THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Dark matter
Non-luminous, non-baryonic matter that outweighs ordinary atoms about five to one and binds galaxies and clusters together.
Debye temperature
The characteristic temperature above which a solid behaves classically and below which its heat capacity collapses toward zero.
Degrees of freedom
The independent ways a molecule can store energy — translation, rotation, vibration — each counted toward its heat capacity.
derivative
The instantaneous rate of change of one quantity with respect to another; geometrically, the slope of the tangent line to a curve.
Dielectric
An insulating material that can be polarized but does not conduct — its bound charges shift locally in response to a field while no current flows.
Dielectric constant
The dimensionless ratio κ = ε/ε₀ of a material's permittivity to that of vacuum. Tells you how much a dielectric amplifies a capacitor's storage capacity.
Dirac quantization condition
The 1931 Dirac result that the existence of a single magnetic monopole anywhere in the universe forces electric charge to be quantised in integer multiples of e = 2πℏ/(g μ₀ c), where g is the magnetic charge. Turns the empirical fact of charge quantisation into a theoretical consequence.
Displacement current
The term ε₀ ∂E/∂t Maxwell added to Ampère's law in 1861 to restore consistency with charge conservation. A changing electric field produces a magnetic field just as a current does — and the term makes Maxwell's equations predict light.
Displacement field
The vector D = ε₀E + P whose divergence equals only the free charge density. Lets you do Gauss's law inside a dielectric without tracking bound charges.
Divergence
A scalar measure of how much a vector field spreads outward from a point, per unit volume. ∇·F = source density.
drag
Resistive force exerted on a body moving through a fluid; linear in velocity at low speeds, quadratic at high speeds.
Dual field tensor
The Hodge dual *F^{μν} = (1/2) ε^{μνρσ} F_{ρσ} of the electromagnetic field tensor, obtained by swapping E and cB (up to signs in mostly-minus signature). Sources magnetic monopoles in the symmetric Maxwell equations; never observed sourced.
Duane-Hunt limit
The sharp high-energy cutoff of the bremsstrahlung X-ray spectrum at E_max = eU, where U is the accelerating voltage of the tube. Discovered by William Duane and Franklin Hunt at Harvard in 1915 and one of the early confirmations that E = hν.
Dulong–Petit law
Most simple solids have a molar heat capacity near 3R — an early hint of energy equipartition among atomic vibrations.
eccentricity
Dimensionless number between 0 and 1 describing how squished an ellipse is.
Einstein equivalence principle
The form of the equivalence principle Einstein needed for general relativity: WEP + local Lorentz invariance + local position invariance. Inside any sufficiently small freely falling laboratory, the laws of physics reduce to special relativity, and any deviation would be measurable by a sensitive enough experiment.
Einstein field equations
G_{μν} = (8πG/c⁴) T_{μν}. Ten coupled nonlinear partial differential equations relating spacetime geometry (left) to matter-energy distribution (right). The defining equations of general relativity. Published November 1915 by Einstein; near-simultaneously derived by Hilbert via the Einstein-Hilbert action.
Einstein relation
D = k_BT/(6πηr) and its kin — the bridge tying random diffusion to viscous drag, the first fluctuation-dissipation theorem.
Einstein tensor
G_{μν} = R_{μν} − (1/2) R g_{μν}. The unique divergence-free combination of Ricci and metric — ∇^μ G_{μν} = 0, a consequence of the contracted Bianchi identities. The geometric side of Einstein's field equations G_{μν} = (8πG/c⁴) T_{μν}.
elastic collision
A collision in which total kinetic energy is conserved as well as total momentum.
Electric charge
The fundamental conserved quantity that produces electric forces. Comes in ± signs. Measured in coulombs.
Electric field
The force per unit charge that a test charge would feel at a given point. A vector field filling all of space. Units: newtons per coulomb, equivalently volts per metre.
Electric potential
The electrostatic potential energy per unit charge at a point. A scalar field measured in volts. V = −∫E·dℓ from a reference point.
Electric susceptibility
The dimensionless coefficient χ_e in P = ε₀χ_e E that measures how easily a dielectric polarizes in response to an applied electric field.
Electromagnetic duality
The symmetry E → cB, cB → −E (equivalently F^{μν} → *F^{μν}) that maps the source-free Maxwell equations to themselves. In a universe with magnetic monopoles, the duality extends to interchanging electric and magnetic charges/currents, restoring perfect E↔B symmetry to the field equations.
Electromagnetic field
The unified field consisting of both the electric field E and the magnetic field B (equivalently, the antisymmetric tensor F^μν). Classical electromagnetism is the study of its dynamics. Full treatment across §07–§08.
Electromagnetic field tensor
The rank-2 antisymmetric 4×4 tensor F^{μν} that packages the three components of E and three components of B into one Lorentz-covariant object, with F^{0i} = E_i/c and F^{ij} = -ε_{ijk} B_k. Also called the Faraday tensor.
Electromagnetic spectrum
The full range of frequencies (or equivalently wavelengths) of EM radiation, from kilohertz radio to zettahertz gamma rays. All regions are the same physical phenomenon — classical EM waves — differing only in ω.
Electromagnetic wave equation
The second-order PDE ∇²E = (1/c²)∂²E/∂t² (and identically for B), derived from Maxwell's equations in source-free vacuum. Its plane-wave solutions propagate at c = 1/√(μ₀ε₀).
Electromotive force (EMF)
The work per unit charge done by a source on charges as they move around a closed circuit, measured in volts. Despite the name, EMF is not a force; it is the energy-per-charge a battery, generator, or induction process supplies.