Which material is typically described as having high permeability and low retentivity and is suitable for transformer cores?

• A. Software iron
• B. Copper
• C. Permanent magnets
• D. Soft iron

Answer: (D)

The material used for transformer cores needs to respond readily to changing magnetic fields and not hold onto magnetization when the field reverses. High permeability means the material easily becomes magnetized, allowing it to carry a large amount of magnetic flux with relatively little magnetizing force. Low retentivity (low remanence) means it won’t stay magnetized once the field is removed, so there’s minimal residual flux and reduced hysteresis losses during each cycle of AC operation. Soft iron fits these requirements well: it has very high permeability and very low coercivity/remanence, so the core efficiently channels flux while keeping losses small as the current alternates. In contrast, copper isn’t magnetic at all, permanent magnets retain magnetization and would cause unwanted residual flux and higher losses, so they’re not suitable for transformer cores.