What is the significance of B-H curve?

Answer 1

H is the symbol for magnetic field strength, which is defined as the magnetomotive force per unit length of a magnetic circuit, where the magnetomotive force is provided by a current-carrying coil, wound around that magnetic circuit. Magnetomotive force is the product of the current flowing through the coil and the number of turns, expressed in amperes (although often spoken as "ampere turns").

The magnetomotive force gives rise to the magnetic flux within the magnetic circuit, the intensity of which is termed flux density (symbol B), expressed in teslas.

A B-H curve plots changes in a magnetic circuit's flux density as the magnetic field strength is gradually increased. The resulting shape indicates how the flux density increases due to the gradual alignment of the magnetic domains (atoms, that behave like tiny magnets) within the magnetic circuit material. When all the domains have aligned, the B-H curve reaches a plateau and the magnetic circuit is said to be saturated. At this point, any further increase in magnetic field strength has no further effect on the flux density. Different magnetic materials, such as iron, steel, etc., have B-H curves with different slopes and points at which saturation occurs.

After reaching saturation, a reduction in the magnetic field strength results in a reduction in the flux density. However, the resulting curve does not quite match the original curve, but 'lags behind' it. This effect is called hysteresis, which is from the Greek, meaning to 'lag behind'.

When the magnetic field strength reaches zero, the resulting curve indicates that the flux density has not, itself, reached zero. The value of flux density remaining is termed the remanence (or residual magnetism) of the magnetic material. 'Soft' magnetic materials, used in the manufacture of transformer cores, etc., will have a very small remanence; whereas 'hard' magnetic materials, used in the manufacture of permanent magnets, will have a very high remanence.

In order to remove any remanence, the magnetic field strength requires to be reversed (by reversing the direction of the current in the coil) and increased in the opposite direction. The amount of 'negative' magnetic field strength necessary to completely remove the remanence is called coercivity.

If we continue to increase the negative magnetic field strength, the magnetic material will again reach saturation in the opposite direction, and the new curve will be a mirror image of the original curve. The complete B-H curve is then usually described as a hysteresis loop. The area contained within a hysteresis loop indicates the energy required to perform the 'magnetise - demagnetise' process.

'Soft' magnetic materials require relatively little energy to become magnetised and demagnetised and, so, have 'narrow' hysteresis loops, whereas 'hard' magnetic materials require a great deal of energy and have 'wide' hysteresis loops.

So, B-H Curves and Hysteresis Loops are a valuable tools for comparing the characteristics and behaviour of different magnetic materials, in order to select them for an appropriate application.