MAGNETISM ( MEMORY CAPSULE )

                                                                    MAGNETISM

1. Like poles repel each other and unlike poles attract each other.

2. Coulomb’s law of magnetic force –

                  It states that the force of attraction or repulsion between two magnetic poles is directly proportional to the product of their pole strength and inversely proportional to the square of the distance between them.

                              F = µ₀ / 4π (M₁ M₂ / r²)

Here µ₀ / 4π = 10^-7 NA^-2 or Wb A^-1 m^-1

          µ₀ = Permeability of free space = 4π x 10^-7  Wb A^-1 m^-1

3. M = m x 2L

Here M = magnetic moment

          m = Pole strength

          2l = Magnetic length

4. S.I. unit of magnetic moment is Ampere. Metre² or Joule / tesla.

5. Magnetic lines of force start from north pole and end on south pole.

6. Two magnetic lines of force never cross each other.

7. The magnetic lines of force have a tendency to contract lengthwise. This explains two unlike poles attract each other.

8. The lines of force have a tendency to expand sidewise. This explains repulsion between two like poles.

9. B _axial = µ₀ / 4π (2M / r³)

10. B _equatorial = µ₀ / 4π (M / r³)

11. τ = MB sin ᴓ = M x B

Here τ = torque

M = Magnetic moment

B = Magnetic induction

12. U = MB (cos ᴓ₁ – cos ᴓ₂)

13. M = I A

Here M = magnetic moment

A = area

14. µ_B = eh / 4πm = 9.27 x 10^-24 Am²

15. The angle between the geographical meridian and the magnetic meridian at a plane is called the magnetic declination at that place.

16.  The angle made by earth’s total magnetic field B with the horizontal direction in the magnetic meridian is called angle of dip at any place.

17. Angle of dip is zero at the magnetic equator.

18. Angle of dip is 90⁰ at the magnetic poles.

19. It is the component of earth’s total magnetic field B in the horizontal direction in the magnetic meridian.

20. Neutral point is the point where the resultant magnetic field intensity is zero.

21. B = F / m

Here B = magnetic induction

          F = force

          m = pole strength

22. I = M / V

 Here I = intensity of magnetisation

          M = magnetic moment

           V = volume

23. I = m / a

Here m = pole strength

          a = area

24. µ= B / H

Here µ = magnetic permeability

          B = magnetic induction

          H = Magnetic intensity

25. µ_r = µ / µ₀

Here µ_r = Relative permeability

          µ = Permeability of medium

          µ₀ = Permeability of free space

26. k = I / H

Here k = magnetic susceptibility

          I = Intensity of magnetisation

          H = Magnetic intensity

27. Magnetic susceptibility is unitless and dimensionless.

28. µ = µ₀ (1 + k)

29. µ₀ = 1 + k

Here µ₀ = permeability of free space

          µ = Permeability of medium

          k = magnetic susceptibility

30. Diamagnetic substances are those which develops feeble magnetisation in the opposite direction of the magnetising field. Such substances are feebly repeled by the magnets and tend to move from stronger to weaker parts of a magnetic field.

Ex – Bismuth, copper, lead, Zinc, Tin etc.

31. Susceptibility of diamagnetic material is small and negative.

32. The relative permeability is less than one for a diamagnetic material.

33. Paramagnetic substances are those which develops feeble magnetisation in the direction of magnetising field. Such substances are feebly attracted by the magnet and tend to move weaker parts to stronger part of a magnetic field.

Ex – Iron, oxygen, Cobalt, Nickel etc.

34. Susceptibility of paramagnetic material is small but positive value.

35. The relative permeability for a paramagnetic material has a value slightly greater than one.

36. Curie’s Law –

                  The magnetic susceptibility of a paramagnetic material varies inversely as the absolute temperature.

                                 k α 1 / T

                     Or,      k = C / T

Here C = Curie constant

37. Ferromagnetic substances are those which develops stronger magnetisation in the direction of magnetising field. They are strongly attracted by the magnets and tend to move from weaker part to stronger part of a magnetic field.

Ex- Iron, Cobalt, Nickel and alloys like alnico

38. Susceptibility of ferromagnetic material has a large positive value.

39. The relative permeability of a ferromagnetic material has a large positive value.

40. The susceptibility of a ferromagnetic material decreases with temperature in accordance to Curie Weiss law.

41. At certain temperature, called curie point, the susceptibility suddenly falls and the ferromagnetic substance becomes paramagnetic.