Browse All 842 Physics Terms A–Z

Heisenberg Uncertainty Principle Quantum & Atomic Physics J·s (Action Units) A fundamental quantum limit stating that canonical conjugate physical variables (such as position and momentum, or energy and time) cannot be simultaneously measured with arbitrary precision. Δx Δp ≥ ℏ / 2, ΔE Δt ≥ ℏ / 2

Helmholtz Free Energy Thermodynamics & Statistical Physics J A thermodynamic potential measuring maximum reversible work obtainable from a closed system at constant temperature and volume. F = U - T S, dF = -S dT - P dV

Hemodynamics Biophysics & Medical Physics L/min (Cardiac Output) & mmHg (Pressure) The physical study of blood flow dynamics, vascular resistance, and pressure relations within the circulatory system. Q = ΔP / R_v, R_v = (8 η L) / (π r⁴) (Poiseuille Flow)

Hermite Polynomials Mathematical & Computational Physics Dimensionless Orthogonal Polynomials A sequence of classical orthogonal polynomials satisfying Hermite's differential equation y'' - 2x y' + 2n y = 0. H_n(x) = (-1)^n e^(x²) (d^n / dx^n) (e^(-x²))

Hertzsprung-Russell (H-R) Diagram Astrophysics, Cosmology & Relativity Luminosity (L_☉) vs Surface Temp (K) A foundational scatter plot of stellar luminosity versus surface temperature (or spectral type) used to trace stellar evolution phases. L ∝ R² T^4 (Plot of Absolute Magnitude vs Spectral Type)

Hessian Matrix Mathematical & Computational Physics Second Derivative Matrix A square matrix of second-order partial derivatives of a scalar-valued function, describing local surface curvature and local extrema stability. H_ij = ∂²f / (∂x_i ∂x_j)

Higgs Boson Nuclear & Particle Physics GeV/c² (Mass Energy) An elementary scalar boson associated with the Higgs field that endows fundamental particles with rest mass via electroweak symmetry breaking. m_H ≈ 125.1 GeV/c², Spin = 0, Scalar Field φ

Hilbert Space Mathematical & Computational Physics Complete Inner Product Space A complete abstract vector space possessing an inner product that allows length and angle to be defined for infinite-dimensional function spaces. ⟨f, g⟩ = ∫ f*(x) g(x) dx, ||f|| = √(⟨f, f⟩)

Holography Optics N/A A technique for recording and reconstructing 3D wavefields by recording phase and amplitude interference patterns between object and reference laser beams. I(x,y) = |E_ref + E_obj|²

Hooke's Law Classical Mechanics N (k in N/m) A law of elasticity stating that restoring force exerted by a spring or elastic material is proportional to its extension or compression distance. F = -k x

Hubble's Law Astrophysics, Cosmology & Relativity km/s/Mpc (Hubble Constant) The cosmological observation that galaxies recede from Earth at velocities directly proportional to their proper distance. v = H₀ d, H₀ ≈ 70 (km/s) / Mpc

Hund's Rule of Maximum Multiplicity Quantum & Atomic Physics Dimensionless Rule A rule stating that when filling degenerate atomic orbitals, electrons occupy separate orbitals singly with parallel spins before pairing up. S_total = ∑ s_i (Maximized spin in degenerate orbitals)

Huygens-Fresnel Principle Waves, Oscillations & Acoustics N/A The wave propagation principle asserting that every point on a wavefront acts as a secondary source of spherical wavelets. New Wavefront = Envelope of Secondary Wavelets

Hydrogen Spectrum Quantum & Atomic Physics m⁻¹ (Wavenumber) The set of discrete electromagnetic emission and absorption lines produced when the electron in a hydrogen atom transitions between energy levels. 1 / λ = R_H (1 / n_f² - 1 / n_i²)

Hydrologic Cycle Geophysics & Environmental Physics Global Water Flux (km³/year) The continuous global movement and transformation of water through Earth's atmosphere, land surface, oceans, and subsurface aquifers. Water Balance: P = E + R + ΔS (Precipitation = Evaporation + Runoff + Storage Change)

Hydrothermal Vents Geophysics & Environmental Physics °C (Vent Fluid Temperature) Fissures on the ocean floor near volcanically active mid-ocean ridges that vent geothermally heated, mineral-rich seawater into the ocean. Geothermally Heated Seawater T ~ 350–400 °C

Hyperfine Structure Quantum & Atomic Physics eV or MHz (Small Energy Splitting) Extremely small energy level splittings in atoms caused by interaction between nuclear magnetic moments and electron magnetic fields. ΔE_HFS ∝ I_nucleus · J_electron

Hypothesis Foundations of Physics N/A A proposed, testable explanation for an observed physical phenomenon, serving as the starting point for experimental investigation.

Ice Core Analysis Geophysics & Environmental Physics Parts per thousand (‰ δ¹⁸O) / ppm (CO₂) The retrieval and paleoclimatological analysis of cylindrical ice samples extracted from deep polar ice sheets and alpine glaciers. Paleotemperature Proxy δ¹⁸O = ((¹⁸O/¹⁶O)_sample / (¹⁸O/¹⁶O)_standard - 1) × 1000‰

Ideal Gas Law Thermodynamics & Statistical Physics Pa·m³ = J The equation of state relating pressure, volume, temperature, and quantity of an ideal gas composed of non-interacting point particles. P V = n R T = N kB T

Impulse Classical Mechanics N·s (or kg·m/s) The vector quantity defined as the integral of a force over the time interval during which it acts, equal to the net change in momentum. J = ∫ F dt = Δp = m vf - m vi

Inclined Plane Classical Mechanics Dimensionless A simple machine consisting of a flat supporting surface tilted at an angle to the horizontal. MA = L / h = 1 / sin θ

Independent Variable Foundations of Physics Various The experimental condition or factor intentionally manipulated by the researcher to observe its effect on an outcome.

Inductive Reactance Electromagnetism & Electronics Ω (Ohms) The opposition offered by an inductor to alternating current flow, directly proportional to AC frequency and inductance. X_L = 2π f L = ω L

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