Experiments for B. Tech. 1st Year Physics Laboratory
Experiment No. 5
B – H curve
Aim :
To study B-H curve and to find out the values of coercivity, retentivity and saturation magnetisation of experimental material. (commercial Nickel).
Apparatus Used :
Set up for B-H curve, experimental material (commercial Nickel), CRO, connecting leads.
Theory Introduction A precise knowledge of various magnetic parameters of ferromagnetic substances and the ability to determine them accurately are important aspects of magnetic studies. These not only have academic significance but are also indispensible for both the manufacturers and users of magnetic materials. The characteristics which are usually used to define the quality of the substance are coercivity, retentivity, saturation magnetisation and hysteresis loss. Furthermore, the understanding of the behaviour of these substances and improvement in their quality demand that the number of magnetic phases present in a system is also known. 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
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Experiments for B. Tech. 1st Year Physics Laboratory 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 or retentivity) 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 applications..
Design Principle
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Experiments for B. Tech. 1st Year Physics Laboratory
When a cylindrical sample is placed coaxially in a periodically varying magnetic field (say by the solenoid) the magnetisation in the sample also undergoes a periodic variation. This variation can be picked up by a pickup coil which is placed coaxially with the sample. Normally, the pickup coil is wound near the central part of the sample so that the demagnetisation factors involved are ballistic rather than the magnetometric.
For the uniform field Ha produced, the effective field H acting in the cylindrical sample will be
H = Ha - NM where M is the magnetisation, or NJ H H a 0 where N is the normalized demagnetisation factor including 4π and J is the magnetic polarization defined by
B 0 H J with magnetic induction B = µH and
B = µ0(H+M) B = µ0 ( H + ) J = (휇 − 휇 )퐻 Based on these eqs. an electronic circuit may be designed to give the values of J and H and hence the Hysterisis loop (J-H loop).