Magnetizing and Demagnetizing the Ferromagnetic Core of a Transformer

In a transformer, the ferromagnetic core plays a crucial role in the transformation of electrical energy between the primary and secondary windings. When the transformer is in operation, the core is continuously subjected to magnetic fields, resulting in magnetization and demagnetization processes. Let's discuss the changes that occur in the core's length during these processes.

**Magnetization of the Ferromagnetic Core:**

- When an alternating current (AC) is passed through the primary winding of a transformer, it creates a magnetic field in the core due to the electromagnetic induction effect.
- The magnetic field induces a magnetization in the ferromagnetic material of the core, aligning its magnetic domains in a specific direction.
- During magnetization, the core experiences a change in its length. This change depends on the magnetic properties of the core material, such as its permeability.
- Ferromagnetic materials have high permeability, which means they can be easily magnetized. When magnetized, the core's length decreases.

**Demagnetization of the Ferromagnetic Core:**

- When the AC current in the primary winding changes direction or is turned off, the magnetic field in the core starts to collapse.
- As the magnetic field collapses, the alignment of the magnetic domains in the core material is disrupted.
- This disruption causes the core to lose its magnetization, leading to the demagnetization process.
- During demagnetization, the core experiences another change in its length. Again, this change depends on the core material's magnetic properties.
- In the case of ferromagnetic materials, when demagnetized, the core's length increases.

**Explanation of the Correct Answer:**

Based on the above explanations, we can conclude that when the ferromagnetic core of a transformer is magnetized, its length decreases. Conversely, when the core is demagnetized, its length increases. Thus, option 'A' - decreases when magnetized and increases when demagnetized - is the correct answer to the question.