Animated Solution for Physics - Magnetic Effects of Current: The materials suitable for making electro- magnets should have

Imagine a massive crane in a junkyard, effortlessly lifting a two-ton car and then dropping it precisely into a crusher. This mechanical magic is powered by an electromagnet. Unlike the permanent magnets stuck to your refrigerator, an electromagnet is a temporary magnet. It only exhibits magnetic properties when an electric current flows through its coils.

For an electromagnet to be effective, it must satisfy two critical, almost contradictory conditions. First, when the current is switched on, it must instantly become an incredibly powerful magnet. Second, the absolute moment the current is switched off, it must lose all its magnetism so it can release whatever it was holding.

To find a material that behaves this way, physicists look at the Hysteresis Loop, also known as the $B-H$ curve. In this graph, the horizontal axis ($H$) represents the external magnetizing force (the current we apply), and the vertical axis ($B$) represents the resulting magnetic field induced in the material.

Two specific points on this curve dictate a material's destiny:

1. Retentivity: This is the $y$-intercept of the curve. It tells us how much magnetic field ($B$) is retained by the material when the external magnetizing force ($H$) is reduced to zero. For our junkyard crane to lift heavy cars, the electromagnet must be exceptionally strong. Therefore, we need a material with high retentivity.

2. Coercivity: This is the $x$-intercept of the curve. It represents the reverse magnetizing force ($H$) required to completely wipe out the residual magnetism and bring $B$ back to zero. Because our crane needs to drop the car instantly when the switch is flipped, the material must be incredibly easy to demagnetize. Therefore, we need a material with low coercivity.

Soft Iron

When we plot the $B-H$ curve for Soft Iron, we see a beautifully tall and narrow loop.

The tallness of the loop indicates a high $y$-intercept, confirming that soft iron has high retentivity. It becomes a fiercely strong magnet when current flows.

The narrowness of the loop indicates a very small $x$-intercept, confirming that soft iron has low coercivity. Its magnetic domains are highly mobile; they align easily when the current is on, and they scramble back into randomness the second the current stops, instantly killing the magnetic field.

Permanent Magnets

What if we used steel instead? Steel is used to make permanent magnets. If you look at the hysteresis loop for steel, it is much wider. While it has good retentivity, it has extremely high coercivity. This means once you magnetize a piece of steel, it stubbornly refuses to let go of its magnetism. If you built a crane out of steel, it would pick up the car, but it would never drop it!

Therefore, the golden rule for electromagnets is: High Retentivity and Low Coercivity. This perfectly aligns with option (c).