Class 11 Physics — Chapter 306: Magnetism

Key concepts: Magnetism (Class 11 Physics)

Magnetism as a property of matter: the bar magnet and its equivalence to a solenoid, the absence of magnetic monopoles, torque and energy of a magnetic dipole, the elements of Earth's magnetic field including declination and dip, and the three classes of magnetic material with the hysteresis loop of a ferromagnet.

Bar magnet and field lines

Magnetic field lines form closed loops running from north to south outside the magnet and from south to north INSIDE it — unlike electric field lines, which begin and end on charges.

No magnetic monopoles

Isolated poles have never been observed; breaking a magnet in half yields two complete magnets. This is why the net magnetic flux through any closed surface is zero.

Magnetic dipole moment

The strength of a magnet or current loop, NIA for a loop, measured in A m². It points from the south pole to the north pole of the magnet.

Bar magnet as a solenoid

A bar magnet produces the same external field pattern as a current-carrying solenoid, which is how the magnetic moment of a magnet is defined at all.

Field of a bar magnet

The axial field is twice the equatorial field at equal distance, and both fall off as the cube of distance — exactly parallel to the electric dipole.

Torque on a magnetic dipole

A uniform field exerts a torque mB sin θ that aligns the dipole with the field, which is why a compass needle swings to point north.

Energy of a magnetic dipole

Equal to −m·B, minimum when the dipole is aligned with the field and maximum when it is anti-parallel.

Earth's magnetism

Earth behaves as a giant dipole whose magnetic south pole lies near the geographic north — which is why a compass needle's north end points north.

Magnetic declination

The angle between the geographic and magnetic meridians at a place, i.e. the correction between true north and compass north.

Angle of dip

The angle the total field makes with the horizontal. It is 0° at the magnetic equator and 90° at the magnetic poles.

Components of Earth's field

The total field resolves into horizontal and vertical components, whose ratio gives the tangent of the dip angle.

Magnetisation and susceptibility

Magnetisation is the net dipole moment per unit volume, and susceptibility is the magnetisation produced per unit magnetising field.

Magnetic permeability

Measures how readily a material carries magnetic flux. Relative permeability is one plus the susceptibility.

Diamagnetic materials

Weakly REPELLED by a magnetic field, with small negative susceptibility. Bismuth, copper and water are examples; superconductors are perfect diamagnets.

Paramagnetic materials

Weakly attracted, with small positive susceptibility that falls as temperature rises according to Curie's law. Aluminium and oxygen are examples.

Ferromagnetic materials

Strongly attracted, with very large susceptibility and permanent magnetisation from aligned domains. They show hysteresis and lose ferromagnetism above the Curie temperature.