B-H Hysteresis Loop
Soft iron (thin loop, low loss) vs hard steel (fat loop). Coercivity, retentivity.
Key Notes
Hysteresis: the B vs H curve of a ferromagnet does NOT retrace itself when H is reversed. It forms a CLOSED LOOP.
Starting from unmagnetised material (B = 0, H = 0): as H increases, B follows the INITIAL MAGNETIZATION CURVE up to saturation B_s.
Reducing H to zero: B does NOT return to zero. It retains REMANENT MAGNETIZATION B_r (residual magnetism).
Reversing H: B decreases through zero at COERCIVE FIELD H_c (the negative H required to demagnetize).
Further reverse H: saturates at −B_s. Then reversing again traces out a closed loop.
Area enclosed by the loop = energy dissipated per unit volume per cycle (heat). Source of transformer core losses.
Soft magnets (Si-iron): narrow loop, small area ⇒ small hysteresis loss. Used in transformers.
Hard magnets (alnico, NdFeB): wide loop ⇒ large H_c ⇒ retain magnetization strongly. Used as permanent magnets.
Formulas
Saturation magnetization
M_s = saturation magnetization; ferromagnet can't go higher.
Remanence
Residual flux density.
Coercivity
Field strength needed to demagnetize.
Hysteresis energy loss / cycle / volume
Equal to area of the B-H loop.
Important Points
Hysteresis loop AREA = energy dissipated per cycle ⇒ heating. Critical for AC transformers (50 Hz × area = power loss).
Soft magnetic materials (silicon steel, mu-metal): narrow loop, low loss, easy to magnetize/demagnetize.
Hard magnetic materials (NdFeB, AlNiCo): wide loop, high H_c — once magnetized, hard to un-magnetize.
Demagnetization methods: (i) heat above T_C; (ii) AC field with decreasing amplitude.
Hysteresis enables magnetic data storage — hard drives, magnetic tape rely on retained magnetization.
Common pitfall: thinking hysteresis is a manufacturing defect — it's an INTRINSIC feature of ferromagnets.
B-H Hysteresis Loop notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.