Revision: Magnetic Effects of Current and Magnetism >> Magnetism and Matter Physics Science (English Medium) Class 12 CBSE

A bar magnet is a short permanent magnet with two poles — North (N) and South (S) — at its opposite ends, producing a magnetic dipole field around it. The pattern of iron filings sprinkled on a glass sheet placed over a bar magnet suggests that the bar magnet is a magnetic dipole.

Magnetic field lines are imaginary continuous curves drawn in a magnetic field such that the tangent at any point on the curve gives the direction of the net magnetic field \[\vec B\] at that point.

A solenoid is a long cylindrical coil of insulated wire wound closely in the shape of a helix. When an electric current is passed through it, it produces a magnetic field similar to that of a bar magnet, with one end acting as a north pole and the other as a south pole.

A magnetic dipole is a system consisting of two equal and opposite magnetic poles separated by a small distance. Any small current loop, a bar magnet, or a compass needle acts as a magnetic dipole.

Magnetic flux is the measure of the magnetic field passing through a surface. It depends on the magnetic field, the area of the surface, and the orientation of the surface.

A Gaussian surface is an imaginary closed surface used to analyse field flux. It may be spherical, cylindrical, cubical, or irregular in shape, but Gauss’s law for magnetism remains valid for every closed surface.

The magnetic field that exists in a vacuum and induces magnetism is called a magnetising field.

The ratio of magnetic permeability of the material (μ) and magnetic permeability of free space (μ₀) is called relative permeability.

The ratio of the strength of the magnetising field to the permeability of free space is called magnetic intensity.

The ratio of the magnitude of intensity of magnetisation to that of magnetic intensity is called magnetic susceptibility.

The ratio of the magnitude of the total field inside the material to that of the intensity of the magnetising field is called magnetic permeability.

The ratio of magnetic moment to the volume of the material is called magnetization.

The ratio of the strength of the magnetizing field to the permeability of free space is called magnetic intensity. 

The magnetic moment developed per unit volume of a material when placed in a magnetising field is called the 'intensity of magnetisation'.

The ability of a magnetising field to magnetise a material medium is called magnetising field intensity.

The ratio of magnetic moment to the volume of the material is called magnetisation.

The total magnetic field inside a magnetic material, which is the sum of the external magnetising field and the additional magnetic field produced due to magnetisation of the material, is called magnetic induction.

Substances which when placed in a magnetic field are feebly magnetised in the direction of the magnetising field are called paramagnetic substances.

Substances which when placed in a magnetising field are strongly magnetised in the direction of the magnetising field are called ferromagnetic substances.

Substances which when placed in a magnetic field are feebly magnetised in a direction opposite to that of the magnetising field are called diamagnetic substances.

B = μ₀​(H + M)

• SI unit: tesla (T)

M = χH

or

χ = HM​

B = μH

• SI unit: H/m

μr ​= \[\frac {μ}{​μ_0}\]

M = \[\frac {\text {magnetic moment​}}{\text {volume}}\]

• SI unit: A/m

H = nI

where n is the number of turns per unit length and I is current.

• SI unit: A/m

Gauss’s law for magnetism: The net magnetic flux through any closed surface is zero.

Mathematical form

For a closed surface,

This expression means that the total magnetic flux entering and leaving a closed surface balances exactly.

Physical meaning

• Magnetic field lines form closed loops.
• No isolated north pole or isolated south pole has been observed.
• A closed surface cannot enclose a net magnetic “charge” in the way it can enclose electric charge.
• Therefore, total outward magnetic flux is always zero.

For ferromagnetic substances above the Curie temperature, the magnetic susceptibility is inversely proportional to (T − T_C), where T_C is the Curie temperature. Mathematically,

On heating beyond the Curie temperature (T_C(iron) = 770 °C), ferromagnetic substances get converted into paramagnetic materials.

The magnetic susceptibility of a paramagnetic material varies inversely with its absolute temperature. Mathematically,

On cooling, paramagnetic substances get converted to ferromagnetic materials at the Curie temperature.

• A bar magnet can be thought of as a large number of atomic circulating currents (Ampere's Hypothesis).
• The far axial magnetic field of both a bar magnet and a solenoid is B = \[\frac {μ_0}{4π}\]\[\frac {2m}{r^3}\]​, confirming equivalence.
• Equivalence holds only at far axial distances (r ≫ a, r ≫ l).
• The magnetic moment of the bar magnet = the magnetic moment of the equivalent solenoid (m = NIA).
• Similarities: field pattern, axial field formula, poles, directive and attractive properties.
• Key differences: a bar magnet is permanent, a solenoid is temporary; solenoid poles are reversible, bar magnet poles are fixed.

• Diamagnetic substances are weakly repelled and have negative susceptibility.
• Paramagnetic substances are weakly attracted and obey Curie law.
• Ferromagnetic substances are strongly attracted and contain domains.
• Ferromagnets become paramagnetic above the Curie temperature.
• The comparison table is the most important revision tool for board preparation.