JEE/NEET Physics · Magnetism & Matter series · Part 4 of 4 · All parts →
- All atoms are tiny dipoles; materials differ in how these dipoles respond to fields
- Diamagnetic: weakly REPELLED — induced moments opposing the field (bismuth, water, copper)
- Paramagnetic: weakly ATTRACTED — existing moments partially align (aluminium, oxygen)
- Ferromagnetic: strongly attracted — domains align and STAY (iron, cobalt, nickel)
- Curie temperature: above it, ferromagnets lose order and become paramagnetic
Put water, aluminium, and iron near a magnet and you get three different stories: water shyly retreats, aluminium leans in gently, iron slams into it and stays magnetized. Every material answers magnetism in its own dialect — and there are exactly three. Part 4 of the Magnetism & Matter series.
- The three temperaments
- Diamagnets: the skeptics
- Paramagnets: the willing
- Ferromagnets: the devoted (and domains)
- Curie temperature
- Solved examples
- Common mistakes
- This physics in your daily life
- Practice set
- Recap
The Three Temperaments
| Class | Behaviour | χ (susceptibility) | Examples |
|---|---|---|---|
| Diamagnetic | weakly repelled | small negative | water, copper, bismuth, gold |
| Paramagnetic | weakly attracted | small positive | aluminium, platinum, O₂ |
| Ferromagnetic | strongly attracted, retains magnetization | large positive | iron, cobalt, nickel, alloys |
Diamagnets: The Skeptics
An applied field induces orbital-current changes that OPPOSE the field (Lenz’s instinct at the atomic scale) — every material has this baseline skepticism; diamagnets have nothing else. Water is diamagnetic: frogs and strawberries have famously levitated in 16 T fields.
Paramagnets: The Willing
Atoms already carry permanent dipole moments (unpaired electrons). Thermal chaos randomizes them; a field gently biases alignment — attraction grows as 1/T (Curie’s law): cool a paramagnet and it tries harder.
Ferromagnets: The Devoted (and Domains)
Quantum exchange coupling locks neighbouring moments PARALLEL into domains (micron-scale magnetic continents). An external field grows favourable domains at the expense of others — and when the field leaves, the alignment largely REMAINS: permanent magnetization, hysteresis, memory.
Curie Temperature
Heat a ferromagnet past its Curie point (iron: 770 °C) and thermal chaos defeats the exchange coupling: domains dissolve, only paramagnetic willingness remains. Magnetization above Curie = forbidden by thermodynamics’ crowd-against-order logic.
Solved Examples
Dia, para, ferro — the three temperaments in one line.
✔
Answer: Dia, para, ferro
B = μ₀(1+χ)H ≈ μ₀×501×(10⁻⁴/μ₀)·… B ≈ 5×10⁻² T — hundreds of times amplified.
This amplification is why iron cores transform electromagnets.
✔
Answer: ≈50 mT
Above 770 °C the domains dissolve (paramagnetic, weak); cooling below Curie re-forms domains — and the earth’s field plus the poker’s history re-aligns them partially.
✔
Answer: Domains die above Curie, revive below
- ‘Non-magnetic’ materials. Nothing is truly non-magnetic: paper and water are weak diamagnets — everything answers.
- Paramagnet = permanent magnet. No — paramagnets lose alignment instantly with the field; only FERROmagnets remember (domains).
- χ sign mix-ups. Dia: negative; para: small positive; ferro: large positive and field-dependent — the sign IS the classification.
- Curie point as melting. Iron loses magnetism at 770 °C, long before melting (1538 °C): two very different thresholds.
This Physics in Your Daily Life
- Fridge doors and hard drives — ferromagnetic memory: domains storing your shopping list and your files alike.
- Neodymium magnets in earbuds and motors — engineered ferromagnets with enormous domain stability: sound and motion by the devoted class.
- MRI contrast agents (gadolinium) — paramagnets enhancing image signals: medicine borrowing paramagnetic willingness.
- The famous levitating frog — water’s diamagnetism at 16 T: scepticism strong enough to float a living thing.
- Demagnetized screwdrivers by heat —Curie physics in the workshop: drop below the point, re-magnetize by stroking on a magnet.
Ask three diners to join a conga line: the skeptic steps AWAY (induced opposition — diamagnet), the polite one joins while you hold their hand but drifts off when you leave (paramagnet), and the enthusiast not only joins but brings friends and keeps dancing after you’ve gone home (ferromagnet). Same invitation, three temperaments.
Susceptibilities: water’s χ ≈ −10⁻⁵ (barely departs); aluminium’s +2×10⁻⁵ (barely joins); iron’s up to ~10⁵ (joins overwhelmingly and stays). Nine orders of magnitude between tap water and transformer steel — the widest range of any material property.
Sketch domains as arrows within crystal grains: ferromagnet’s arrows locked parallel per region, randomly oriented between regions; the field flips whole regions to consensus. The paramagnet: free-spinning arrows weakly biased by field, thermal arrows fighting back. The diamagnet: no arrows at all — the field INDUCES counter-arrows by Lenz’s instinct.
Practice set (answers hidden — try first)
(NEET-level) Classify bismuth:
(JEE Main-level) A material retains magnetization after the field leaves:
(NEET-level) Above the Curie point, iron becomes:
(Concept) Every material has a diamagnetic contribution because:
(JEE Main-level) Paramagnetic susceptibility grows when:
- three classes by χ’s sign and size
- dia: induced opposition (all matter’s baseline)
- para: thermal moments, weakly willing (1/T)
- ferro: domains, memory, huge χ
- Curie point dissolves domains
- 🔁 classification table
- 🔁 origin of each class
- 🔁 domain picture
- 🧠 Chant: ‘skeptic, polite, devoted’.
- 🧠 Memory test: ‘only ferro remembers’.
- 🏠 Daily: hard drives = domain memory.
- 🏠 Daily: levitating frog = water’s diamagnetism.
Quick revision
- All atoms are tiny dipoles; materials differ in how these dipoles respond to fields
- Diamagnetic: weakly REPELLED — induced moments opposing the field (bismuth, water, copper)
- Paramagnetic: weakly ATTRACTED — existing moments partially align (aluminium, oxygen)
- Ferromagnetic: strongly attracted — domains align and STAY (iron, cobalt, nickel)
- Curie temperature: above it, ferromagnets lose order and become paramagnetic
- Ferromagnets: the devoted (and domains)
Have a doubt on this topic?
