Hysteresis, Coercivity, and Magnetic Materials
A hysteresis loop is the cyclic B-H or M-H response of a ferromagnet. Coercive field H_c is the magnitude of reverse magnetizing field required to reduce B or M to the specified zero crossing after saturation.
Why this shows up in the exam
Demagnetizing bar magnets · Choosing transformer-core materials · Choosing hard materials for permanent magnets
Learn the idea
Hysteresis records magnetic history, and coercivity is the reverse H needed to remove residual magnetization. Ferromagnetic domains do not always return immediately when the applied field is removed. A reverse field is needed to undo remanence; soft materials need little reversal, while hard materials resist demagnetization.
🧠 Memory hook: Coercivity is the reverse push needed to erase.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- H_solenoid = n I = N I/L — reverse field produced by a long demagnetizing solenoid
- I = H_c L/N — current needed to supply the coercive field
- energy loss per cycle per volume = loop area — hysteresis loss represented by the B-H loop area
How to approach it
- 1Identify the required reverse H from coercivity
- 2Compute solenoid turns per unit length
- 3Choose soft or hard material from the intended use
Common slip-ups that cost marks
- •Confusing coercivity with retentivity
- •Using total turns instead of turns per unit length in H = nI
- •Choosing a large hysteresis loop for low-loss transformer cores
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A charge of 2 microC moves perpendicular to a 3 T magnetic field at 4 x 10^5 m/s. Find the magnetic force.
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