When Magnetic domains remain aligned or extended after the influence of a magnetic field is removed, he material is said to be magnetized. This residual field is called Remanence or a Residual magnetic field. The ability of materials to retain magnetism after the magnetic force has been removed is called retentivity.
Although described as magnetic lines, the magnetic field within and surrounding magnetized materials is continuous. When a paper is placed over a magnet and fine iron particles are sprinkled on the paper, the iron particles align with the magnetic field in distinct lines of equipotential magnetic intensity and appear to form lines. Therefore, the magnetic field is referred to as lines or lines of force.
All magnets have two poles, North and South. A permanent bar magnet exhibits polarity; if freely suspended, one end of the bar will point toward the earth's magnetic North Pole. This north-seeking end of the magnet is called the South Pole; the opposite end is the North Pole.
If a bar magnet is U-shaped (horseshoe), the polarity remains but the magnetic field and the lines of force are more concentrated in the gap between the ends of the bar. If the magnetized bar is formed into a close or fused loop, the magnetic field is fully contained within a closed circuit in the magnetic material and no external magnetic lines of field exists.
Magnetic lines of force have the following properties:
- They formed closed loops
- They do not cross one another
- They seek paths of least magnetic resistance
- Their density decreases as distance from the poles increases
- They are considered to have direction by convention, from north to south external to the magnet.
When two magnets are moved into close proximity to each other, a reaction occurs. The like poles repel each other and unlike poles attract one another.