THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Critical temperature
The temperature T_c below which a superconductor exhibits zero resistance and the Meißner effect. Ranges from 1.2 K (aluminium) to 135 K (cuprate high-T_c materials) to above 250 K in hydride compounds under pressure.
Cross product
The vector a × b perpendicular to both a and b, with magnitude |a||b|sin θ. The operation that produces torque and angular momentum.
Curie temperature
The critical temperature T_c above which a ferromagnet loses its spontaneous magnetization and becomes an ordinary paramagnet. 1043 K for iron, 627 K for nickel, 1388 K for cobalt.
Curl
A vector operation ∇× that measures how much a vector field circulates around a point. Nonzero curl means the field has rotational structure; zero curl means it is conservative.
Current density
The vector J = nqv giving the charge passing per unit time through a unit area perpendicular to the flow direction. Units: amperes per square metre.
cycloid
Curve traced by a point on a rolling circle; solves both tautochrone and brachistochrone.
damping
Energy dissipation causing oscillation amplitude to decay exponentially.
derivative
The instantaneous rate of change of one quantity with respect to another; geometrically, the slope of the tangent line to a curve.
Destructive interference
The superposition of two or more coherent waves in anti-phase, producing an amplitude (and intensity) less than the sum. Total cancellation requires equal amplitudes and a path-length difference of (m+½)λ.
Diamagnetism
The universal property of all matter to develop a weak magnetization *opposite* to an applied field. In most materials it is swamped by para- or ferromagnetism, but in closed-shell atoms it dominates.
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.
Diffraction (EM)
The bending of light around obstacles and the spreading of light beyond apertures, resulting from the wave nature of EM radiation. Sets the resolution limit of every imaging system at about λ/NA.
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.
Dispersion
The dependence of wave speed on wavelength or frequency — the reason a pulse spreads and a prism makes a rainbow.
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.
Doppler effect
The shift in observed frequency when a wave source and observer move relative to each other.
Double-slit diffraction
Young's 1801 experiment. Two coherent slits a distance d apart produce an interference pattern of bright fringes at d sin θ = mλ, modulated by the single-slit envelope of each slit's width.
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.
eccentricity
Dimensionless number between 0 and 1 describing how squished an ellipse is.
Eddy current
A circulating current induced inside a bulk conductor by a changing magnetic flux. Eddy currents dissipate energy as heat, and Lenz's law ensures the force on the conductor always opposes the relative motion of source and conductor.
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.