Magnetic

A magnet is material or object that produces a magnetic field. A "hard" or "permanent" magnet is one which stays magnetized for a long time, such as magnets often used in refrigerator doors. A "soft" or "impermanent" magnet is one which loses its memory of previous magnetizations. more...

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"Soft" magnetic materials are often used in electromagnets to enhance (often hundreds or thousands of times) the magnetic field of a wire that carries an electrical current and is wrapped around the magnet; the field of the "soft" magnet increases with the current. Permanent magnets occur naturally in some rocks, particularly lodestone, but are now more commonly manufactured. A magnet's magnetism decreases when it is heated and increases when it is cooled.

Two measures of a material's magnetic properties are its magnetic moment and its magnetization. A material without a permanent magnetic moment can, in the presence of magnetic fields, be attracted (paramagnetic), or repelled (diamagnetic). Liquid oxygen is paramagnetic; graphite is diamagnetic. Paramagnets tend to intensify the magnetic field in their vicinity, whereas diamagnets tend to weaken it. "Soft" magnets, which are strongly attracted to magnetic fields can be thought of as strongly paramagnetic; superconductors, which are strongly repelled by magnetic fields, can be thought of as strongly diamagnetic.

Quantities

The magnetic field, magnetic moment, and magnetization are vectors, meaning they have direction and magnitude. The magnetic moment and magnetization are properties only of the magnet, while the magnetic field it produces depends on the position relative to the magnet. The magnetic moment points from its south pole to its north pole. Also, its north pole points towards the Earth's geographic north pole, which is a magnetic south pole. A compass needle is approximately a bar magnet.

Magnetic field strength

A magnetic field can be measured using a good magnetic compass (this is a small permanent magnet). The direction of the field at a point in space is the direction in which the compass needle points when it passes through that point and is in equilibrium. The magnitude (or strength, usually denoted by the symbol B) of a magnetic field can also be measured using a compass, if the field is, like the Earth's, nearly uniform over the volume occupied by the needle. The needle is rotated about its center, and this makes it oscillate about its equilibrium position. The period t of oscillation is measured. For small oscillation angles, the frequency of the oscillation, 1/t, is proportional to the square root of B. This is a result from the theory of rotational motion and the theory of the torque on a magnet, and can be tested by creating an electromagnet, which makes a magnetic field proportional to the electric current that it carries. The common unit of magnetic field is the tesla, denoted "T", equal to one N/Amp-m (Force/Current-Distance), and about 20,000 times the Earth's magnetic field. Technically, B should be called the magnetic induction field, because changing B induces an electric field, by Faraday's Law of electromagnetic induction.

Read more at Wikipedia.org