Illustrated Physics
physics, drawn
The divisions

Fields

The divisions physics is conventionally cut into. They are partly an accident of history — the same differential equation turns up in four of them — but they are how a reader arrives, so the collection carries them and then spends most of its time crossing them.
The pendulum's phase portrait. Angle plotted against angular velocity. Closed loops are swinging back and forth; the open curves above and below are rotating all the way round; the dashed curve between them is the separatrix.

Mechanics

Motion, force, and the quantities that refuse to change.

57 essays A travelling wave, caught at one instant. A sine wave plotted against position at a fixed moment. The wavelength is the distance between repeats. The ghosted curve is the same wave a moment later.

Waves

Oscillation, and everything that turns out to be an oscillation.

57 essays Refraction from n = 1 into n = 1.5. A ray crossing a boundary between media of refractive index 1 and 1.5, bending by the amount Snell's law requires.

Optics

Light, and the small number of rules it obeys.

57 essays The field of a dipole. Field lines traced from a positive charge toward a negative one. Every line does eventually close on the negative charge, but the outer ones loop far outside any frame, so this picture is a crop rather than the whole field.

Electromagnetism

Charge, field, and the lines drawn between them.

58 essays Molecular speeds at 4 temperatures. The distribution of molecular speeds in a gas, with each curve enclosing the same area. Raising the temperature moves the peak right and lowers it: the same molecules, spread over a wider range of speeds.

Thermodynamics

Heat, disorder, and the one law with a direction in it.

57 essays A spacetime diagram at β = 0.5. Position across, time up, in units where light travels at 45°. The shaded wedges are the future and past reachable by light; the tilted axes belong to an observer moving at 0.5 of the speed of light.

Relativity

Space and time, drawn on the same axes.

57 essays The blackbody spectrum, against what classical physics predicted. Spectral exitance against wavelength for a blackbody at 3000, 4000, 5000 kelvin, in kilowatts per square metre per nanometre. Each curve peaks at the wavelength Wien's displacement law gives — 966 nm at 3000 K, 724 nm at 4000 K, 580 nm at 5000 K — and falls to nothing at short wavelengths.

Quantum

Where the continuous picture runs out, and what replaces it.

58 essays Three vessels, one pressure. Three vessels filled to the same depth of 3 m. The pressure on each base is 29.4 kPa — identical, because pressure is set by depth — while the weight of water each holds differs by a factor of 4.7. The base of the flaring vessel carries more force than the water standing over it weighs.

Fluids

Matter that will not hold a shape, and the forces that act in it anyway.

57 essays Light crossing an accelerating box. A pulse crosses a box 6 m wide while the box accelerates at 9.81 m/s². The crossing takes 2·10⁻⁸ s, in which the far wall gains 1.96·10⁻⁷ m/s, so the pulse lands 1.96·10⁻¹⁵ m below the height it left at — and the path is a parabola. An observer sealed inside cannot tell that from a beam of light bending in a gravitational field, and the equivalence principle says there is nothing to tell. The sag is drawn 5.6·10¹⁴ times its true size.

Astrophysics

Gravity read as geometry, and the laws carried where no laboratory can follow.

57 essays

A field says what an essay is about. A ladder says what else there is to say about one idea, and a concept is a named object with every essay that touches it. Themes cut across all four.