CBSE Class 12 Physics (Physics) Chapter 5 Magnetism and Matter Notes, Download PDF

5.1 INTRODUCTION

The word magnet is derived from the name of an island in Greece called magnesia where magnetic ore deposits were found, as early as 600 BC.

Some of the commonly known ideas regarding magnetism are:

(i) The earth behaves as a magnet with the magnetic field pointing approximately from the geographic south to the north.

(ii) When a bar magnet is freely suspended, it points in the north-south direction. The tip which points to the geographic north is called the north pole and the tip which points to the geographic south is called the south pole of the magnet.

(iii) There is a repulsive force when the north poles ( or south poles ) of two magnets are brought close together. Conversely, there is an attractive force between the north pole of one magnet and the south pole of the other.

(iv) We cannot isolate the north, or south pole of a magnet. If a bar magnet is broken into two halves, we get two similar bar magnets with somewhat weaker properties. Unlike electric charges, isolated magnetic north and south poles known as magnetic monopoles do not exist.

(v) It is possible to make magnets out of iron and its alloys.

5.2 THE BAR MAGNET

A bar magnet has two poles, similar to the positive and negative charge of an electric dipole. One pole is designated the North pole and the other, the South pole. When suspended freely, these poles point approximately towards the geographic north and south poles, respectively. A similar pattern of iron filings is observed around a current carrying solenoid.

5.2.1 The magnetic field lines

The pattern of iron filings permits us to plot the magnetic field lines.

Properties of magnetic field lines:

(i) The magnetic field lines of a magnet (or a solenoid) form continuous closed loops. This is unlike the electric dipole where these field lines begin from a positive charge and end on the negative charge or escape to infinity.

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

(iii) The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field B.

(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.

5.2.2 Bar magnet as an equivalent solenoid

Cutting a bar magnet in half is like cutting a solenoid. We get two smaller solenoids with weaker magnetic properties. The field lines remain continuous, emerging from one face of the solenoid and entering into the other face.

5.2.3 The dipole in a uniform magnetic field

Place a small compass needle of known magnetic moment m and allowing it to oscillate in the magnetic field.

5.2.4 The electrostatic analog

5.3 MAGNETISM AND GAUSS’S LAW

Gauss’s law for magnetism is:

The net magnetic flux through any closed surface is zero.

The difference between Gauss’s law of magnetism and that for electrostatics is a reflection of the fact that isolated magnetic poles (also called monopoles) are not known to exist. There are no sources or sinks of B; the simplest magnetic element is a dipole or a current loop. All magnetic phenomena can be explained in terms of an arrangement of dipoles and/or current loops.

5.4 MAGNETISATION AND MAGNETIC INTENSITY

Magnetisation M of a sample is equal to its net magnetic moment per unit volume

5.5 MAGNETIC PROPERTIES OF MATERIALS

In terms of the susceptibility X, a material is diamagnetic if X is negative, para- if X is positive and small, and ferro- if c is large and positive.

5.5.1 Diamagnetism

Diamagnetic substances are those which have tendency to move from stronger to the weaker part of the external magnetic field. 

Unlike the way a magnet attracts metals like iron, it would repel a diamagnetic substance. 

The field lines are repelled or expelled and the field inside the material is reduced.

When placed in a non-uniform magnetic field, the bar will tend to move from high to low field.

Some diamagnetic materials are bismuth, copper, lead, silicon, nitrogen (at STP), water and sodium chloride.

The phenomenon of perfect diamagnetism in superconductors is called the Meissner effect, after the name of its discoverer.

5.5.2 Paramagnetism

Paramagnetic substances are those which get weakly magnetised when placed in an external magnetic field. They have tendency to move from a region of weak magnetic field to strong magnetic field, i.e., they get weakly attracted to a magnet.

The individual atoms (or ions or molecules) of a paramagnetic material possess a permanent magnetic dipole moment of their own.

When placed in a non-uniform magnetic field, the bar will tend to move from weak field to strong.

Some paramagnetic materials are aluminium, sodium, calcium, oxygen (at STP) and copper chloride

5.5.3 Ferromagnetism

Ferromagnetic substances are those which gets strongly magnetised when placed in an external magnetic field.

They have strong tendency to move from 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.

Thus, in a ferromagnetic material the field lines are highly concentrated. In non-uniform magnetic field, the sample tends to move towards the region of high field.

In some ferromagnetic materials the magnetisation persists. Such materials are called hard magnetic materials or hard ferromagnets. 

Alnico, an alloy of iron, aluminium, nickel, cobalt and copper, is one such material.

The naturally occurring lodestone is another. Such materials form permanent magnets to be used among other things as a compass needle. On the other hand, there is a class of ferromagnetic materials in which the magnetisation disappears on removal of the external field. Soft iron is one such material. Appropriately enough, such materials are called soft ferromagnetic materials. There are a number of elements, which are ferromagnetic: iron, cobalt, nickel, gadolinium, etc.

The ferromagnetic property depends on temperature. At high enough temperature, a ferromagnet becomes a paramagnet. The domain structure disintegrates with temperature. This disappearance of magnetisation with temperature is gradual.