In a magnetic material hysteresis loss takes place primarily due to

Question

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Option A: High retentivity

Explanation & Concepts:
Hysteresis loss is the energy dissipated as heat when a ferromagnetic material is subjected to a cycle of magnetization and demagnetization. This energy loss is directly proportional to the area enclosed by the B-H hysteresis loop, which is determined by the material's magnetic properties including retentivity and coercivity.

Working Principle: As the magnetizing force (H) varies, the domain walls within the ferromagnetic material experience resistance due to molecular friction or crystal lattice defects. This causes the flux density (B) to lag behind the magnetizing force, resulting in a closed loop known as the B-H curve. The work done to overcome this internal friction during each reversal of the magnetic field is dissipated as heat, representing the hysteresis loss.

Key Formulas: $W_h = \eta B_{max}^{1.6} f V$ — Steinmetz hysteresis loss formula where $\eta$ is the Steinmetz coefficient, $B_{max}$ is maximum flux density, $f$ is frequency, and $V$ is volume.

Key Points: Hysteresis loss is independent of the resistivity of the material.; It is directly proportional to the frequency of the magnetic field reversals.; The area of the B-H loop represents the energy dissipated per unit volume per cycle.; Materials with high retentivity and high coercivity have larger hysteresis loops and higher losses.

Advantages: Used in permanent magnets (high retentivity/coercivity materials); Hysteresis loops assist in characterizing magnetic material suitability for specific applications

Disadvantages: Reduces efficiency in electrical machines (transformers, motors); Causes unwanted heating in iron cores

Applications: Magnetic storage media; Transformer cores (choosing low-hysteresis loss soft iron)

Additional Info: The question states 'It high retentivity' which is a simplified way of identifying materials with a wide hysteresis loop, characteristic of hard magnetic materials.. Option C (Flux density lagging) is a symptom/definition of hysteresis, but the loss specifically occurs due to the energy consumed during that process.. Option D (Rapid reversals) increases the total power loss per second because the energy lost per cycle is multiplied by the frequency.

Answer: A is correct — High retentivity often implies a wide B-H loop, which is associated with higher energy dissipation per cycle in ferromagnetic materials.

Exam Tip: For transformer core design, always look for materials with low coercivity and narrow B-H loops to minimize hysteresis losses, whereas permanent magnets require the opposite.

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Option B: Molecular friction

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Option C: Flux density lagging behind the magnetising force

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Option D: Rapid reversals of its magnetisation

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