Why is the core of an electromagnet usually made of soft iron rather than steel or other materials?

Introduction / Context:
Electromagnets are coils of wire that become strongly magnetised when electric current flows through them. The material placed as the core inside the coil has a major effect on how strong and how controllable the magnetic field is. In practical devices such as relays, electric bells, and lifting magnets, the core material is usually soft iron. This question checks whether you understand the magnetic properties that make soft iron suitable, especially the ideas of magnetic susceptibility and retentivity.

Given Data / Assumptions:

• The core is part of a typical electromagnet used in basic electrical devices.
• Soft iron and other magnetic materials such as steel differ in magnetic susceptibility and retentivity.
• We want the electromagnet to magnetise strongly when current flows and demagnetise quickly when current is switched off.
• We assume standard definitions of susceptibility and retentivity from magnetism theory.

Concept / Approach:
Magnetic susceptibility measures how easily a material becomes magnetised in an external magnetic field. Retentivity measures how strongly a material retains magnetisation when the external field is removed. For an electromagnet, we want a core that quickly becomes strongly magnetised when current is applied, which requires large susceptibility. At the same time, we want it to lose magnetisation almost completely when current is switched off, which requires small retentivity. Soft iron has exactly this combination: high susceptibility and low retentivity. Hard steel, in contrast, has high retentivity and is used for permanent magnets instead, not for electromagnets.

Step-by-Step Solution:
Step 1: Recall that in an electromagnet, current through the coil creates a magnetic field that magnetises the core. Step 2: For a strong magnet when the current flows, the core material should become magnetised easily; this means it should have large magnetic susceptibility. Step 3: When the current is switched off, we generally want the electromagnet to lose its magnetism, so that the device can reset or release objects. Step 4: To lose magnetisation easily, the core must have small retentivity, so that it does not behave like a permanent magnet. Step 5: Soft iron is known to have large susceptibility and small retentivity, making it ideal for electromagnet cores. Step 6: Therefore, the correct reason is that soft iron has large magnetic susceptibility and small retentivity.

Verification / Alternative check:
If we used hard steel instead of soft iron, the electromagnet would retain significant magnetisation after the current is switched off, behaving more like a permanent magnet. This is desirable in devices like permanent magnets but not in switching devices or instruments where rapid control of magnetism is needed. Experimental B H curves for soft iron show steep initial rises (high susceptibility) and narrow hysteresis loops with small remanence (low retentivity). This confirms that soft iron gives strong but easily reversible magnetisation, matching the requirements of an electromagnet core.

Why Other Options Are Wrong:
Small magnetic susceptibility and small retentivity: This material would not magnetise strongly and would give a weak electromagnet.
Large density and large retentivity: Density is not the main factor here, and large retentivity would make the core behave like a permanent magnet, which is not desired.
Small density and large retentivity: Again, retentivity is too high in this case, and density does not determine magnetic behaviour directly.

Common Pitfalls:
Learners sometimes focus on density or mechanical strength of the core material instead of its magnetic characteristics. Another common confusion is between the needs of a permanent magnet and an electromagnet. Permanent magnets require high retentivity, while electromagnets require low retentivity for easy switching. Remember that soft iron is the standard choice for electromagnet cores precisely because it magnetises and demagnetises readily.

Final Answer:
The core of an electromagnet is made of soft iron because it has large magnetic susceptibility and small retentivity.