Physics 3 — Electricity & Magnetism
Coulomb’s law, fields, magnetic force on moving charge, the solenoid, and the electromagnetic wave.
Two invisible fields, one unified story. Electric fields push on charges; magnetic fields push on moving charges; and Maxwell's punchline — a changing one creates the other — is why light exists. (Working in plain SI numbers here; the units are noted per line.)
- 01 Coulomb’s law — the inverse square Charges push with F = k·q₁q₂/r² — same shape as gravity, but ~10³⁶ times stronger. The inverse square means doubling the distance quarters the force:
- 02 The field — force per unit charge Instead of asking "what force on THIS charge", map what any charge would feel: E = k·q/r², in newtons per coulomb. The field exists whether or not…
- 03 Magnetic force — only on the moving A magnetic field does nothing to a parked charge. Move, and it pushes sideways — perpendicular to both v and B: F = q·v·B. Sideways force means circles, which…
- 04 Making fields: the solenoid Wrap wire into a coil and current makes a uniform field inside: B = μ₀·n·I. More turns per meter, more field — this is the electromagnet, the relay, the MRI…
- 05 The payoff: light A changing E makes B; a changing B makes E; the disturbance feeds itself and propagates at exactly c = 1/√(μ₀ε₀). Maxwell computed that speed, recognized the…
next course: Physics 4 — Thermo & Fluids →