Correct Answer

Concept & PrincipleElectricalElectrical Materials

Option C

$Weber/m^2$

Quick Summary: Retentivity (or remanence) is the ability of a magnetic material to retain its magnetization after the external magnetizing force is removed. It is measured as the residual magnetic flux density ($B_r$) when the magnetic field intensity ($H$) is reduced to zero in the $B-H$ curve of a material.

💡 Explanation

Retentivity (or remanence) is the ability of a magnetic material to retain its magnetization after the external magnetizing force is removed. It is measured as the residual magnetic flux density ( $B_r$ ) when the magnetic field intensity ( $H$ ) is reduced to zero in the $B-H$ curve of a material.

🔢 Key Formulas

$B_r = \frac{\Phi_r}{A}$ — Where $B_r$ is retentivity, $\Phi_r$ is residual flux, and $A$ is the cross-sectional area.

⚙️ Working Principle

When a ferromagnetic material is subjected to a magnetizing force, the magnetic domains align. Upon removing the external force, the internal crystalline structure resists the complete randomization of these domains. The remaining flux density, measured in Tesla or $Weber/m^2$ , represents the material's 'memory' of the magnetic state.

📌 Key Points

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Retentivity is a property of soft and hard magnetic materials.
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Permanent magnets require high retentivity and high coercivity.
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Electromagnets (like transformer cores) generally require low retentivity to minimize hysteresis loss.
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The unit $Weber/m^2$ is equivalent to the Tesla (T).

✅ Advantages

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Useful for manufacturing permanent magnets.
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Enables magnetic data storage technologies.

❌ Disadvantages / Limitations

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Causes hysteresis losses in AC applications.
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Residual magnetism can interfere with sensitive electronic measurements.

🛠️ Applications / Uses

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Permanent magnet production
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Magnetic recording media
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Relays and memory elements

📄 Additional Information

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Option B (Ampere turn/metre) is the unit of magnetizing force ( $H$ ).
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Option A (Ampere turn) is the unit of magnetomotive force (MMF).
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Option D (Weber) is the unit of magnetic flux ( $\Phi$ ).

📊 Diagram / Illustration

✅

C is correct — Retentivity is a measure of residual magnetic flux density, which is defined as flux per unit area, expressed in $Weber/m^2$ or Tesla.

Core Concepts Used

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Hysteresis Loop Residual Magnetism Magnetic Flux Density

💡 EXAM TIP

Always distinguish between magnetizing force ( $H$ , unit: $AT/m$ ) and magnetic flux density ( $B$ , unit: $T$ or $Wb/m^2$ ). In the $B-H$ loop, the intercept on the $B$ -axis is retentivity, while the intercept on the $H$ -axis is coercivity.

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