Question

The materials suitable for making electromagnets should have

• A. high retentivity and high coercivity
• B. low retentivity and low coercivity
• C. high retentivity and low coercivity
• D. low retentivity and high coercivity

Hint:

Logic Tip: Compare this to Permanent Magnets! A permanent magnet (like steel) needs exactly the opposite: High Retentivity (to hold a strong magnetic field indefinitely) and High Coercivity (to resist being demagnetized by stray external fields or physical shocks).

Answer 1

The Correct Option is B

Concept:
An electromagnet is a temporary magnet that generates a magnetic field only when an electric current flows through its wire coil. The core material determines how effectively and quickly the electromagnet turns "on" and "off". The suitability of a magnetic material is defined by its hysteresis loop characteristics:

• Retentivity (Remanence): The residual magnetism left in the material after the external magnetizing field is removed.
• Coercivity: The reverse magnetic field required to completely demagnetize the material.

Step 1: Evaluate the Retentivity requirement.
When the electric current is switched off, an ideal electromagnet should immediately lose all its magnetism to release whatever it was holding. If the material had high retentivity, it would remain strongly magnetized even after the power was cut, defeating the purpose of an electromagnet. Therefore, the material must have low retentivity.
Step 2: Evaluate the Coercivity requirement.
Electromagnets are often used in alternating current (AC) applications where the magnetic field reverses direction continuously (e.g., transformers). A material with high coercivity would require a massive amount of energy just to overcome its residual magnetism and flip its magnetic domains, leading to severe energy losses (hysteresis loss). To ensure it can be easily and efficiently magnetized and demagnetized with minimal energy waste, the material must have low coercivity. Thus, the ideal material (like soft iron) possesses low retentivity and low coercivity.