Definitions [12]
Define the following term regarding a bar magnet:
Magnetic axis
An imaginary line (XY) passing through the magnetic north pole and magnetic south pole of a bar magnet is called its magnetic axis.
Define the following term:
Angle of dip
The angle between the horizontal and earth’s magnetic field is known as the angle of dip.
Define natural magnet.
It is a piece of lodestone, which is a black iron oxide (Fe3 O4) called magnetite. The word lodestone means a leading stone.
Define the following term regarding a bar magnet:
Magnetic field
The region around a magnet where its magnetic force can be experienced is called the magnetic field.
Define artificial magnet.
Pieces of iron and other magnetic materials which can be made to acquire the properties of natural magnets are called artificial magnets.
Define electromagnet.
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.
Define the following term regarding a bar magnet:
Effective length
The distance (NS) between the north pole and south pole of a magnet is called the length or effective length of the magnet.
Define the terms magnet and magnetism.
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.
Define the following term regarding a bar magnet:
Magnetic equator
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.
Theorems and Laws [2]
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 − TC), where TC is the Curie temperature. Mathematically,
On heating beyond the Curie temperature (TC(iron) = 770 °C), ferromagnetic substances get converted into paramagnetic materials.
Key Points
- Relative permeability ranges: μr ≫ 1, of the order of 102; μ ≫ μ0
- Diamagnetic: B ≫ B0; Bm ≫ B0
- Magnetic susceptibility (χ): positive and high, χ ≈ 102; very large, positive, temperature dependent, χm ∝ \[\frac {1}{T−T_C}\] (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: μr < 1 (as B is less than μ₀H); also 1 > μr > 0, μ < μ0
- Diamagnetic: B < B0; Bm < B0
- Magnetic susceptibility (χ): low and negative, ∣χ∣ ≈ 1; small, negative and temperature-independent, χm ∝ 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: μr > 1 (as B is slightly greater than μ₀H); (1 + ε) ≥ μr > 1, μ > μ0
- Diamagnetic: B < B0; Bm < B0
- Magnetic susceptibility (χ): low and positive, χ ≈ 1; small, positive, varies inversely with temperature, χm ∝ \[\frac {1}{T}\] (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)
