Magnetic Materials and Earth's Magnetism
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Direct answer
Diamagnetic, paramagnetic and ferromagnetic behaviour is graded by susceptibility chi: diamagnets carry a tiny negative chi (about −10^-5), are feebly repelled from stronger fields and include bismuth, copper and water; paramagnets carry a small positive chi (about +10^-5) that obeys Curie's law chi proportional to 1/T; ferromagnets such as iron have chi in the hundreds to thousands, respond through domains, follow hysteresis loops, and above the Curie temperature (1043 K for iron) collapse into paramagnetism obeying Curie–Weiss behaviour. Earth behaves as a huge magnet with its north-type pole near the geographic south; the three magnetic elements are declination D, dip I and horizontal component B(H), linked by B(H) = B cos(I) with B(V) = B sin(I).
What you must remember
- The trio at a glance: diamagnetic — chi small negative, moves from stronger to weaker field, magnetisation opposite B; paramagnetic — chi small positive, aligns along B; ferromagnetic — chi large positive, domains align, strongly attracted.
- Curie laws: chi proportional to 1/T for paramagnets; ferromagnets above the Curie temperature become paramagnetic with chi proportional to 1/(T − T(C)); iron's Curie point is 1043 K.
- Superconductor edge case: a perfect diamagnet, chi = −1, expelling flux entirely (Meissner effect) — the strongest diamagnetic response possible.
- Hysteresis loop: B versus H traces a loop; the loop area is energy lost per cycle per unit volume; soft iron (narrow loop) suits electromagnets and transformers, hard steel (fat loop) suits permanent magnets.
- Intensity relations: I = chi H, B = mu(0)(H + I) = mu H, mu(r) = 1 + chi; only for linear materials is chi a constant.
- Earth's elements: declination D (angle between geographic and magnetic meridians), dip I (angle of the field with the horizontal), B(H) = B cos(I), B(V) = B sin(I), tan(I) = B(V)/B(H); at the magnetic equator I = 0, at the magnetic poles I = 90 degrees.
- Neutral points: where a magnet's field exactly cancels B(H) — a compass there stands anywhere; their count differs for N–S versus E–W placement of the bar magnet, a classic Main question.
- Pattern note: Main asks the classification table and dip/B(H) arithmetic; Advanced asks hysteresis energy and neutral-point geometry.
Deciding what a mystery specimen is
Lower a specimen toward a strong field gap and watch three personalities. It is pushed out of the gap — diamagnet; the induced moments oppose the field (Lenz at the atomic scale) and the material retreats to weaker field. It is feebly drawn in, less when heated — paramagnet; permanent atomic moments thermal-agitate out of alignment, and Curie's law makes the attraction fade as 1/T. It slams into the gap with memorable violence and stays magnetised after removal — ferromagnet; domains have switched walls and rotated, and the remanence records its history in a hysteresis loop. Liquid oxygen visibly bridges the gap between pole pieces — the standard paramagnet viva exhibit.
Earth magnetism uses the same numbers in navigation dress. At a place where B(H) = 3 × 10^-5 T and dip I = 30 degrees, the total field is B = B(H)/cos(I) = 3.46 × 10^-5 T and B(V) = B tan(I) = 1.73 × 10^-5 T; a dip circle's needle reads 30 degrees there, zero at the magnetic equator and 90 degrees at the magnetic poles.
Where students slip
The direction of induced magnetisation decides the sign of chi, and candidates forget that diamagnets are repelled — weakly, but genuinely — so a material moving out of a strong field is not "non-magnetic", it is diamagnetic. Second, paramagnetism versus ferromagnetism is not a strength knob alone: paramagnets lose their alignment with the field removed and obey Curie's law, ferromagnets keep domains and show hysteresis; the loop area — energy dissipated per cycle — is the quantitative separator Advanced examines. Third, Earth's "north magnetic pole" is a south-type pole of the equivalent magnet (that is why the compass's north-seeking end points there); mixing this in declination questions scrambles the geometry.
Frequently asked questions
How are dia-, para- and ferromagnetic materials distinguished?
By susceptibility and response: chi slightly negative and repulsion (diamagnetic), chi slightly positive with Curie-law temperature dependence (paramagnetic), chi large with domains and hysteresis (ferromagnetic).
What happens to a ferromagnet above its Curie temperature?
Thermal agitation destroys domain order; it becomes paramagnetic with susceptibility following chi proportional to 1/(T − T(C)), the Curie–Weiss law.
Why does a paramagnetic specimen's attraction weaken on heating?
Alignment of permanent atomic moments competes with thermal randomisation, and Curie's law chi proportional to 1/T dilutes the response as temperature rises.
What are the three elements of Earth's magnetism?
Declination D (angle between geographic and magnetic meridians), dip or inclination I (angle of the field with horizontal), and the horizontal component B(H); together they specify the local field completely.
What is a neutral point in magnetism?
A location where the magnet's field exactly cancels Earth's horizontal component so the net horizontal field vanishes and a compass needle stays in any orientation.