Browse All 842 Physics Terms A–Z

Equivalent Dose Biophysics & Medical Physics Sv (Sievert = J/kg) or rem A radiation protection dose quantity weighing absorbed dose by the specific biological effectiveness factor w_R of the radiation type. H = D × w_R (Radiation Weighting Factor w_R)

Error Propagation Mathematical & Computational Physics Uncertainty Units The mathematical evaluation of how uncertainties in measured input variables propagate into the final calculated physical quantity. σ_f² = ∑ (∂f / ∂x_i)² σ_x_i² + 2 ∑ (∂f/∂x_i)(∂f/∂x_j) Cov(x_i, x_j)

Escape Velocity Classical Mechanics m/s The minimum speed required for a ballistic object to escape a massive body's gravitational field without further propulsion. v_esc = √(2 G M / R)

Euler Method Mathematical & Computational Physics First-Order Numerical ODE Integration A basic first-order numerical procedure for solving ordinary differential equations with a given initial value. y_(n+1) = y_n + h f(t_n, y_n)

Euler-Lagrange Equation Mathematical & Computational Physics Calculus of Variations Equation The fundamental partial differential equation of the calculus of variations that a function must satisfy to make a functional stationary. (d / dt) (∂L / ∂q̇_i) - (∂L / ∂q_i) = 0

Event Horizon Astrophysics, Cosmology & Relativity km or m (Boundary Radius) The boundary surrounding a black hole beyond which escape velocity exceeds the speed of light, preventing any matter or signal from escaping. R_s = 2 G M / c²

Exciton Condensed Matter & Materials Physics eV (Binding Energy) A neutral quasiparticle consisting of a conduction electron bound to a valence band hole by electrostatic Coulomb attraction. E_exciton = E_g - E_b (where E_b = μ e⁴ / (8 ε² h² n²))

Exoplanet Astrophysics, Cosmology & Relativity Earth Masses (M_⊕) or Jupiter Masses (M_J) A planet situated outside the Solar System that orbits a star, stellar remnant, or brown dwarf. Transit Depth ΔF / F = (R_planet / R_star)²

Expectation Value Quantum & Atomic Physics Same as measured quantity The probabilistic average or expected value of many physical measurements performed on an ensemble of identically prepared quantum states. ⟨A⟩ = ∫ ψ* Â ψ dx / ∫ ψ* ψ dx

Extrinsic Semiconductor Condensed Matter & Materials Physics cm⁻³ (Dopant Carrier Density) A semiconductor whose electrical conductivity is dominated by intentional impurity dopants rather than thermal excitation across the band gap. n p = n_i² (Mass Action Law)

Faraday's Law of Induction Electromagnetism & Electronics V (Volts) Maxwell's third equation stating that a time-varying magnetic flux through a closed conductive loop induces an electromotive force (EMF) around the loop. E = -dΦ_B / dt, ∇ × E = -∂B/∂t

Fast Fourier Transform (FFT) Mathematical & Computational Physics Computational FFT Algorithm An efficient algorithm that computes the Discrete Fourier Transform (DFT) of a sequence in O(N log N) operations instead of O(N²). Cooley-Tukey Divide-and-Conquer: O(N log N) vs O(N²)

Fatigue Condensed Matter & Materials Physics Cycles to Failure N_f The progressive structural weakening and microcrack failure of a material subjected to repeated cyclic loading stresses below its ultimate tensile strength. Basquin's Relation: S_a = σ'_f (2 N_f)^b

Fermat's Principle Optics s (or m) The fundamental principle stating that light traveling between two points follows the path that requires the stationary (usually minimum) optical path time. Optical Path Length L = ∫ n(s) ds = extremum

Fermi Energy Quantum & Atomic Physics eV or Joules The highest occupied kinetic energy state of non-interacting fermions in a quantum system at absolute zero temperature (0 K). E_F = (ℏ² / (2 m)) (3 π² n)^(2/3)

Fermi Level Condensed Matter & Materials Physics eV (Chemical Potential) The thermodynamic Fermi-Dirac chemical potential representing the energy level at which the probability of electron occupancy is exactly 50% at finite temperature. f(E) = 1 / (e^((E - E_F) / (k_B T)) + 1)

Fermi Paradox Astrophysics, Cosmology & Relativity Astronomical Paradox The apparent contradiction between high probabilistic estimates for the existence of extraterrestrial civilizations and the complete lack of physical evidence or contact. Enrico Fermi: 'Where is everybody?'

Fermions Quantum & Atomic Physics Dimensionless Particle Class Subatomic particles possessing half-integer intrinsic spin that obey Fermi-Dirac statistics and the Pauli Exclusion Principle. Spin = ½, ³/₂, ⁵/₂...

Ferromagnetism Electromagnetism & Electronics Dimensionless A strong magnetic phenomenon in which atomic magnetic moments spontaneously align parallel into localized magnetic domains, producing permanent magnetization. μ_r ≫ 1 (μ_r ~ 1,000–100,000)

Feynman Diagram Nuclear & Particle Physics Calculational Diagram Visual pictorial representations of mathematical expressions describing the quantum interaction and scattering behavior of subatomic particles. Pictorial matrix elements M_fi in QFT perturbational expansions

Fick's Laws of Diffusion Biophysics & Medical Physics mol/(m²·s) (Flux J) & m²/s (Diffusion Coefficient D) Physical laws describing solute diffusion mass transport across biological concentration gradients. 1st: J = -D (dC / dx); 2nd: ∂C / ∂t = D (∂²C / ∂x²)

Field-Reversed Configuration (FRC) Plasma Physics Dimensionless High Beta ⟨β⟩ ~ 0.9 A high-beta compact toroid magnetic confinement concept where a closed-poloidal-field plasma torus is sustained inside a simple linear solenoid magnet. Average Beta ⟨β⟩ ≈ 1 (High-Beta Compact Toroid)

Fine Structure Quantum & Atomic Physics eV (Energy Splitting) The small splitting of atomic spectral lines resulting from relativistic mass corrections and spin-orbit coupling. E_fine ~ α² E_n, α ≈ 1 / 137

Finite Difference Method Mathematical & Computational Physics Grid Discretization Method A numerical method for solving differential equations by approximating derivatives with finite difference quotients on a discretized grid. f'(x) ≈ (f(x + h) - f(x - h)) / (2 h), f''(x) ≈ (f(x+h) - 2f(x) + f(x-h)) / h²

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