Magnetization and magnetic intensity.

Magnetization, also called magnetic polarization, is a vector quantity that gives the measure of the density of permanent or induced dipole moment in a given magnetic material.

We have seen that electrons in an atom have a magnetic moment. In bulk, these magnetic moments add up vectorially and they give a net magnetic moment.

We define magnetization M of a sample to be equal to its net magnetic moment per unit volume.

 

 

Consider a long  current-carrying solenoid, Magnetic field inside the solenoid is given by

When the interior is filled with magnetic material with non-zero magnetization. Then field inside the solenoid  B will be greater than B0

B = BBm, Where  Bm is a field contributed by the magnetic cores. Bm is proportional to the magnetization M.

Now introducing another vector field H called the magnetic intensity, which is defined by

Magnetization M  is proportional to magnetic intensity H.

Where χ is a dimensionless quantity and called magnetic susceptibility.

Magnetic materials

Materials that are attracted by the magnet are called magnetic materials. And materials that are not attracted to a magnet are called non-magnetic materials.

Classification of Magnetic materials

Magnetic materials are classified into the following types. In this chapter, we will discuss diamagnetic, paramagnetic and ferromagnetic materials in detail.

Diamagnetism

  • Materials that have a tendency to move from the stronger to the weaker part of the external magnetic field.
  •  These materials are weakly repelled by the magnetic field.

Cause of Diamagnetism

 Electrons revolve around the nucleus in an orbit possessing orbital angular momentum. The orbiting electrons are equivalent to the current-carrying loop and thus possess a magnetic moment. In diamagnetic materials, the resultant magnetic moment of the atom is zero.

When the magnetic field is applied, those electrons having orbital magnetic moments in the same direction slow down and those in opposite directions speed up in accordance with Lenz’s law.

Thus the substance develops a net magnetic moment in the direction opposite to the applied field and is hence repelled by the field.

Paramagnetism

  • Paramagnetic substances are those which get weakly magnetized when placed in an external magnetic field.
  • They have a tendency to move from a region of the weaker magnetic field to a stronger magnetic field, these are weakly attracted by the external field.

Cause of paramagnetism

The individual atoms (ions or molecules) of a paramagnetic material possess magnetic moments of their own. In the absence of external B, these magnetic moments are oriented in a random direction and hence net magnetic moment is zero. When we apply an external magnetic field, the individual magnetic dipoles orient themselves in the direction of the magnetic field and hence get weakly magnetized

Ferromagnetism

  • Ferromagnetic materials are those which get strongly magnetized when placed in an external magnetic field.
  • Ferromagnetic materials have a strong tendency to move from a region of the weaker magnetic field to a stronger field when placed in an external magnetic field.
  • These materials got strongly attracted by the external magnetic field.

Cause of ferromagnetism

In ferromagnetic materials, individual atoms are associated with large magnetic moments. The magnetic moments of the neighboring atoms interact with each other and align themselves spontaneously in a common direction over microscopic regions called domains.

In a ferromagnetic material in the unmagnetized state, atomic dipoles in small regions called domains are aligned in the same direction. The domain exhibits a net magnetic moment even in the absence of an external magnetizing field.

However, the magnetic moment of the neighboring domains are oriented in opposite directions and they cancel out and therefore the net magnetic moment of the material is zero.

On applying an external magnetic field these domains all align themselves in the direction of the applied field. In this way, the material is strongly magnetized in a direction parallel to the magnetizing field.

Hysteresis

When a ferromagnetic sample is placed in a magnetizing field, the sample gets magnetized by induction. As the magnetizing field intensity H carries, the magnetic induction B does not vary linearly with H.

The figure given below shows the variation of magnetic induction B with magnetizing field intensity H.

  • The origin represents the initial unmagnetized state of a ferromagnetic sample. As the magnetizing field intensity H increases, the magnetic induction B first gradually increases and attains a constant value.
  • Now if the magnetizing field intensity H is gradually decreased to zero, B decreases but along a new path AB. The sample is not demagnetized even when the magnetizing field has been removed.
  • The magnetic induction left behind in the sample after the magnetizing field has been removed is called residual magnetism or retentivity.
  • To reduce the magnetism to zero, the field H id gradually increases in the reverse direction, the induction B decreases and becomes zero at a value of H= OC.
  • The value of reverse magnetizing field intensity H required for the residual magnetism of a sample to become zero is called coercivity of the sample.

A study of the hysteresis loop provides us with information about retentivity, coercivity and hysteresis loss of magnetic. This helps in the proper selection of material for designing cores of transformers and electromagnets and making permanent magnets.

Soft ferromagnetic materials

These are the ferromagnetic materials in which the magnetization disappears on the removal of the external magnetization field. Such materials have narrow hysteresis loops.

Example: soft iron.

Hard ferromagnetic material

These are the ferromagnetic materials that retain magnetization even after the removal of the external magnetizing field.

Example: Steel, lodestone etc.

Permanent magnets and electromagnets

Permanent magnets

Substances which at room temperature retain their ferromagnetic property for a long period of time are called permanent magnets.

Methods of making permanent magnet :

  • One can hold an iron rod in a north-south direction and hammer it repeatedly.
  • 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 permanent magnets.
  • An efficient way to make a permanent magnet is to place a ferromagnetic rod in a solenoid and pass a current. The magnetic field of the solenoid magnetizes the rod.

Desired property of a material to make permanent magnets

Hysteresis curve allows us to select suitable material for the permanent magnet.

  • Material should have high retentivity so that the magnet is strong.
  • Material should have high coercivity so that the magnetization is not erased by any stray magnetic field, temperature fluctuations etc.
  • Steel is the favorable choice of material for a permanent magnet, Other suitable materials for permanent magnet are alnico, cobalt steel and ticonal.

Electromagnets

Electromagnets are the type of magnets where a magnetic field is produced by electric current. It is made up of a coil of wire which acts as a magnet when an electric current passes through it.

  • An electromagnet only displays magnetic properties when an electric current is applied to it.
  • The strength of an electromagnet can be adjusted by the amount of electric current allowed to flow into it.
  • Cores of electromagnets are made of ferromagnetic material which have high permeability and low retentivity. Soft iron is a suitable material for electromagnets.

Application of the electromagnets

  • It is used in generators and motors, which are necessary for the conversion of mechanical energy into electrical energy.
  • Electric buzzers and bells.
  • Headphones and loudspeakers use electromagnets.
  • Data storage devices like VCR’s, tape recorders, hard discs etc.
  • MRI machines also used electromagnets.