Answer: Low retentivity and a narrow hysteresis loop, minimising hysteresis energy loss per AC cycle.
- A High retentivity and a wide hysteresis loop, much like a permanent bar magnet as widely reported
- B Low retentivity and a narrow hysteresis loop, minimising hysteresis energy loss per AC cycle
- C Zero magnetic permeability, behaving magnetically just like a non-magnetic vacuum in standard practice
- D Negative magnetic susceptibility, behaving magnetically like a diamagnetic substance under most conditions encountered
Correct answer: B. Low retentivity and a narrow hysteresis loop, minimising hysteresis energy loss per AC cycle
Explanation: Transformer cores undergo continuous magnetisation and demagnetisation as the AC current alternates, so a material with a narrow hysteresis loop (like soft iron) is used to minimise the energy lost as heat in each cycle. Materials with a wide hysteresis loop (like steel) are better suited for permanent magnets.
Magnetic field of a bar magnet: field lines emerge from the north pole and re-enter at the south pole outside the magnet, forming continuous closed loops. Image: Geek3, CC BY-SA 3.0, via Wikimedia Commons.
Concept context
Bar magnets, Earth's magnetism, and how different materials respond to an external magnetic field - diamagnetic, paramagnetic, and ferromagnetic behaviour.