Magnet and Magnetism

Magnetism describes interactions involving magnetic fields, moving electric charge, magnetic moments and magnetic materials. Motors, generators, transformers, sensors and many other electrical devices use these interactions. A permanent magnet produces a field without continuous external power. It strongly attracts ferromagnetic materials such as iron, cobalt, nickel and many of their alloys. A freely suspended small magnet tends to point along the local magnetic field, which near Earth’s surface is approximately north-south rather than exactly along geographic north and south.

Magnetic Poles

A bar magnet’s external field is strongest near the regions called its poles. By convention, the north-seeking end is the north pole and the other end is the south pole. Opposite poles attract and like poles repel. No isolated magnetic monopole has been observed, so cutting a magnet produces smaller magnetic dipoles rather than separate north and south pieces.

Magnetic Materials

Ferromagnetic materials respond strongly to an applied field because magnetic domains can align. Iron, cobalt, nickel and many alloys are examples. Their permeability is nonlinear, depends on field history and changes as the material approaches saturation.
Paramagnetic materials develop weak magnetisation in the direction of an applied field, so they are weakly attracted where the field is stronger. Their relative permeability is slightly greater than one. Aluminium, magnesium, manganese and platinum are examples under ordinary conditions.
Diamagnetic materials develop an induced magnetic moment opposite to the applied field, so a non-uniform field weakly repels them. Their relative permeability is slightly less than one. Copper, silver, lead, mercury, sulfur and zinc are common examples. Magnetic response can depend on temperature, crystal form and measurement conditions.

Properties of Magnets

1. A magnet strongly attracts suitable ferromagnetic material; it does not attract every metal.
2. A freely suspended magnet points along the local magnetic field, including Earth’s field.
3. Like magnetic poles repel each other, while unlike poles attract.
4. Heating above the Curie temperature removes ferromagnetic order. Lower heating, hard impact or an opposing field can also reduce permanent magnetisation, depending on the material.
5. Dividing an ordinary magnet makes smaller pieces with both north and south poles; it does not isolate one pole.

Types of Magnets

Lodestone is naturally magnetised magnetite and was used in early compasses. The word magnet is commonly linked to Magnesia in Asia Minor, although the exact historical origin is less certain than the traditional account suggests. Naturally magnetised material is now much less common in engineering than manufactured magnets.
Artificial magnets include permanent magnets, soft magnetic cores and electromagnets. Permanent magnets use hard magnetic materials with enough coercivity to retain magnetisation, including ferrites, alnico and rare-earth alloys. An electromagnet uses current in a coil to create a controllable field, often strengthened by a soft iron core. Magnetising ordinary iron does not necessarily make a stable permanent magnet because remanence and coercivity depend on the material.

Applications of Permanent Magnet

Permanent magnets provide a field without a continuously energised coil. They are used in loudspeakers, small motors and generators, position and speed sensors, magnetic latches, measuring instruments and laboratory demonstrations. The required material and shape depend on flux density, temperature, corrosion resistance, size and demagnetising fields.

Differences between Permanent Magnets and Temporary Magnets

1. A permanent magnet retains useful remanent magnetisation without continuous power. An electromagnet’s field is controlled mainly by coil current, turns and magnetic-circuit geometry.
2. A permanent magnet’s pole orientation is set by its magnetisation direction. Reversing electromagnet current reverses its pole orientation when the core and circuit permit it.
3. Removing coil current greatly reduces an electromagnet’s field, but a ferromagnetic core can retain residual magnetisation. The field does not always vanish instantly because inductance delays current change.
4. Neither type is universally stronger. Field strength depends on material limits, size, air gaps, coil heating, current and magnetic saturation.
5. Excess heat, strong opposing fields or mechanical shock can weaken a permanent magnet. An electromagnet needs current for its controlled field, but its core can also be affected by heat, hysteresis and remanence.