Animated Solution for Physics - Magnetic Effects of Current: Hysteresis loops for two magnetic materials A and B are as given below These materials are used to make magnets for electric generators, transformer core and electromagnet core. Then, it is proper to use

When we place a ferromagnetic material inside an external magnetic field $H$, the material gets magnetized, and a net magnetic field $B$ is established inside it. However, the relationship between $B$ and $H$ is not linear. If we increase $H$ and then decrease it back to zero, the material does not completely lose its magnetism. This lagging of magnetic flux density $B$ behind the magnetic field intensity $H$ is known as magnetic hysteresis.

Let's break down the anatomy of a hysteresis loop. The point where the curve intersects the y-axis (when $H=0$) is called Retentivity. It represents the residual magnetism left in the material even after the external magnetizing field is removed. The point where the curve intersects the negative x-axis (when $B=0$) is called Coercivity. It represents the reverse magnetic field required to completely wipe out the residual magnetism.

Most importantly, the area enclosed by the hysteresis loop has a profound physical significance. It is directly proportional to the net energy absorbed per unit volume by the material during one complete cycle of magnetization and demagnetization. This energy is dissipated as heat, which we call hysteresis loss.

Now, let's apply this to real-world devices. Transformers and electromagnets operate on alternating current (AC). This means the magnetic core inside them is subjected to rapid, continuous cycles of magnetization and demagnetization—often 50 or 60 times a second!

If we use a material with a large hysteresis loop area (like Material A), the core will dissipate a massive amount of energy as heat, leading to severe inefficiency and dangerous overheating. Therefore, for transformers and electromagnets, we strictly require a material with a narrow hysteresis loop (like Material B).

Furthermore, an electromagnet must be able to turn its magnetic field on and off instantly. This requires the material to have low retentivity (so it doesn't stay magnetized when switched off) and low coercivity (so it's easy to demagnetize). Material B perfectly fits all these criteria.

Material A has a broad loop, meaning it has high retentivity and high coercivity. While it would be terrible for a transformer, these exact properties make it the ultimate choice for making permanent magnets. A permanent magnet needs to hold onto its magnetism strongly (high retentivity) and resist being demagnetized by stray external fields or physical shocks (high coercivity).

By understanding the geometry of the B-H curve, we can engineer the perfect magnetic materials for any technological application!