需要Yahoo的愛因斯坦助我解答這磁石問題

磁石是否多數用鋼做的呢？

Magnets are made from ferromagnetic materials or alloys containing such material. Only the elements iron, nickel and cobalt are ferromagnetic.
----------------------------------------
那麼做一粒磁石成本是否好高呢？

It depends on the strength and size of the magnet. A small bar magnet, e.g. those used in compass as compass needles, is not expensive. However, the cost of a large permanent magnet used in industry can be quite high.
---------------------------------------------
咁點解一粒磁石沒通電都會有磁性的呢？

The magnetic property of a permanent manget is due to the alignment of atomic dipoles inside the magnetic material.

Atoms in ferromagnetic substances are in the form of dipoles. A dipole is an atom (or molecule) that has manetic north and south poles resemble to a minute bar magnet (this is due to unbalanced electronic structure within the atom).

These dipoles are originally oriented randomly in the substance so that there apears no net magnetic strength of the substance. When the substance is being magnetized, e.g. by passing a strong external magnetic field through it, the dipoles are all aligned in the same direction. The weak magnetic strength of each dipoles then add up together to form a strong magnetic field observable outside the substance.
----------------------------------------------------------------
但係佢d磁力點解會減弱呢？

Thermal agitation of the dipoles may cause them to dis-oriented from the aligned state, especially when the permanent magnet is placed at places of high temperature. When most of the dipoles return to the state of random orientation, the magnet strength of the manget would vanish.
----------------------------------------------------------------
我們我否電電佢令佢重生呢？

You can placed the weakened magnet in a strong external field, which could be produced by passing electric current through a solenoid, to align the dipoles again. This could restore the magnetic strength of the bar magnet.