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Permanent magnets and Electro magnets

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Permanent magnets and Electro magnets - Lesson Summary

Permanent magnets are substances that can retain their ferromagnetic property for a long period of time at room temperature. We cannot make permanent magnets from all types of materials. The Hysteresis curve helps us to select a suitable material for it. Materials such as steel and alnico that have high retentivity, high coercivity and high relative magnetic permeability are used for making permanent magnets.

Hold a steel rod and rub it with one end of a bar magnet repeatedly in a particular direction. Thus, a permanent magnet is formed. This method of magnetisation is known as single-touch method. However, the most efficient method of making a permanent magnet is to place a suitable magnetic material in the core of a solenoid and pass direct current through it. Thus, the magnetic field due to the current carrying solenoid magnetises the material placed in the core of the solenoid. This method of magnetisation is known as electrical method. Permanent magnets have many applications such as in magnetic door stoppers and magnetic screwdrivers.

The materials used in electromagnets should have high magnetic relative permeability and low retentivity. However, the hysteresis loop for this type of material is narrow.

Considering all these properties, soft iron is the most suitable material for making electromagnets. Place a soft iron rod in the core of a solenoid and pass current through it. The magnetism of the solenoid increases by many times due to the soft iron rod in the core of the solenoid. When the current through the solenoid is switched off, the magnetism due to the solenoid is also switched off because of low retentivity of soft iron. Thus, the electromagnets are referred to as temporary magnets. Electromagnets have their usage in many applications, such as electric bells, loud speakers and telephone diaphragms. Huge electromagnets are used in cranes to lift bulk quantities of iron, steel and machinery.

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