What are permanent magnets? What is an efficient way of preparing a permanent magnet? Write two characteristic properties of materials which are required to select them for permanent magnets.
Solution
A permanent magnet is a piece of ferromagnetic material that retains its magnetisation for a long time at room temperature without needing an external field.
Efficient preparation: place a ferromagnetic rod (e.g. steel) inside a solenoid and pass a strong direct current; the solenoid's field magnetises the rod to saturation, after which the rod stays magnetised. (Stroking with a magnet or placing a steel rod in a strong field also works.)
Required material properties:
(i) High retentivity, so it retains a large magnetisation after the field is removed.
(ii) High coercivity, so the magnetisation is not easily destroyed by stray fields.
Efficient preparation: place a ferromagnetic rod (e.g. steel) inside a solenoid and pass a strong direct current; the solenoid's field magnetises the rod to saturation, after which the rod stays magnetised. (Stroking with a magnet or placing a steel rod in a strong field also works.)
Required material properties:
(i) High retentivity, so it retains a large magnetisation after the field is removed.
(ii) High coercivity, so the magnetisation is not easily destroyed by stray fields.
CBSE Class 12 Board · 2008 · Subjective