Revision: Magnetism Science SSLC (English Medium) Class 8 Tamil Nadu Board of Secondary Education

An imaginary line (XY) passing through the magnetic north pole and magnetic south pole of a bar magnet is called its magnetic axis.

The angle between the horizontal and earth’s magnetic field is known as the angle of dip.

It is a piece of lodestone, which is a black iron oxide (Fe₃ O₄) called magnetite. The word lodestone means a leading stone.

The region around a magnet where its magnetic force can be experienced is called the magnetic field.

Pieces of iron and other magnetic materials which can be made to acquire the properties of natural magnets are called artificial magnets.

lt is a solenoid with a soft iron core placed inside it. When current is passed through the solenoid, the soft iron core becomes a temporary magnet.

The distance (NS) between the north pole and south pole of a magnet is called the length or effective length of the magnet.

The substances which have the property of attracting small pieces of iron, nickel, cobalt, etc, are called magnets, and this property of attraction is called magnetism.

An imaginary line (PQ) bisecting the effective length of a magnet is called the magnetic equator of the magnet.

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.

The process of stopping the magnetic field from entering a region is called magnetic shielding.

The number of magnetic field lines crossing unit area kept normal to the direction of field lines is called magnetic flux density. Its unit is Wb/m²

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.

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.

• Relative permeability ranges:   μ_r ≫ 1, of the order of 10²; μ ≫ μ₀
• Diamagnetic: B ≫ B₀​; B_m ≫ B₀
• Magnetic susceptibility (χ): positive and high, χ ≈ 10²; very large, positive, temperature dependent, χ_m ∝ \[\frac {1}{T−T_C}\]​ (Curie–Weiss law)
• Magnetic moment: very high
• Intensity of magnetisation (I) vs H: I is very large, positive, varies non-linearly with H (I is in the direction of H, value of I is very high)

• Relative permeability ranges: μ_r < 1 (as B is less than μ₀H); also 1 > μ_r > 0, μ < μ₀
• Diamagnetic: B < B₀​; B_m < B₀
• Magnetic susceptibility (χ): low and negative, ∣χ∣ ≈ 1; small, negative and temperature-independent, χ_m ∝ T₀
• Magnetic moment: very low (≈ 0)
• Intensity of magnetisation (I) vs H: I is small, negative, varies linearly with H (I and H in opposite direction, I is negative with respect to H)

• Relative permeability ranges:  μ_r > 1 (as B is slightly greater than μ₀H); (1 + ε) ≥ μ_r > 1, μ > μ₀
• Diamagnetic: B < B₀​; B_m < B₀
• Magnetic susceptibility (χ): low and positive, χ ≈ 1; small, positive, varies inversely with temperature, χ_m ∝ \[\frac {1}{T}\]​ (Curie law)
• Magnetic moment: very low but not zero
• Intensity of magnetisation (I) vs H: I is small, positive, varies linearly with H (I and H in same direction, value of I is low)