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Magnetism and Matter — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsMagnetism and Matter5 Marks
Question
Write the properties of diamagnetic, paramagnetic and ferromagnetic materials.
✓

Answer

Properties of Diamagnetic Materials :
(i) When a bar of diamagnetic material is suspended freely between two magnetic poles of a magnet then it turns to become perpendicular to the magnetic field and the poles produced on the two sides of the bar are similar to the nearer magnetic poles.
(ii) When a diamagnetic material is placed in a strong uniform external magnetising field, the field lines prefer to pass through the surrounding air than to pass through the material itself i.e. field lines get repelled or expelled. Consequently, the magnetic induction 'B' inside material becomes less than magnetising field H. Thus permeability for diamagnetic substance is less but never negative.
(iii) In a non-uniform field a diamagnetic substance moves from stronger to the weaker parts of the field. For instance, if a diamagnetic liquid is placed in a watch glass resting on the pole pieces not more than 2 mm apart, the liquid accumulates on the sides where the field is weaker and creates depression in the middle. The reverse is the effect when the poles are far apart because now the field is stronger near the poles.
(iv) If a diamagnetic solution is poured into a U-tube and one arm of this U-tube is placed between the poles of a strong magnet, the level of the solution in that arm is depressed.
(v) A diamagnetic gas when allowed to ascend in between the poles of a magnet spreads across the field.
(vi) The susceptibility of a diamagnetic substance is independent of temperature and has a negative value e.g. for lead it is $-1.7 \times 10^5$.
Properties of Paramagnetic Materials :
(i) When a rod of a paramagnetic materials is suspen-ded freely between two magnetic poles of a magnet then it rotates until its axis becomes parallel to the magnetising field. The poles produced at the ends of the rod are opposite to the nearer magnetic poles of the magnet.
(ii) When a rod of paramagnetic substance is placed in a strong external magnetising field, the field lines prefer to pass through it than through the surrounding air, i.e. these field lines get slightly more concentrated inside the material. Consequently, the magnetic induction B inside the paramagnetic substance becomes slightly greater than the magnetising field H. Thus permeability for paramagnetic substance is greater than 1, but increase is very small, about I part in $10^5$. For Al and platinum the value of $\chi$ is 1.76 x $10^{-6}$ and 2.88 × $10^{6}$ respectively.
(iii) In a non-uniform field they experience an attractive force towards the stronger part of the field. For example, if a paramagnetic liquid be placed in a watch glass resting on the pole pieces not more than 2 mm apart, the liquid accumulates in the middle where the field is strongest. If the pole-pieces are far apart say, 2 cm or so, the field is strongest near the poles, then the liquid moves away from the centre producing a depression in the middle.
(iv) If the solution of paramagnetic substance is poured in a U-tube and one-arm of the U-tube is placed between two strong poles the level of the solution in that arm rises.
(v) When paramagnetic gases (like traces of ammonia and hydrogen chloride) are allowed to ascend between the poles of a magnet they spread along the field.
(vi) The susceptibility $\chi$ for paramagnetic susbtances is positive and mall e.g. for A l it is $1.76 \times 10^{-6}$ and for platnium it is 2.88 × 10-6.
(vii) $\chi$ and $\mu$ for paramagnetic substances do not change with variation in magnetising field H. $\chi$ varies inversely with the absolute temperature. At some high temperature becomes negative and substance become diamagnetic.
(viii) A paramagnetic substance acts like a diamagnetic substance if it is surrounded by a medium which is more paramagnetic than itself.
Properties of Ferromagnetic Materials
(i) The dipole moment of atoms is permanent and exists in domains.
(ii) The alignment of atomic dipoles and external magnetic fields are in the same direction.
(iii) There is a significant strength of magnetic dipole moment.
(iv) The magnetisation intensity varies linearly with the magnetising field. Moreover, the magnetisation intensity is quite high and positive.
(v) Magnetic susceptibility is relatively high and positive.
(vi) The magnetic flux density is also high and positive. The magnetic field lines inside ferromagnetic materials are dense.
(vii) The relative permeability is also relatively high. It varies linearly with the magnetic field. The magnetic field inside the material is substantially more vital than outside the material. They tend to pull in many force lines from the material.
(viii) The field aggressively attracts ferromagnetic materials. They tend to adhere to the poles where the field is more potent in a non-uniform field.
(ix) Because the field is more vital at poles, if the ferromagnetic powder is placed in a watch glass between two properly faraway poles, powder accumulates on the sides, and the centre is depressed.
(x) At high temperatures, a ferromagnetic substance loses its ferromagnetic characteristics.

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