Physics: Electricity and Magnetism

For most of history, electricity and magnetism looked like two totally different things. Then, in the 1800s, scientists made a world-changing discovery: they're two aspects of a single force, **electromagnetism**. Every electric motor, every radio wave, every computer chip, and every particle of light in the universe is an electromagnetic phenomenon. Understanding the basics is a gateway to modern physics.

**Electric charge** is a property of matter, like mass. There are two kinds:\n\n- **Positive** (carried by protons)\n- **Negative** (carried by electrons)\n\nOpposite charges attract. Like charges repel. This force is described by **Coulomb's Law**:\n\n**F = k q₁q₂ / r²**\n\n- F is the force\n- q₁ and q₂ are the charges\n- r is the distance between them\n- k is Coulomb's constant (≈ 9×10⁹ N·m²/C²)\n\nNotice: it's an inverse-square law — the same mathematical form as gravity. Same structure, vastly different strength: the electric force between two electrons is about 10⁴² times stronger than their gravitational attraction.

When charges flow, you have an **electric current** (symbol I, measured in amperes or amps).\n\n**Ohm's Law** describes simple circuits:\n\n**V = IR**\n\n- V = voltage (measured in volts) — the "push" driving current\n- I = current (amps) — flow of charge per second\n- R = resistance (ohms) — how much the material opposes current\n\nHigher voltage = more current (for given R). Higher resistance = less current (for given V). These relationships let engineers design every circuit from flashlights to supercomputers.

**Magnetic fields** are created by moving charges (electric currents) and by certain materials (ferromagnets like iron, nickel, cobalt). They exert force on other moving charges and magnetic materials.\n\nA magnet has a **north** and **south** pole. Opposite poles attract; like poles repel. You cannot isolate a single pole (a "magnetic monopole") — if you cut a bar magnet in half, you get two smaller bar magnets, each with north and south poles. This is fundamentally different from electric charges, which CAN be isolated.

The key insight of 19th-century physics: **moving electric charges create magnetic fields**, and **changing magnetic fields create electric fields**.\n\n- **Oersted (1820)**: a wire carrying current deflects a compass needle — current creates magnetism.\n- **Faraday (1831)**: a magnet moving near a wire induces current — magnetism creates electricity.\n- **Maxwell (1860s)**: wrote four equations unifying all electric and magnetic phenomena, and predicted that electromagnetic waves — light! — travel at the speed of light c = 3 × 10⁸ m/s.\n\nThat prediction, that light is electromagnetic radiation, was one of the greatest scientific insights in history. It meant radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays are all the same kind of wave at different frequencies.

These two discoveries — current-makes-magnetism and magnet-motion-makes-current — power everything modern.\n\n**Motor**: Pass current through a coil of wire in a magnetic field → the coil experiences force and spins. Result: electrical energy → mechanical motion. Every fan, every electric car, every washing machine.\n\n**Generator**: Spin a coil of wire in a magnetic field → current is induced in the coil. Result: mechanical motion → electrical energy. Every power plant — nuclear, coal, wind, hydroelectric — is essentially a generator of this type.\n\nThat's the same phenomenon, running in opposite directions. Brilliant engineering built on brilliant physics.

When a magnetic field through a loop of wire CHANGES, current is induced in the wire. The faster the change, the bigger the current. The more loops, the bigger the current.\n\nThis is why power plants work (changing the magnetic field through rotating coils). It's also why transformers work (stepping voltage up for long-distance transmission, stepping down for your wall outlet). And it's why induction stovetops, wireless phone chargers, and induction loop hearing systems in public spaces work.\n\nOne principle. Enormous consequences.

Electromagnetism is one of the four fundamental forces of nature, alongside gravity, the weak, and strong nuclear forces. It's the force behind chemistry, biology (nerve signals are electrical!), and nearly all of modern engineering. The unification of electricity and magnetism was one of humanity's most profound intellectual achievements — and the base on which Einstein later built special relativity and modern physics.