Permanent manets

The atoms in magnetic material, such as iron are arranged in Domains.

Domains are groups of atoms which are aligned in the same direction and therefore the electrons in all of the atoms contribute to the field.

Random magnetic domains

Unmagnetised nail in which the domains have random orientation

 

 

aligned magnetic domains

Magnetised nail in which the domains are aligned

 

 

 

The classic method of revealing the shape of a magnetic field is to use iron filing on a sheet of paper near the magnet. On a simple bar magnet the shape is like this:

Field picture for bar magnet

 

 

 

 

 

Field diagram for bar magnet

 

 

 

 

 

 

The strength of the field between the poles of a U shaped magnet is much stronger because the air gap is smaller. The iron or metal alloy of the metal has a high permeability whereas that of the air is very low.

Field of a U magnet

U magnet

 

 

 

 

 

 

The definition of a magnetic field is that it is the space around a magnetic which exerts a force on a moving electric charge. The field is a vector, it has size or strength and direction.

Magnets and magnetic fields, video lesson

Electromagnetism

An electric current flowing through a wire produces a magnetic field around it. The field around a single wire is generally very small. The strength depends on the size of the current and the distance from the wire. In reality, unless the current is very large it is hard to see the field using iron filings, but can easily be detected with a magnetic compass. The field is circular and clockwise, looking in the direction of conventional current flow.

However a strong magnetic field can be made if we use a coil of wire and stronger still if we wrap the wire around a piece of iron like a nail.

 

Magnetic fields around a coil

The combined fields around each coil produce a very strong field in the centre of the coil. If a highly permeable iron nail is placed in the centre of the coil the air path of the field lines is shorter and the field is stronger.