic north pole. Similarly, the north pole of the compass needle is south-seeking and always points to the Earth's south magnetic pole.
The Earth itself acts as a magnet with two poles and an enormous magnetic field. At some places on the Earth's surface, its magnetic force is greater than at others. Moreover, the magnetic strength changes with the passage of the Moon around the Earth. The magnetic poles also shift their positions slightly from year to year. The Magnetic North Pole and the Geographic North Pole do not coincide.
Jeff Danger,
Science Ranger
Cool Science Show For Kids