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Materials having good retentivity are?
A) Strong magnets
B) Weak magnets
C) Temporary magnets
D) Permanent magnets
Permanent magnets
Materials with high retentivity retain a large portion of their residual magnetism even after the external magnetizing force is removed. This characteristic hysteresis property makes them suitable for manufacturing permanent magnets that maintain a stable magnetic field indefinitely.
Materials with high retentivity retain a large portion of their residual magnetism even after the external magnetizing force is removed. This characteristic hysteresis property makes them suitable for manufacturing permanent magnets that maintain a stable magnetic field indefinitely.
BrтАЛ=Residual┬аFlux┬аDensity┬а(Retentivity) тАФ measure of magnetism retained at H=0
HcтАЛ=Coercive┬аForce┬а(Coercivity) тАФ reverse field required to reduce flux density to zero
When a ferromagnetic material is magnetized to saturation and the magnetizing force H is reduced to zero, the remaining magnetic flux density B is called remanence or retentivity. Materials with high retentivity and high coercivity resist demagnetization, keeping their magnetic domains aligned.
High retentivity ensures that the material holds significant magnetism after the field is removed.
High coercivity is required alongside high retentivity so the permanent magnet resists demagnetization.
Common materials used for permanent magnets include Alnico, Hard Ferrites, and Rare-Earth alloys (e.g., NdFeB).
Provides a continuous magnetic field without requiring electrical power consumption.
Generates zero continuous heat loss (I2R) compared to electromagnets.
Magnetic field strength cannot be easily controlled or turned off.
Properties can degrade over time due to high temperature, physical impact, or opposing fields.
DC motor field poles and permanent magnet synchronous motors (PMSM)
Loudspeakers, microphones, and magnetic sensors
Analog measuring instruments (PMMC meter movement)
| Feature | Permanent Magnets | Temporary Magnets |
|---|---|---|
Retentivity | High (BrтАЛ) | Low (BrтАЛ) |
Coercivity | High (HcтАЛ) | Low (HcтАЛ) |
Hysteresis Loss | Large loop area | Small loop area |
Core Material | Alnico, NdFeB, Hard Ferrite | Silicon Steel, Soft Iron |
Option A & B: Strong or weak magnets refer to the strength of the magnetic field produced, not directly to the property of retaining magnetism.
Option C: Temporary magnets (like soft iron) have low retentivity and low coercivity, losing their magnetism quickly once the external magnetizing force ceases.
D is correct тАФ Materials with high retentivity retain magnetic flux density after the magnetizing field is removed, making them ideal for permanent magnets.
Remember: Permanent magnets require both high retentivity and high coercivity (wide hysteresis loop), while transformer cores require low retentivity and low coercivity (narrow loop) to minimize hysteresis loss.