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What is hysteresis? I only know its related to the frequency.
Hysterisis mens “Lagging Behind”
Hysteresis phenomena occur in magnetic materials, (as well as in the elastic, electric, and magnetic behavior of materials,) in which a lag occurs between the application and the removal of a field (or force) and its subsequent effect
The lack of retraceability of the magnetization curve is the property called hysteresis
When an external magnetic field is applied to a ferromagnetic substance, the atomic dipoles align themselves with the external field. Even when the external field is removed, part of the alignment will be retained: the material has become magnetized.
The relationship between magnetic field strength (H)(EXTERNAL) and magnetic flux density (B) (ACQUIRED) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it 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.
If the magnetic field is now reduced linearly, 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 remanence (RETENTIVITY).
If this relationship is plotted for all strengths of applied magnetic field the result is a sort of S- shaped loop. (See the diagram) The ‘thickness’ of the middle bit of the S describes the amount of hysteresis, related to the coercivity of the material.
“It is said that an iron piece behaves as a magnet when an external magnetic field is applied. Then otherwise why doesn’t it behave as a magnet even though the earth’s magnetic field is acting on it.”
Every molecule of iron possess magnetic behavior. But in the absence of an external strong magnetic field, the individual molecular magnets form closed loops or they are arranged in a random fashion in such a way that the net magnetic effect is zero.
Since the magnetic dipole moments of all the molecular magnets inside the iron add up to zero, the iron rod does not respond to earth’s magnetic field like a magnet.
Further, the strength of earth’s magnetic field is not so large to magnetize a piece of iron. The coercivity of iron is also very less (it can easily lose any acquired magnetism)
However, steel buried in North South direction is found to acquire some magnetism.