The unit of retentivity is

Question

Examoogle · Accepted Answer

$Weber/m^2$

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.

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

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

Advantages: Useful for manufacturing permanent magnets.; Enables magnetic data storage technologies.

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

Applications: Permanent magnet production; Magnetic recording media; Relays and memory elements

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

Answer: 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.

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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