1. Introduction to Magnetism

• Definition: Magnetism is a fundamental force of nature that manifests as an attractive or repulsive force between electrically charged particles in motion. It also describes the behavior of certain materials (magnets) that produce magnetic fields.
• Magnetic Poles: Every magnet has two poles: a North (N) pole and a South (S) pole.
  • Monopoles Do Not Exist: Unlike electric charges, isolated magnetic poles (monopoles) have never been observed. If a magnet is broken, new North and South poles will form at the break points.
  • Interaction of Poles:
    • Like poles repel (N-N, S-S).
    • Unlike poles attract (N-S).
• Magnetic Field: A region around a magnet or a current-carrying conductor where magnetic forces can be observed.
  • Vector Quantity: Has both magnitude and direction.
  • SI Unit: Tesla (T).
    • Another common unit: Gauss (G). (1 T=104 G).

2. Magnetic Field Lines

• Definition: Imaginary lines used to visualize the direction and strength of a magnetic field.
• Properties:
  • Originate and Terminate: Originate from the North pole and terminate on the South pole outside the magnet.
  • Continuous Loops: Form continuous closed loops, passing from South to North inside the magnet.
  • Never Cross: Magnetic field lines never intersect each other.
  • Density of Lines: The density of field lines indicates the strength of the magnetic field (closer lines = stronger field).
  • Tangent: The tangent to a magnetic field line at any point gives the direction of the magnetic field at that point.

3. Sources of Magnetism

• 3.1. Permanent Magnets:
  • Materials (e.g., iron, nickel, cobalt, and their alloys like Alnico, Neodymium magnets) that retain their magnetism after being magnetized.
  • Composed of magnetic domains (regions where atomic magnetic moments are aligned). In an unmagnetized state, these domains are randomly oriented; in a magnetized state, they are aligned.
• 3.2. Electric Current (Electromagnetism):
  • Oersted’s Discovery (1820): An electric current flowing through a conductor produces a magnetic field around it.
  • Right-Hand Thumb Rule (for straight conductor): If you point your right thumb in the direction of conventional current, your curled fingers indicate the direction of the magnetic field lines.
  • Solenoid: A coil of wire wound into a tightly packed helix.
    • When current passes through it, it produces a strong, nearly uniform magnetic field inside, similar to a bar magnet.
    • The strength of the field depends on the current, number of turns, and core material.
  • Electromagnet: A temporary magnet created by wrapping a coil around a ferromagnetic core (e.g., soft iron).
    • Can be turned on/off and its strength varied.
    • Applications: Cranes, doorbells, circuit breakers, relays, MRI machines.
• 3.3. Earth’s Magnetism (Geomagnetism):
  • Earth itself acts as a giant magnet, with its magnetic North pole near the geographic South pole and vice versa.
  • Caused by convection currents of molten iron and nickel in the Earth’s outer core (dynamo effect).
  • Magnetic Declination: The angle between the geographic North and magnetic North at a given location.
  • Magnetic Inclination (Dip Angle): The angle between the Earth’s magnetic field lines and the horizontal plane. It is 0 at the magnetic equator and 90 degrees at the magnetic poles.
  • Importance: Protects Earth from harmful solar winds and cosmic rays; basis for compass navigation.

4. Magnetic Properties of Materials

• 4.1. Diamagnetic Materials:
  • Weakly repelled by a magnetic field.
  • Do not have permanent magnetic dipoles.
  • Example: Bismuth, copper, silver, gold, water, air, noble gases.
• 4.2. Paramagnetic Materials:
  • Weakly attracted by a magnetic field.
  • Have permanent magnetic dipoles that become weakly aligned with an external field.
  • Example: Aluminum, sodium, platinum, oxygen, manganese.
• 4.3. Ferromagnetic Materials:
  • Strongly attracted by a magnetic field.
  • Can be permanently magnetized.
  • Contain magnetic domains that align strongly with an external field, leading to strong magnetic effects.
  • Example: Iron, nickel, cobalt, and their alloys.
  • Curie Temperature: The temperature above which a ferromagnetic material loses its ferromagnetism and becomes paramagnetic.

