Magnetic Fields The region where the magnetic forces act is called the “magnetic field”
EARTH’SMAGNETISMThe Earth itself has a hugemagnetic field - as if it had ahuge bar magnet embeddedat its centre. The Earth’smagnetic field lines emergefrom near the geographicalnorth pole and re-enter it atthe south pole. The nature ofthe field around the Earthvaries in both strength anddirection. The Earth’smagnetic field is strongest atthe magnetic poles andweakest near the Equator.
Why does the Earth have a magnetic field? The Earth has, at its centre, a dense liquid core, of about half the radius of the Earth, with a solid inner core. This core is though to be mostly made of molten iron and nickel perhaps mixed with some lighter elements. Circulating ions of iron and nickel in highly conducting liquid region of earth’s core might be forming current loops and producing earth’s magnetism.
Magnetic eleMents Magnetic Declination Magnetic Inclination or Magnetic Dip
Magnetic Declination The small angle between magnetic axis and geographic axis at a place is defined as the magnetic declination.
Magnetic Inclination orMagnetic Dip The angle which the direction of total strength of earth’s magnetic field makes with a horizontal line in magnetic meridian.
Atoms themselves have magnetic properties due to the spin of the atom’s electrons. Groups of atoms join so that their magnetic fields are all going in the same direction These areas of atoms are called “domains”
When an unmagnetized substance is placed in a magnetic field, the substance can become magnetized. This happens when the spinning electrons line up in the same direction.
The metals affected by magnetism consist of tiny regions called Domains which behave like tiny magnets. Normally they are arranged in the magnetic material all pointing in different directions in a completely random fashion and so their magnetic effects cancel each other out. If an object is magnetized it is because the domains are all made to point in the same direction. This can be done by stroking the magnetic material with a magnet (or magnets) as shown in the diagram. When aligned the domains reinforce one another and create north and south poles at either end.
Classification of magnetic materials Diamagnetic Substances Paramagnetic substances Ferromagnetic substances
Diamagnetic substances• The diamagnetic substances are those in which the individual atoms or ions do not possess any net magnetic moment on their own.• When such substances are placed in an external magnetizing field, they get feebly magnetized in a direction opposite to a magnetizing field.
Paramagnetic Substances Paramagnetic substances are those in which each individual atom or molecule or ion has a net non zero magnetic moment of its own. When such substances are placed in an external magnetic field, they get feebly magnetized in the direction of the magnetizing field.
Ferromagnetic materialsFerromagneticDomains inMaterials Ferromagnetic material are those in which each individual atom or molecule has a non zero magnetic moment
Hysteresis Curve The relationship between magnetic field strength (H) and magnetic flux density (B) will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation till point (a)
• the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or Retentity as shown in graph at point (b)• The thickness of the middle, describes the amount of hysteresis, related to the coercivity of the material as from (c) to (f)
Hysteresis curve of soft and steel The retentivity of soft iron > retentivity of steel Soft iron is more strongly magnetized than steel Coercivity of soft iron < Coercivity of steel Hence, soft iron loses its magnetism more rapidly than steel does.
Anelectromagnet A soft iron rod has no magnetic field When current flows in the wire the soft iron becomes magnetized so a magnetic field is detected by the plotting compasses.