Q.Which one is correct for a diamagnetic substance ?

Diamagnetics are weakly repelled by a field ($\mu_r<1$, e.g. bismuth); paramagnetics are weakly attracted ($\mu_r>1$, e.g. aluminium).

Diamagnetic materials. These have no net atomic magnetic moment. In an external field a weak moment is induced opposite to the field, so they are feebly repelled and tend to move from stronger to weaker field regions. Their relative permeability $\mu_r < 1$ and susceptibility is small and negative. Examples: bismuth, copper, gold, water, mercury.

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Nickel is one of the three common ferromagnetic elements (Fe, Co, Ni).

Ferromagnetic materials have strongly interacting atomic magnetic moments that align within domains, giving very large positive susceptibility and permanent magnetisation. The classic ferromagnetic elements at room temperature are iron, cobalt, and **n …

Cobalt, iron and nickel are the three common ferromagnetic elements; bismuth is diamagnetic.

Ferromagnetic materials (iron, cobalt, nickel and a few alloys) have unpaired electron spins whose magnetic moments spontaneously align with each other within small regions called domains, even without an external field, giving a large positive susceptibility ($\chi \gg 1$). Bismuth, in contrast, is a classic diamagnetic material — all its electron shells are paired, it develops a weak magnetisation opposed to any applied field, and has a smal …

Diamagnetic materials have susceptibility χ < 0 and μ_r = 1 + χ slightly less than 1, so the field B they support is slightly less than for vacuum — B < H. Option (b).

Step 1 — magnetic properties (NCERT/CBSE Class 12 Physics, Magnetism and Matter): B = μ₀(H + M) = μ₀μ_r H, where μ_r = 1 + χ.

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Nickel is a ferromagnetic material.

Ferromagnetic substances are strongly attracted by a magnet and can be magnetised, owing to spontaneously aligned magnetic domains. The three common elemental ferromagnets are iron, cobalt, and nick …

Diamagnetic materials are weakly magnetised opposite to the applied field, giving them a small negative susceptibility — exactly the signature here.

Reasoning

Magnetic susceptibility $\chi$ classifies materials as:

• Diamagnetic: small negative $\chi$ (typically $-10^{-5}$ to $-10^{-6}$), weakly repelled by a magnetic field.
• Paramagnetic: small positive $\chi$ (typically $10^{-3}$ to $10^{-5}$). …

A diamagnetic material weakly opposes an applied magnetic field, so its relative permeability is slightly less than that of free space.

For a diamagnetic substance, the induced magnetisation is weak and opposite to the applied field, so the material's relative magnetic permeability μr is slightly LESS than 1 (though very cl …