Electromagnets & Permanent magnets

Electromagnets

They are normally used for the purpose of lifting ferromagnetic substance.

 Properties of the material of an electromagnet :

(1)  The material should have low retentivity so that it gets demagnetised easily.

(2)  For the electromagnet to be very strong, it should have a high value of saturation magnetisation.

(3)  It should have low coercivity .

(4)  Hysteresis loss should be small. It should not be heated up during magnetisation and demagnetisation.

The above-mentioned properties are reasonably met with soft iron and so soft iron is usually preferred for an electromagnet.

 Permanent magnets

Normally used in measuring instruments.

Properties of the material of a permanent magnet :  

(1)  It should have high retentivity so that it remains magnetised in the absence of the magnetising field.

(2)  It should have high saturation magnetisation.

(3)  It should have high coercivity so that it does not get demagnetised easily.

Note:Compared to soft iron, steel has got low retentivity. Even then steel is preferred for a permanent magnet as it has got high coercivity.

 Trasformer cores

As the materials used for making transformer cores are subjected to many cyclic changes per second, so they are preferred to have low hysteresis loss. Moreover magnetic field B should be large. As soft iron shows these properties, hence it is always preferred to develop transformer cores.

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