If magnetic fields are produced by moving charges, how is it possible to understand the magnetic field produced by a permanent bar magnet?  Furthermore, how can it explain why heating a piece of iron will cause it to not be attracted to a magnet, but the attractive force will return when the piece of iron is cooled?


As you learned in previous lessons, the moving charges associated with matter are the electrons.  In some atoms, the electron movement generates a very small magnetic field.   Ferromagnetic material -such as iron, nickel, and cobalt-has many adjacent atoms with electron movement that, taken together, form a small region with an intense magnetic field.

 

Each region, about 1 mm across, contains billions of atoms and is referred to as a  domain .  Within the domain, the tiny magnetic field associated with the movement of each electron is additive, producing a significant magnetic field.  When all the domains of a bar magnet, for example, align together, it is said to be magnetized.  When the domains are erratically arranged they cancel one another out and the overall magnetic properties no longer exist.

ferromagnetic material:  a metal with adjacent atoms having electron movement that, taken together, form a small region with an intense magnetic field. Examples include iron, nickel and cobalt.


domain:  the region of a material in which the magnetic fields of most of the atoms are aligned


 

 

 

The domain model helps explain the effect of heat on the magnetic properties of a permanent iron magnet.  When heated, the atoms composing the domains become agitated and the electron movement changes, effectively destroying the alignment of the domains, so the iron loses its magnetic field.  When the iron cools, the unaligned domains are locked in place and the magnet is a regular piece of iron.

 

To change the piece of iron back into a permanent magnet it must be heated, which frees the domains to change alignment, placed in a strong external magnetic field and cooled.  The external magnetic field causes the excited domains to align and they are locked into position as the iron cools.  The domains of the piece of iron are aligned again and it becomes a magnet again.

 

The domain theory also explains why non-magnetized materials, such as iron, can become magnetized in the presence of another magnet.  The magnetic field lines emanating from a permanent magnet can cause the alignment of some of the domains that exist in other metals, thereby giving them magnetic properties too.  This explains why metal containing iron or nickel are attracted to permanent magnets.  The external magnetic field causes an alignment of some of the domains in the iron or nickel, giving it weak magnetic properties, which can then interact with the external magnetic field that caused the alignment.  When the magnet is moved away from the iron, the domains quickly lose their alignment and the iron is non-magnetic again.


Read 
There are many devices that use electromagnets.  Some common examples are explained in the document " Applications of the Electromagnet ."  More information on hand rules and electromagnets can be found on pages 587-589 of the textbook.