Browse All 842 Physics Terms A–Z

Drag Force Classical Mechanics N The fluid resistance force opposing the motion of an object moving through a liquid or gas. Fd = ½ ρ v² Cd A

Drake Equation Astrophysics, Cosmology & Relativity Number of Civilizations N A probabilistic equation formulated by Frank Drake in 1961 to estimate the number N of active, communicative extraterrestrial civilizations in the Milky Way. N = R* · f_p · n_e · f_l · f_i · f_c · L

Ductility Condensed Matter & Materials Physics % (Percent Elongation or Area Reduction) A material's mechanical capability to undergo significant plastic strain deformation under tensile stress before fracturing. % Elongation = ((L_break - L₀) / L₀) × 100%

Dusty Plasma Plasma Physics μm (Dust Size) & Charged State A complex plasma containing solid nanometer-to-micrometer scale dust grains that collect high negative charges from surrounding plasma electrons. Q_dust ~ - 10³ e to - 10⁵ e (Charged Micron Dust Grains)

Dynamic Equilibrium Classical Mechanics N, N·m The state of a physical system moving at constant linear velocity and constant angular velocity with zero net force and zero net torque. ∑ F = 0 (v = const ≠ 0), ∑ τ = 0 (ω = const)

Earth's Energy Budget Geophysics & Environmental Physics W/m² (Watts per square meter) The precise balance between incoming solar radiation absorbed by Earth and outgoing thermal infrared radiation emitted back into space. F_solar (340 W/m²) = F_reflected (100 W/m²) + F_emitted_IR (240 W/m²)

Earth's Inner Core Geophysics & Environmental Physics GPa (Pressure) & Kelvin (~5700 K) The solid, innermost spherical core of Earth composed primarily of a crystallized iron-nickel alloy at intense pressure. Radius R ≈ 1220 km, Pressure P ≈ 330–360 GPa

Earth's Outer Core Geophysics & Environmental Physics Pa·s (Liquid Viscosity ~10⁻³ Pa·s) The fluid interior layer of Earth ~2,260 km thick composed of liquid iron-nickel alloy surrounding the solid inner core. Radius ~ 3480 km to 1220 km (Liquid Iron-Nickel Alloy)

Echo Waves, Oscillations & Acoustics m A distinct reflected sound wave received at the listener's ear with sufficient time delay after the direct sound. d = v t / 2

Eddy Currents Electromagnetism & Electronics W (Watts) Loops of electric current induced within solid bulk conductors by a changing magnetic field according to Faraday's law. P_eddy ∝ f² B_max² t²

Edge Localized Modes (ELMs) Plasma Physics MJ (Energy Loss Pulse per ELM) Periodic magnetohydrodynamic bursts occurring at the boundary edge pedestal of H-mode tokamak plasmas, expelling heat and particles toward reactor walls. Magnetohydrodynamic Peeling-Ballooning Boundary Instability

Effective Dose Biophysics & Medical Physics mSv (Millisieverts) A radiation protection quantity representing the overall stochastic health risk to the whole body by weighting organ equivalent doses by tissue sensitivity factors w_T. E = ∑ H_T × w_T = ∑ D_T × w_R × w_T

Effective Mass Condensed Matter & Materials Physics kg or Fraction of Rest Mass m_e A parameter that simplifies modeling electron and hole dynamics in a crystal lattice by treating them as free particles with a modified mass determined by energy band curvature. m* = ℏ² / (d²E / dk²)

Efficiency Classical Mechanics % The ratio of useful work output to total energy input for a machine or system. η = (W_output / E_input) × 100%

Eigenvalue Problem Mathematical & Computational Physics Eigenvalues λ (Scalar Units) The linear algebra problem of finding non-zero eigenvectors v that, when operated on by a linear operator Â, yield a scalar multiple λ of themselves. Â v = λ v, det(A - λ I) = 0

Eigenvalues and Eigenvectors Mathematical & Computational Physics Linear Algebra Object Special vectors v associated with a linear transformation matrix A that maintain their spatial direction when operated on, scaled only by factor λ. A v = λ v (where v ≠ 0)

Einstein Field Equations Astrophysics, Cosmology & Relativity Non-linear 4D Tensor Equations The set of 10 non-linear partial differential equations in general relativity relating spacetime curvature (Ricci tensor R_μν) to stress-energy-momentum T_μν. R_μν - ½ R g_μν + Λ g_μν = (8 π G / c⁴) T_μν

Elastic Collision Classical Mechanics kg·m/s, J A collision between bodies in which both total linear momentum and total kinetic energy are conserved. ∑ p_i = ∑ p_f, ∑ KE_i = ∑ KE_f

Elasticity Classical Mechanics Pa (N/m²) The mechanical property of a material to deform under applied stress and return to its original shape when stress is removed. Stress / Strain = Elastic Modulus (E)

Electric Charge Electromagnetism & Electronics C (Coulombs) A fundamental intrinsic physical property of subatomic particles that causes matter to experience an electrostatic force when placed in an electromagnetic field. q = n e, e = 1.602 × 10⁻¹⁹ C

Electric Current Electromagnetism & Electronics A (Amperes = C/s) The rate of net flow of electric charge through a cross-sectional area of a conductor per unit time. I = dq/dt = n q v_d A

Electric Dipole Electromagnetism & Electronics C·m (Coulomb-meters) A system of two equal and opposite electric charges (+q and -q) separated by a small distance d. p = q d, V = (1 / (4π ε₀)) (p cos θ / r²)

Electric Dipole Moment Electromagnetism & Electronics C·m (or Debye) A vector quantity measuring the strength and orientation of an electric dipole, pointing from the negative charge toward the positive charge. p = q d, τ = p × E, U = -p · E

Electric Field Electromagnetism & Electronics N/C (or V/m) A vector field surrounding an electric charge that exerts an electrostatic force on any other charge placed within the field. E = F / q = -∇V = (1 / (4π ε₀)) q / r² r̂

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