magnetism

Magnetic phenomena are universal in nature.Magnetism is one aspect of the combined electromagnetic force. It refers to physical phenomena arising from the force caused by magnets, objects that produce fields that attract or repel other objects. ... Permanent magnets, made from materials such as iron, experience the strongest effects, known as ferromagnetism.

 The magnetic field lines

             The pattern of iron filings permits us to plot the magnetic field lines*. This isshown both for the bar-magnet and the current-carrying solenoid. Electric fieldlines of an electric dipole are also  magnetic fieldlines and these lines are a visual and intuitive realisation of the magnetic field. 

Their properties are:

(i)The magnetic field lines of a magnet (or a solenoid) form continuousclosed loops. This is unlike the electric dipole where these field linesbegin from a positive charge and end on the negative charge or escapeto infinity.

(ii)The tangent to the field line at a given point represents the direction ofthe net magnetic field B at that point.

(iii)The larger the number of field lines crossing per unit area, the strongeris the magnitude of the magnetic field B. B is largeraround region  ii  than in region  i  .

(iv)The magnetic field lines do not intersect, for if they did, the direction of the magnetic field would not be unique at the point of intersection.One can plot the magnetic field lines in a variety of ways. One way is to place a small magnetic compass needle at various positions and note its orientation. This gives us an idea of the magnetic field direction atvarious points in space.

PERMANENT MAGNETS AND ELECTROMAGNETS 

Substances which at room temperature retain their ferromagnetic propertyfor a long period of time are called permanent magnets. Permanent magnets can be made in a variety of ways. One can hold aniron rod in the north-south direction and hammer it repeatedly.The method of the illustration is from a 400 year old book to emphasise that the making of permanent magnets is an old art. One can also hold a steel rod and stroke it with one end of a bar magnet a large number of times, always in the same sense to make a permanent magnet.

An efficient way to make a permanent magnet is to place aferromagnetic rod in a solenoid and pass a current. Themagnetic field of the solenoid magnetises the rod.The hysteresis curve allows us to select suitable materials for permanent magnets. The material should have high retentivity so that the magnet is strong and high coercivity so that the magnetisation is not erased by stray magnetic fields,temperature fluctuations or minor mechanical damage.Further, the material should have a high permeability. Steel is one-favoured choice. It has a slightly smaller retentivity thansoft iron but this is outweighed by the much smaller coercivityof soft iron. Other suitable materials for permanent magnetsare alnico, cobalt steel and ticonal.

Ferromagnetism

Ferromagnetic substances are those which gets strongly magnetised whenplaced in an external magnetic field. They have strong tendency to movefrom a region of weak magnetic field to strong magnetic field, i.e., they get strongly attracted to a magnet.

The individual atoms (or ions or molecules) in a ferromagnetic material possess a dipole moment as in a paramagnetic material. However, they interact with one another in such a way that they spontaneously align themselves in a common direction over a macroscopic volume called domain. The explanation of this cooperative effect requires quantum mechanics and is beyond the scope of this textbook. Each domain has a net magnetisation. Typical domain size is 1mm and the domain contains about 1011 atoms. In the first instant, the magnetisation varies randomly from domain to domain and there is no bulk magnetisation. When we apply an external magnetic field B0, the domains orient themselves in the direction of B0 and simultaneously the domainoriented in the direction of B0 grow in size. This existence of domains andtheir motion in B0 are not speculations. One may observe this under amicroscope after sprinkling a liquid suspension of powdered