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.
(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)
![](https://i0.wp.com/drrajkumars.com/drla/wp-content/uploads/2020/12/3a-1.png?resize=36%2C35)
![](https://i0.wp.com/drrajkumars.com/drla/wp-content/uploads/2020/12/3b-1.png?resize=31%2C35)
Gauss’s law for magnetism states that the net magnetic flux through any closed surface is zero
![](https://i0.wp.com/drrajkumars.com/drla/wp-content/uploads/2020/12/4a-1.png?resize=173%2C50)
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)
![](https://i0.wp.com/drrajkumars.com/drla/wp-content/uploads/2020/12/6a-3.png?resize=49%2C40)
Magnetic susceptibility:
For a linear material
. So that B = µH and
is called the magnetic susceptibility of the material.
![](https://i0.wp.com/drrajkumars.com/drla/wp-content/uploads/2020/12/7a-3.png?resize=49%2C18)
![](https://i0.wp.com/drrajkumars.com/drla/wp-content/uploads/2020/12/7b-2.png?resize=10%2C20)
- The three quantities, magnetic susceptibility,
the relative magnetic permeability, µr and the magnetic permeability, µ are related as follows:
![](https://i0.wp.com/drrajkumars.com/drla/wp-content/uploads/2020/12/7c-2.png?resize=55%2C20)
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.