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⚛️ NEET Physics❓ Question

Curie's law, Curie temperature and hysteresis loops, why soft iron for electromagnets and steel for permanent magnets?

A third-attempt student re-learning magnetic materials from scratch.

Rahul DubeyDropper Sample

9 Aug 2026 · 1 min read

Third attempt, so I am trying to learn the chapters properly instead of passing my eyes over them. Magnetic materials are on my "keeps coming wrong" list.

What I have so far:

  • Diamagnetic: χ\chi small and negative, weakly repelled.
  • Paramagnetic: χ\chi small and positive, weakly attracted. Curie's law: χ=C/T\chi = C/T.
  • Ferromagnetic: χ\chi large and positive, domains.

What I don't have clear:

  1. What happens to a ferromagnet above its Curie temperature? I wrote "it becomes diamagnetic" and I was told that's wrong.
  2. Does Curie's law apply to ferromagnets too? A test question said "magnetic susceptibility is inversely proportional to temperature for" and I wasn't sure.
  3. Hysteresis: define retentivity and coercivity, and why do electromagnet cores use soft iron while permanent magnets use steel or alnico? I know the answer is something about the loop shape but I can't say it clearly in two lines.
  4. Is the energy lost in hysteresis related to the area of the loop?

I have a lot of old notes that say contradictory things, so please check my first part as well.

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7 answers & replies

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  • [+]Arjun Nair12thSample· 2mo ago

    Your first part is good. Add: diamagnetism is present in all materials but is overwhelmed in the other two; susceptibility of diamagnets is roughly independent of temperature. Superconductors are perfect diamagnets, χ=−1\chi=-1.

    1. Above the Curie temperature TcT_c a ferromagnet turns paramagnetic (domains break up by thermal motion). In that range χ=CT−Tc\chi = \dfrac{C}{T-T_c} (Curie-Weiss form).
  • [+]Kabir Joshi11thSample· 2mo ago

    For Q2 the answer is "paramagnetic materials", right? since Curie's law is χ=C/T\chi = C/T and that applies to all magnetic materials.

    • [+]Arjun Nair12thSample· 2mo ago

      Careful. χ∝1/T\chi\propto1/T is for paramagnets. Diamagnets do not follow it (they are almost temperature-independent), and ferromagnets only roughly follow the Curie-Weiss form above TcT_c. If an option says "paramagnetic substances" pick that.

      • [+]Kabir Joshi11thSample· 2mo ago

        Okay, my "all" was too much. Paramagnetic only.

  • [+]Pooja Iyer12thSample· 2mo ago

    Hysteresis definitions that worked for me:

    • Retentivity (remanence): the B left in the material after you bring the external field H back to zero. "Retained magnetism".
    • Coercivity: the reverse H you need to apply to bring the B back to zero.

    Mnemonic: Retentivity = what it retains. Coercivity = the coercion (force) needed to erase it. ✨

  • [+]Zoya Khan12thSample· 2mo ago

    Why soft iron vs steel is a loop-shape answer, yes:

    Soft iron (electromagnet, transformer core): high permeability, low coercivity, narrow loop. It magnetises and demagnetises easily and quickly. A narrow loop means little area, so little energy lost as heat per cycle when the current alternates.

    Steel / alnico (permanent magnets): high retentivity and high coercivity, wide loop. Once magnetised, it stays magnetised and resists demagnetising.

    And on Q4: yes, the energy dissipated per cycle per unit volume equals the area of the B-H loop.

    • [+]Rahul DubeyDropperSample· 2mo agoAuthor

      That is the cleanest two-line answer I've seen: wide loop = keep it, narrow loop = switch it fast. Rewrote the whole page of notes. Thanks everyone, this thread replaces my old mess.

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