## MAGNETISM AND MATTER

 Bar magnet Magnetic monopoles Properties of magnetic field lines Bar magnet as an equivalent solenoid Dipole in a uniform magnetic field The electrostatic analog Gauss law and magnetism Statement and explanation Earth’s magnetic field and its elements Magnetic declination Dip and Earth’s horizontal component BH and their variation Magnetisation (M) Magnetic intensity (H) magnetic susceptibility ( χ ) and permeability Magnetic properties of materials Para magnetism Diamagnetism Ferromagnetism Curie’s law and Curie temperature Hysteresis loop Retentivity and coercivity Permanent magnets and electromagnets

## Magnetic materials:

• Magnetic materials tend to point in the north-south direction.
• Like magnetic poles repel and unlike ones attract.
• Cutting a bar magnet in two leads to two smaller magnets.
• Magnetic poles cannot be isolated
When a bar magnet of dipole moment ‘m’ is placed in a uniform magnetic field B,
(a) the force on it is zero
(b) the torque on it is m × B
(c) its potential energy is. B, where we choose the zero of energy at the orientation when m is perpendicular to B.
The magnetic field B due to the bar magnet is,
(along axis)
(along equator)
Gauss’s law for magnetism states that the net magnetic flux through any closed surface is zero
Three quantities are needed to specify the magnetic field of the earth on its surface – the horizontal component, the magnetic declination and the magnetic dip. These are known as the elements of the earth magnetic field.
Consider a material placed in an external magnetic field B0. The magnetic intensity is defined as, The magnetisation M of the material is its dipole moment per unit volume. The magnetic field B in the material is, B =µ0 (H + M)

## Magnetic susceptibility:

For a linear material . So that B = µH and is called the magnetic susceptibility of the material.
• The three quantities, magnetic susceptibility, the relative magnetic permeability, µr and the magnetic permeability, µ are related as follows:
µ= µ0 µr
Magnetic materials are broadly classified as: diamagnetic, paramagnetic, and ferromagnetic.
• For diamagnetic materials c is negative and small.
• For paramagnetic materials it is positive and small.
• Ferromagnetic materials have large c and are characterised by non-linear relation between B and H. They show the property of hysteresis.
• Substances, which at room temperature, retain their ferromagnetic property for a long period of time are called permanent magnets.