Question

Which of the following materials is best suited for making magnetic keepers?
A. Steel
B. Iron
C. Alnico
D. Wood

Answer 1

Hint: A magnetic keeper is a bar, aligned to connect the stray poles of a permanent magnet to retain the strength of the magnet by completing the magnetic circuit. Magnets with low coercivity are easily influenced by any external magnetic fields such as the Earth's magnetic field or field produced by a nearby current-carrying conductor. These external fields disorient the domain in the permanent magnet leading to demagnetization.
Magnetic keepers also help to protect other metals from getting attracted towards these poles by closing the magnetic circuit.

Complete step by step solution:

The material best suited for making magnetic keepers should be magnetic enough to strengthen the main magnet but does not cause interference with the field it produces. The fields of the permanent magnet should pass through the magnetic keeper uninterrupted.
Therefore, a wooden bar cannot be used as a magnetic keeper as it is diamagnetic. An Alnico material is highly coercive to the magnetic field and will hence interfere with the domain of the permanent magnet.
Steel and Iron are suitable for making a magnetic keeper as they have low enough coercivity and susceptibility. Iron is preferred to steel as Iron gets magnetized easily and completes the circuit and does not retain any field when it is reattached to the permanent magnet.
Therefore the correct option is B.

Additional information:
A diamagnetic material slightly repels the magnetic field and the material does not retain the magnetic properties. The coercivity of a magnetic material is the field intensity necessary to demagnetize the fully magnetized material.

Note:
1. Coercivity is the ability for the material to resist the change in magnetization of the magnet.
2. Steel is also used as a magnetic keeper but the remaining magnetic field in the steel causes interference.
3. Wood materials are placed between the magnets to prevent them since wood is not a magnetic material. So they are not able to provide the magnetic keeper functions.