Browse All 842 Physics Terms A–Z

Debye Length Plasma Physics m (Meters) The characteristic spatial scale over which mobile charge carriers in a plasma screen out localized electrostatic potentials. λ_D = √(ε₀ k_B T_e / (n_e e²))

Decay Constant Nuclear & Particle Physics s⁻¹ or years⁻¹ The probability per unit time that an individual radioactive nucleus will undergo spontaneous nuclear decay. λ = -(1 / N) (dN / dt) = ln(2) / t_½

Decibel Scale Waves, Oscillations & Acoustics dB A logarithmic scale used to express sound intensity or sound pressure level relative to a reference threshold. β = 10 log10(I / I0), I0 = 10⁻¹² W/m²

Dendrochronology Geophysics & Environmental Physics Annual Calendar Years The scientific discipline of dating annual tree rings to reconstruct past local climate conditions and calendar dates. Tree Ring Width w = f(Temperature, Precipitation, Soil Moisture)

Dense Plasma Focus Plasma Physics kJ (Capacitor Energy) & Short Pulses (ns) A pulsed plasma device that uses electromagnetic forces to accelerate and pinch a plasma column into a short-lived, high-density hot spot. E_k ~ 10–100 kJ (Electromagnetic Acceleration and Pinch)

Density Foundations of Physics kg/m³ The mass per unit volume of a substance under specified temperature and pressure conditions. ρ = m / V

Dependent Variable Foundations of Physics Various The factor measured or observed in an experiment that changes in response to manipulations of the independent variable.

Deposition Thermodynamics & Statistical Physics J/kg or J/mol The direct phase transition of a gas into a solid without passing through an intermediate liquid phase. ΔH_dep = -ΔH_sub

Derived Units Foundations of Physics Various Units of measurement formed as algebraic combinations of the seven fundamental SI base units.

Diamagnetism Electromagnetism & Electronics Dimensionless A universal weak magnetic property of matter where applied magnetic fields induce opposing atomic orbital currents that weakly repel the material. χ_m < 0 (χ_m ~ -10⁻⁵)

Differential Equation Mathematical & Computational Physics Mathematical Equation Class A mathematical equation relating an unknown function to its derivatives with respect to one or more independent variables. F(x, y, y', y'', ..., y^(n)) = 0, d²y/dx² + ω² y = 0

Diffraction Optics m The bending, spreading, and interference of light waves around the edges of an obstacle or through a narrow aperture. a sin θ = m λ (Single Slit Minima)

Diffraction Grating Optics m (slits per mm) An optical element with a periodic structure of thousands of closely spaced parallel rulings or lines that disperses light into its component spectral wavelengths. d sin θ = m λ (m = 0, ±1, ±2, ...)

Dimensional Analysis Foundations of Physics Dimensionless An analytical method used to verify equation consistency and establish mathematical relationships between physical quantities by examining fundamental dimensions. [M]^a [L]^b [T]^c

Dirac Delta Function Mathematical & Computational Physics 1 / [x Unit] A generalized function (distribution) that is zero everywhere except at x = 0 where it is infinite, with an integral over all space equal to 1. δ(x) = 0 for x ≠ 0, ∫_(-∞)^(∞) δ(x) dx = 1, ∫ f(x) δ(x - a) dx = f(a)

Dislocation Condensed Matter & Materials Physics nm (Burgers Vector Magnitude) A 1D line defect or irregularity in a crystal lattice where atomic planes are misaligned, governing plastic deformation in metals. Burgers Vector b (Edge: b ⊥ line; Screw: b ∥ line)

Dispersive Power Optics Dimensionless A dimensionless material property measuring a prism or lens material's ability to separate different spectral wavelengths. ω = (n_v - n_r) / (n_y - 1)

Displacement Current Electromagnetism & Electronics A (Amperes) A fictitious effective current term introduced by Maxwell to account for magnetic field generation by time-varying electric fields. I_d = ε₀ (dΦ_E / dt)

Divergence Mathematical & Computational Physics [Vector Field Unit] / m A scalar differential operator measuring the net outward flux density of a vector field from an infinitesimal volume around a given point. div F = ∇ · F = (∂F_x/∂x) + (∂F_y/∂y) + (∂F_z/∂z)

Divergence Theorem Mathematical & Computational Physics Vector Integral Theorem A fundamental theorem of vector calculus stating that the outward flux of a vector field through a closed surface ∂V equals the volume integral of its divergence over the enclosed region V. ∯_(∂V) F · dA = ∭_V (∇ · F) dV

Divertor Plasma Physics MW/m² (Heat Flux Capacity) A component in a fusion tokamak that uses magnetic null points (X-points) to extract helium ash and heat exhaust from the plasma edge. Magnetic X-point Separatrix Boundary Geometry

DNA Biophysics Biophysics & Medical Physics nm (Persistence Length) & pN (Stretching Force) The physical study of mechanical stiffness, elasticity, topology, and base-pairing thermodynamics of double-stranded DNA. Persistence Length A_p ≈ 50 nm, Contour Length L

Doping Condensed Matter & Materials Physics Atoms/cm³ (Dopant Concentration) The intentional insertion of trace impurity atoms into an intrinsic semiconductor lattice to drastically modulate its electrical conductivity and charge carrier density. N_D (Donors) or N_A (Acceptors) ~ 10¹⁴ to 10¹⁹ cm⁻³

Doppler Effect Waves, Oscillations & Acoustics Hz The apparent shift in observed wave frequency caused by relative motion between the wave source and the observer. f_obs = f_src (v ± v_obs) / (v ∓ v_src)

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