THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Unitarity
The principle that quantum time evolution preserves probability and never destroys information.
Universality
The principle that critical behaviour depends only on dimensionality and symmetry — never on what the system is made of.
Vapour-compression cycle
The loop inside every real refrigerator — and the reason it boils something.
vector
A quantity with both magnitude and direction; represented as an arrow and added tip-to-tail.
Vector potential
The vector field A whose curl gives the magnetic field: B = ∇×A. Lets you compute B from a scalar-like integral over the source currents.
velocity
The rate of change of position with respect to time; a vector in 2D and 3D, a signed scalar in 1D.
Venturi effect
The drop in static pressure observed when a fluid flows through a constricted section of pipe.
vis viva equation
v² = GM(2/r − 1/a) — gives the orbital speed at any distance r for an orbit with semi-major axis a.
Viscosity
A fluid's internal resistance to shear. For Newtonian fluids, shear stress = η·(du/dy). Unit: Pa·s.
Volt
The SI unit of electric potential difference. One volt equals one joule of work per coulomb of charge transported. Symbol: V.
Voltage divider
Two resistors in series between a voltage source and ground, with the output voltage taken at the midpoint. V_out = V_in · R₂/(R₁+R₂). The simplest non-trivial circuit.
Vortex
A coherent region of swirling fluid — the basic building block of turbulent and near-turbulent flow.
Wave equation
The linear PDE whose solutions are any right-mover plus any left-mover at speed v.
Wave packet
A localised wave formed by superposing many plane-wave components with a narrow band of wavenumbers.
Waveguide mode
A specific transverse field pattern that propagates along a waveguide without distortion, characterised by a propagation constant β and a cutoff frequency below which the mode is evanescent.
Wavelength
The spatial period of a wave — distance crest to crest, symbol λ.
Wavelength (EM)
The spatial period λ of an EM plane wave, the distance between successive points of equal phase. Related to frequency by λ = c/f in vacuum, λ = c/(nf) in a medium. Ranges from kilometres (longwave radio) to femtometres (gamma).
Wavenumber
k = 2π/λ, the spatial angular frequency of a wave, measured in rad/m. The wavevector k has magnitude k and direction along propagation. In spectroscopy, wavenumber often means k̃ = 1/λ in cm⁻¹.
Weak equivalence principle
The statement that gravitational mass equals inertial mass for any test particle, regardless of composition — equivalently, that all bodies in vacuum fall with identical acceleration. Galileo's Pisa-tower observation, Eötvös's torsion-balance precision result, and the MICROSCOPE 2017 satellite null result confirm it to ≲ 10⁻¹⁵.
Weber
The SI unit of magnetic flux. One weber is the flux through a one-square-metre area of a one-tesla field. Symbol: Wb.
work
The energy transferred to a body by a force acting over a distance: W = F · d · cos θ.
Working substance
The matter — gas, vapour, or fluid — that an engine repeatedly expands and compresses to convert heat into work.
World-line
The locus of events in spacetime traced out by a particle as it moves; a 1D curve in the 4D manifold. Massive particles have timelike world-lines; photons have null world-lines; the proper time elapsed on the particle's clock is the arc length of its world-line in the Minkowski metric.
Yang-Mills equations
The non-abelian generalisation of Maxwell's equations: D_μ F^{aμν} = J^{aν}, where the covariant derivative D_μ = ∂_μ + i g [A_μ, ·] couples the gauge field to itself through the structure constants of the gauge group. The field equations of every non-abelian gauge theory in physics.
Young's interference experiment
Thomas Young's 1801 double-slit experiment that demonstrated the wave nature of light by producing interference fringes. Full treatment in §09.7 interference and §09.8 diffraction-and-the-double-slit.
Zeroth law of thermodynamics
If A is in thermal equilibrium with B, and B with C, then A is in equilibrium with C — the transitivity that makes temperature meaningful.