THE MAGNETIC FIELD & ELECTROMAGNETISM

Magnetism Basics

  • Poles: north & south; like repel, opposites attract.

  • No magnetic monopoles; field lines form closed loops (N → S).

  • Compass aligns with local magnetic field.

Atomic Magnetism

  • Electron motion/spin → atomic magnets.

  • Nonmagnetic: random orientation.

  • Ferromagnetic (Fe, Ni): aligned domains.

  • Paramagnetic: temporary alignment in a field.

Magnetic Fields & Current

  • Moving charges create magnetic fields (Oersted).

  • Right-hand rule: thumb = current, fingers = field direction.

  • Field around straight wire

  • Solenoid: coil acts like a bar magnet

Earth’s Magnetic Field & Auroras

  • Caused by molten iron in the core (dynamo effect).

  • Protects from solar wind; poles drift and reverse.

  • Solar particles near poles excite atoms → auroras (O = green, N/H = red/purple).

  • Mars lost its field → atmosphere stripped.

Magnetic Force

  • Acts on moving charges; ⟂ to both v & B.

  • Causes circular motion (centripetal).

  • Used in mass spectrometers and CRTs.

Electromagnetic Induction (Faraday’s Law)

  • Changing magnetic field induces voltage/current.

  • Motion required: magnet near wire or wire near magnet.

  • Lenz’s Law: induced current opposes the change.

  • Moving magnet → wire current opposes motion; solenoid repels or attracts to resist change.

  • Prevents perpetual motion — requires external energy.

Transformers & Power Grid

  • AC in primary coil induces voltage in secondary.

  • More loops → higher voltage.

  • Step-up: ↑V, ↓I (efficient transmission).

  • Step-down: ↓V for safe home use (120 V).

  • High V, low I reduces heat loss in power lines.

Circuit Components & Resonance

  • Resistor: opposes current.

  • Capacitor: stores charge, smooths AC→DC.

  • Inductor: resists current changes (self-induction).

  • LC circuit oscillates naturally

  • Resonance: max current when AC frequency matches natural frequency (used in radios).

Current Wars

  • Edison: DC systems (short range).

  • Westinghouse & Tesla: AC → efficient long-distance power.

  • AC + transformers = modern electrical grid.

  • Tesla coil demonstrates high-voltage discharge.

Key Concepts

  • Magnetism from moving charges.

  • Magnetic fields form loops; no monopoles.

  • Changing fields → induced currents (Faraday’s Law).

  • Lenz’s Law conserves energy.

  • Earth’s field shields life; auroras at poles.

  • AC and transformers enable efficient power distribution.

  • Inductors & capacitors control AC circuit behavior.