5. Force on Charged Particles and Conductors in Magnetic Fields

• 5.1. Force on a Moving Charge (Lorentz Force):
  • A charged particle moving in a magnetic field experiences a force.
  • Formula: F=q(v×B) or F=qvBsinθ
    • q: Charge of the particle.
    • v: Velocity of the particle.
    • B: Magnetic field strength.
    • θ: Angle between v and B.
  • Direction: Perpendicular to both velocity and magnetic field (determined by right-hand rule for positive charge, left-hand for negative).
  • If v is parallel or anti-parallel to B, force is zero.
  • If v is perpendicular to B, force is maximum (F=qvB), causing circular motion.
  • Applications: Mass spectrometers, particle accelerators, cathode ray tubes.
• 5.2. Force on a Current-Carrying Conductor:
  • A conductor carrying current in a magnetic field experiences a force.
  • Formula: F=I(L×B) or F=BILsinθ
    • I: Current.
    • L: Length of the conductor in the field.
    • B: Magnetic field strength.
    • θ: Angle between current direction and magnetic field.
  • Direction: Fleming’s Left-Hand Rule is used to determine the direction of force, magnetic field, and current.
  • Applications: Electric motors, galvanometers, loudspeakers.

6. Electromagnetic Induction (EMI)

• Faraday’s Laws of Electromagnetic Induction:
  • First Law: Whenever the magnetic flux linked with a coil changes, an electromotive force (EMF) is induced in the coil.
  • Second Law: The magnitude of the induced EMF is directly proportional to the rate of change of magnetic flux.
    • Formula: E=−NdtdΦB​​
      • E: Induced EMF.
      • N: Number of turns in the coil.
      • ΦB​: Magnetic flux (ΦB​=B⋅A=BAcosθ).
      • Magnetic Flux Unit: Weber (Wb).
      • The negative sign indicates Lenz’s Law.
• Lenz’s Law: The direction of the induced current (or EMF) is such that it opposes the change in magnetic flux that produced it.
  • This is a consequence of the conservation of energy.
• Motional EMF: EMF induced when a conductor moves through a magnetic field.
  • Formula: E=BLvsinθ
• Eddy Currents: Circulating currents induced in bulk conductors when exposed to changing magnetic fields.
  • Can cause undesirable heating (e.g., in transformer cores), but also used in damping and induction heating.
• Self-Induction: The phenomenon where a changing current in a coil induces an EMF in itself.
• Mutual Induction: The phenomenon where a changing current in one coil induces an EMF in an adjacent coil.
• Applications of EMI:
  • Generators (Alternators/Dynamos): Convert mechanical energy into electrical energy by rotating coils in a magnetic field.
  • Transformers: Devices that change AC voltage levels based on mutual induction. Crucial for power transmission.
  • Induction Motors: Operate on the principle of rotating magnetic fields created by AC.
  • Magnetic Levitation (Maglev) Trains: Use powerful electromagnets and principles of repulsion/attraction.
  • Credit Card Readers: Read magnetic strips.

7. Electromagnetism and its Unified Nature

• Maxwell’s Equations: A set of four equations that form the foundation of classical electromagnetism, describing how electric and magnetic fields are generated and altered by each other and by charges and currents.
• Electromagnetic Waves: Disturbances in electric and magnetic fields that propagate through space as waves.
  • Do not require a medium to travel (can travel through vacuum).
  • Travel at the speed of light (c=3×108 m/s).
  • Examples: Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
  • Show wave-particle duality.

8. Applications of Magnetism in Technology

• Data Storage: Hard drives, magnetic tapes, floppy disks (rely on magnetizing small regions).
• Medical Imaging: Magnetic Resonance Imaging (MRI) uses strong magnetic fields and radio waves to create detailed images of organs and tissues.
• Navigation: Compasses (Earth’s magnetic field), GPS (though primarily satellite-based, relies on precise timekeeping influenced by relativistic effects related to fields).
• Electrical Devices: Motors, generators, transformers, relays, loudspeakers, microphones.
• Security: Metal detectors, magnetic locks.
• Research: Particle accelerators, fusion reactors (magnetic confinement).