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Retentivity is a property of a material that indicates its ability to retain magnetization. This is particularly relevant in the context of magnetic materials, where retentivity determines how strongly a material can remain magnetized after the external magnetic field is removed.

In terms of electromagnets, low retentivity is desirable. This is because you want the electromagnet to lose its magnetism quickly when the electric current is turned off. High retentivity would mean the material continues to be magnetized even without the current, which is counterproductive for electromagnets that need to switch on and off easily.

It helps to understand retentivity by thinking of materials like soft iron, which have low retentivity and are often used in electromagnets for their quick demagnetizing attributes. They don't "hold on" to their magnetism, making them perfect for applications that require flexibility and responsiveness.

Coercivity refers to the amount of external magnetic field needed to bring a magnetized material to zero magnetization. In simple terms, it's a measure of the resistance of a magnetic material to becoming demagnetized.

For electromagnets, a low coercivity is essential. This means that very little energy is required to change the magnetization of the material, allowing it to start and stop being magnetic quickly as the electrical current is applied or removed.

Materials with low coercivity like soft iron are preferred in building electromagnets. They allow for quick changes in magnetization with minimal energy, ideal for devices that require rapid on/off cycles. This property is part of why electric motors and transformers often utilize materials with low coercivity.

Magnetization signifies how much a material is magnetized in response to an external magnetic field. It is essentially the degree to which a material can be magnetized.

In an electromagnet, magnetization occurs when an electric current flows through a coil of wire wound around a magnetic core, usually made of a material with low coercivity and low retentivity like soft iron.

The magnetization process is temporary in electromagnets. When the external electric current is removed, the magnetization decreases swiftly if the material is suited for this purpose. Understanding magnetization helps in improving electromagnet efficiency, by selecting materials that easily achieve high magnetization with a given current but lose this magnetization quickly when the current is cut.