Properties of Diamagnetic Materials – Magnetism & Matters | Class 12 Physics Notes

Here we will study about the Paramagnetic and Diamagnetic Materials. Properties of Diamagnetic Materials & Paramagnetic Substances also discussed in this article.

### Paramagnetic Substances

The substances which when placed in a magnetic field are feebly magnetised in the direction of magnetising field are called paramagnetic substances.

1. The property of paramagnetism is found to exist in substances whose atoms or molecules have an excess of electron spinning in same direction.
2. Atoms of paramagnetic substances possess a permanent magnetic dipole moment and thus behave like small bar magnets called atomic magnets.
3. In absence of external magnetic field paramagnetic substances to not show any magnetism because atomic magnets are randomly oriented so net magnetic dipole moment is zero.
4. In presence of external field each atomic magnet experiences a torque which tries to rotate and align them parallel to direction of magnetic field. The substance acquires net dipole moment and thus gets magnetised in direction of field.
5. The property of paramagnetism is temperature dependent. The thermal agitation on increase of temperature spoils the alignment of atomic magnets which reduces net magnetic dipole moment.

6. Some paramagnetic substances are $A \ell, N a,$ Sb, Pt, $\operatorname{CuC} \ell_{2}, M n,$ Cr, liquid oxygen etc.

### Diamagnetic Materials

1. The substances which when placed in a magnetising field get feebly magnetised in a direction opposite to magnetising field are called diamagnetic.
2. The substances are weakly repelled by the field so in a non uniform field these have a tendency to move from strong to weak field.
3. A diamagnetic rod sets itself perpendicular to field because field is strongest at poles.
4. A diamagnetic liquid in a U-tube depresses in the limb which is between the poles of magnet.
5. Intensity of magnetisation I is very small, negative and proportional to magnetising field.
6. Magnetic susceptibility $\chi_{\mathrm{m}}=\frac{\mathrm{I}}{\mathrm{H}}$ is small and negative $\left(\approx 10^{-5}\right)$
7. The relative permeability $\mu_{\mathrm{r}}=\frac{\mu}{\mu_{0}}$ is slightly less than unity.
8. The field inside the material $\mathrm{B}$ is less than magnetising field H. They have a tendency to expel lines of force.
9. Magnetic dipole moment (M) is small and opposite to magnetising field H.
10. Diamagnetic substances do not obey Curie’s law and show no transition
11. at Curie temperature $X_{m}$ is indepedent of temperature.
12. The origin of diamagnetism is the induced dipole moment due to change in orbital motion of electrons in atoms by applied field.
13. Imp. Diamagnetism is present in all materials and is independent of temperature. As it is weak it is often masked by para and ferromagnetic effects.
14. The atoms do not have any permanent dipole moment i.e. paired spin.
15. Diamagnetism is exhibited by solids, liquids and gases.
16. If a diamagnetic liquid is placed in a watch glass placed. on two pole pieces which are quite close to each other then liquid accumulates at sides and shows depression in the middle, where field is strongest.
17. If a diamagnetic liquid is placed in a watch glass placed on two pole pieces which are sufficiently apart then liquid accumulates in the middle where field is weakest.

Diamagnetic substances are $\mathrm{cu}, \mathrm{zn}, \mathrm{Bi}, \mathrm{Ag},$ Au, Pb, He, $\mathrm{Ar}, \mathrm{NaC} \ell, \mathrm{H}_{2} \mathrm{O},$ marble,glass etc.

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