THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Polarization density
The vector P = (dipole moment)/(volume), measured in coulombs per square metre, that summarises how strongly a dielectric is polarized at each point.
potential energy
Energy stored in the configuration of a system against a conservative force, retrievable by reversing that configuration.
potential well
Region of potential energy that traps a system; shape determines oscillation character.
power
The rate at which work is done or energy is transferred: P = dW/dt, measured in watts (J/s).
Poynting vector
S = (1/μ₀)·E×B. The vector whose magnitude gives the energy-flux density (W/m²) of the electromagnetic field and whose direction gives the flow direction. Introduced by Poynting in 1884.
Poynting's theorem
∂u/∂t + ∇·S = −J·E. The local statement of energy conservation for the electromagnetic field: rate of change of field-energy density plus divergence of energy flux equals the negative of work done by fields on charges.
precession
The slow conical sweep of a spinning body's axis of rotation when a torque acts perpendicular to its angular momentum.
Precession of the equinoxes
The slow 26,000-year conical sweep of Earth's rotation axis, which makes the equinoxes drift through the zodiac.
Precision Lorentz tests
The modern experimental program that constrains hypothetical Lorentz-violating extensions of special relativity to parts in 10⁻¹⁸ or better — Hughes-Drever magnetic-resonance comparisons, Kennedy-Thorndike interferometric asymmetries, and modern atomic-clock-comparison searches for any frame-dependence of fundamental physics.
Pressure
Force per unit area acting perpendicular to a surface. Scalar. Unit: pascal (Pa = N/m²).
principal axes
Three mutually perpendicular body-fixed axes about which the inertia tensor is diagonal; spinning about them produces no wobble.
Principle of least action
Of all paths a system could take between two fixed events, the one realised in nature is the path for which the action S is stationary.
projectile motion
Motion of an object fired into the air under gravity alone; the trajectory is a parabola in vacuum.
Proper time
The time τ measured by a clock carried along a particle's world-line. Related to coordinate time by dτ = dt/γ, so dτ = √(1 − β²) dt; integrated along a timelike world-line gives the arc length in the Minkowski metric. Lorentz-invariant; the geometric content of time dilation.
quadrant
Pre-telescope astronomical instrument for measuring the angular position of stars and planets.
quality factor
Quality factor Q = ω₀/γ; number of oscillations before energy drops to 1/e.
Quality factor (Q)
Dimensionless number Q = ω₀L/R = 1/(ω₀RC) = (1/R)·√(L/C) for an RLC circuit, measuring how sharply resonant the response is. Equivalently Q = 2π · (energy stored) / (energy lost per cycle).
Radiation pattern
The angular distribution of an antenna's far-field radiated power as a function of direction, dP/dΩ(θ, φ). For a short dipole the pattern is the sin²θ doughnut; for arrays and apertures it is the magnitude-squared Fourier transform of the current distribution.
Radiation pressure
The mechanical pressure an electromagnetic wave exerts on a surface it strikes: I/c for absorbers, 2I/c for perfect reflectors, where I is the intensity in W/m². Discovered in principle by Maxwell (1871), measured by Lebedev (1901).
radius of gyration
The distance k = √(I/M) at which a point mass equal to the body's total mass would have the same moment of inertia.
range
Horizontal distance covered by a projectile before it returns to its launch height; maximised at 45° in vacuum.
RC time constant
τ = RC. The characteristic time for an RC circuit to charge to 1−1/e ≈ 63% of its final voltage, or discharge to 1/e ≈ 37% of its initial voltage. In seconds when R is in ohms and C in farads.
Reactance
The imaginary part X of impedance Z = R + jX. Inductive reactance X_L = ωL is positive; capacitive reactance X_C = −1/(ωC) is negative. Reactance stores energy without dissipating it.
Redshift
The stretching of a wave's wavelength when source and observer move apart.
Refractive index
The ratio n = c/v_p of the vacuum speed of light to the phase velocity in the medium.
Relativistic beaming
The forward concentration of radiation from a relativistic source into a cone of half-angle ≈ 1/γ, caused by the Lorentz transformation of solid angles. Responsible for pulsar pulsed emission, blazar flux variability, and the lighthouse behaviour of synchrotron sources.
Relativistic Doppler effect
The frequency shift of light from a moving source observed in any inertial frame, including the time-dilation factor γ alongside the classical motion contribution. For a source receding at radial velocity v: f' = f √((1 − β)/(1 + β)). Reduces to the classical Doppler formula plus a γ correction.
Relativistic velocity addition
The rule for combining velocities in special relativity. For collinear motion: u' = (u − v)/(1 − uv/c²). Replaces the Galilean rule u' = u − v; ensures no combination of subluminal velocities exceeds c.
Relativity of simultaneity
The result that two spatially-separated events judged simultaneous in one inertial frame are not simultaneous in any other frame moving relative to the first. The deepest break with Newtonian intuition; the geometric content of the Lorentz transformation's time-mixing.
Resistance
The ratio R = V/I for a conductor obeying Ohm's law. Measured in ohms (Ω). Determined by the conductor's geometry (R = ρℓ/A) and material resistivity ρ.