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The process of attraction of various substances by matter is called magnetism. The substances are steel, iron, and nickel. Magnetic materials are those materials that show some magnetic properties when kept beside a magnet.
The word ‘magnetism’ is derived from the name of an island in Greece called Magnesia.
Matter and magnetism
Matter and magnetism are combined with some references. Magnetism occurs due to magnetic fields surrounding a matter. This leads to the formation of magnetic materials that react in a certain way.
These are some common properties of magnets.
- Magnets tend to attract substances such as iron, nickel, and steel.
- Magnetic materials have a directive property and thus align in a north–south direction.
- Like magnetic poles repel and unlike ones attract each other.
- Magnetic poles cannot be isolated, which means that they always exist in pairs.
- Magnets have a magnetic field represented by magnetic field lines, which are curved lines that tell the direction of a magnetic field.
- Magnets tend to induce some magnetic property to a substance that is placed nearby. This process is called magnetic induction.
- The setup of two equal and opposite magnetic poles which are separated by some distance is known as a magnetic dipole.
Magnetic dipole moment
It is calculated as the product of its pole strength and magnetic length.
m=qm2l
Where qm is the magnitude of charge and 2l is the distance between the charges.
Gauss’s law for magnetism
According to Gauss’s law, the total magnetic flux in a closed surface is equal to zero.
B=all area elementsB.S=0
Where B is the magnetic field strength,
S is the small area element.
Earth’s magnetic field
Physical Quantity | Electrostatic | Magnetism |
Dipole Moment | p | m |
Axial Field | 140.2pr3 | 04.2mr3 |
Equatorial Field | -140.2pr3 | -04.2mr3 |
Torque in External Field | pm | mB |
PE In External Field | -p.m | -m.B |
Poles
- The magnetic south pole is the pole near the geographic North Pole.
- Similarly, the magnetic north pole is the geographic South Pole.
- The value of the magnetic field on the Earth’s surface = 4 x 10-5 T.
Elements of the Earth’s magnetic field
Three elements are used to define the magnetic field on the surface of the Earth.
- The horizontal component of the Earth’s magnetic field (BH): It is the component in the horizontal direction.
- The magnetic declination (): The angle between the geographic meridian and the magnetic meridian at a particular point is called the magnetic declination at that place.
- The angle of dip or magnetic inclination (): It is the degree of the angle made by the Earth’s total magnetic field B with the horizontal direction in the magnetic meridian.
BH= B
BVBH=B
B = BH2+BV2
- Neutral point: It is the point on the Earth’s surface where the magnetic field is equal but opposite to its horizontal component. The resultant component is zero at this point.
- The intensity of magnetisation: It is calculated as the dipole moment per unit volume.
- Magnetic induction: Magnetic induction is known as the total number of magnetic lines which is crossing per unit area.
- Magnetic permeability (): It is calculated as the ratio of its magnetic induction and magnetic intensity.
- Magnetic intensity of magnetising field: It is the power of a magnetic field to magnetise a material. Let us assume that a material is placed in an external magnetic field, then its magnetic intensity is:
= BH
Classification of magnetic materials
Magnetic materials are broadly divided into the following categories based on their properties.
Property | Diamagnetic substances | Paramagnetic substances | Ferromagnetic substances | |
1. | Effect of magnets | Slowly repelled by magnets | Weakly attracted by magnets | Strongly magnetised |
2. | In an external magnetic field | Gain weak magnetisation in the opposite direction of the field | Gain weak magnetisation in the same direction as that of magnetising fields | Gain strong magnetisation in the same direction as that of magnetising fields |
3. | In a non-uniform magnetic field | Move slowly from stronger to weaker sections in the magnetic field | Move slowly to weaker portions from stronger ones in the same magnetic field | Move rapidly from weaker to stronger portions of the same magnetic field |
4. | In a uniform magnetic field | A freely suspended diamagnetic rod lines up perpendicular to the magnetic field. | It lines up parallel to the field. | It behaves the same as paramagnetic material. |
5. | Susceptibility value (m) | Susceptibility is small. | It is positive and small. -1<m< | Susceptibility is positive and shows a large value. m>1000 |
6. | Relative permeability value (r) | A bit less than 1 0<r<1 | A bit greater than 1 1<r<1+ | Of the order of r> 1+ |
7. | Permeability value | <0 | <0 | >>0 |
8. | Effect of temperature | The susceptibility of the material is not dependent on temperature. | Susceptibility is inverse to temperature. m1T | Susceptibility decreases with temperature in a complex way. m1T-TC, (T>TC) |
9. | Removal of the magnetising field | The magnetisation will be there until the magnetising field is applied. | When the magnetising field is lost, the magnetisation will also be lost. | The magnetisation is retained after the field is lost. |
10. | Variation of M concerning H | M changes linearly with H. | M changes linearly with H. | M changes non-linearly with H. |
11. | Hysteresis effect | It shows no hysteresis. | B shows no hysteresis. | B shows some hysteresis. |
12. | The physical state of the material | Gas, liquid, or solid | Gas, liquid, or solid | Usually solids only |
13. | Examples | Bi, Cu, Pb, NaCl, Si, H2O, N2(at STP) | Na, O2(at STP), CuCl2, Al, Ca | Ni, Co, Fe2O3, Alnico, Fe, Gd |
Conclusion
Magnetic material has the property to point in a north–south direction. Like poles of a magnet repel and unlike poles attract each other. The Earth resembles a large magnet having magnetic properties. Magnetic materials are those which possess magnetic properties. These are of three types: diamagnetic, ferromagnetic, and paramagnetic. All of them acquire different properties when kept in different conditions.
