PRELIMS

Magnetism

Important Magnetic Concepts

• Domain: Atoms of ferromagnetic substances form regions ( atoms) where dipole moments are aligned.
• Curie Temperature: Temperature at which a ferromagnetic substance changes into a paramagnetic substance.
• Core Material:
  • Permanent Magnets: Made of steel, cobalt steel, ticonal, alcomax, alnico.
  • Electromagnets & Transformers: Made of soft iron, mu-metal, stalloy.

Terrestrial (Earth's) Magnetism

Earth behaves as a powerful magnet with its south pole near the geographical North pole and its north pole near the geographical South pole.

1. Declination: Angle between magnetic meridian and geographical meridian.
2. Dip (Inclination): Angle the resultant Earth's field makes with the horizontal.
   • At Poles: .
   • At Equator: .
3. Horizontal Component: Earth's field along the horizontal in the magnetic meridian. Approx 0.4 Gauss ( Tesla).

Magnetic Field Lines

• Outside the magnet: Emerge from North pole and enter at South pole.
• Inside the magnet: Run from South pole to North pole (forming closed loops).
• Properties: Never intersect each other; closer lines indicate a stronger field; tangent at any point gives field direction.

• Classification by Magnetic Behaviour:
  1. Diamagnetic: Weakly repelled by a magnetic field (e.g., bismuth, copper, water).
  2. Paramagnetic: Weakly attracted (e.g., aluminium, platinum, sodium).
  3. Ferromagnetic: Strongly attracted, retains magnetism (e.g., iron, cobalt, nickel).

Electromagnet vs Permanent Magnet

• Electromagnet: Temporary magnet produced by passing current through a coil wound on a soft iron core; strength can be varied/switched off; used in electric bells, cranes, MRI machines, electric motors.
• Permanent Magnet: Retains magnetism without continuous current; made of steel/alnico (high retentivity); used in compasses, loudspeakers.
• Right-Hand Thumb Rule (Maxwell's rule): If a current-carrying conductor is held in the right hand with the thumb pointing in the direction of current flow, the curled fingers show the direction of the magnetic field around it.
• Fleming's Left-Hand Rule: Gives the direction of force on a current-carrying conductor in a magnetic field (motor effect) — thumb: Force, forefinger: Field, middle finger: Current.
• Fleming's Right-Hand Rule: Gives the direction of induced current in a conductor moving in a magnetic field (generator effect).

UPSC Relevance

Magnetism is fundamental to various modern technologies.

• Material Choice: Soft iron for electromagnets (high permeability, low retentivity) vs. steel for permanent magnets.
• Earth's Field: The concept of Dip (0 at equator, 90 at poles) is a standard Prelims science fact.
• Common Trap: Direction of lines of force inside the magnet (S to N) vs. outside (N to S); Fleming's Left Hand (motor/force) vs Right Hand (generator/current) Rule.