Browse All 842 Physics Terms A–Z

Inelastic Collision Classical Mechanics kg·m/s, J A collision in which total linear momentum is conserved, but total kinetic energy is not conserved. ∑ p_i = ∑ p_f, KE_final < KE_initial

Inertia Classical Mechanics kg The natural resistance of any physical object to any change in its velocity or state of motion. Quantified by Mass (m)

Inertial Confinement Fusion Plasma Physics g/cm² (Areal Density) & Mbar (Implosion Pressure) A fusion energy approach that uses intense laser or ion beam pulses to rapidly compress and heat a millimeter-scale fuel capsule to fusion ignition conditions. ρ R ≥ 1 g/cm² (Compressed Areal Density)

Inertial Mass Foundations of Physics kg A measure of an object's intrinsic resistance to changes in its velocity when subjected to a net force. m = F / a

Insolation Geophysics & Environmental Physics W/m² or kWh/m²/day The solar electromagnetic radiation power reaching a specified surface area per unit time. S = S₀ (R₀ / R)² cos Z

Intensive vs Extensive Properties Thermodynamics & Statistical Physics Various Classification of thermodynamic properties based on whether they depend on system size or amount of matter. Extensive: U, V, S, m, H; Intensive: T, P, ρ, μ

Interchange & Flute Instabilities Plasma Physics MHD Flute Mode Instability A magnetohydrodynamic (MHD) instability where high-pressure plasma and low-density magnetic fields interchange spatial positions in regions of unfavorable magnetic curvature. Bad Curvature Condition: (∇P) · (∇B) > 0

Internal Energy Thermodynamics & Statistical Physics J The total microscopic energy contained within a thermodynamic system, comprising kinetic energies of particle translational/rotational/vibrational motion and potential energies of intermolecular forces. U = (f/2) n R T (for ideal gas)

Intrinsic Semiconductor Condensed Matter & Materials Physics cm⁻³ (Carrier Density) An ultra-pure semiconductor crystal containing no significant dopant impurity species, where carrier concentration is determined purely by thermal excitation across the band gap. n = p = n_i = √(N_c N_v) e^(-E_g / (2 k_B T))

Ion Channels Biophysics & Medical Physics pS (Picosiemens Conductance) & pA (Current) Pore-forming transmembrane proteins that allow selective passive passage of specific ions down their electrochemical gradient. I_channel = g_channel (V_m - E_ion)

Ionization Energy Plasma Physics eV (Electron-Volts) The minimum energy required to detach an electron from a neutral atom or ion, initiating gas-to-plasma phase transformation. E_ion = h ν - K_e (Saha Ionization Equation)

Ionosphere Geophysics & Environmental Physics Electrons/m³ (Electron Density n_e) An ionized layer of Earth's upper atmosphere (~60 km to 1,000 km) where extreme solar UV and X-ray radiation ionize atmospheric gas atoms into free electrons and ions. Plasma Frequency f_p = (1 / 2π) √(n_e e² / (ε₀ m_e))

Ionospheric Scintillation Geophysics & Environmental Physics Dimensionless S₄ Index Rapid fluctuations in amplitude and phase of satellite radio signals caused by small-scale electron density irregularities in the ionosphere. Scintillation Index S₄ = √(⟨I²⟩ - ⟨I⟩²) / ⟨I⟩

Isobaric Process Thermodynamics & Statistical Physics J A thermodynamic change in state occurring under constant pressure conditions. ΔP = 0, W = P ΔV, Q = ΔH = n Cp ΔT

Isobars Nuclear & Particle Physics Dimensionless Species Nuclides of different chemical elements that share the exact same mass number A (total number of nucleons) while having different atomic numbers Z. A = Z + N = Constant

Isochoric Process Thermodynamics & Statistical Physics J A thermodynamic process occurring at constant volume, during which no mechanical boundary work is performed. ΔV = 0 ⇒ W = 0, Q = ΔU = n Cv ΔT

Isostasy Geophysics & Environmental Physics Meters (Crustal Root Depth) The state of gravitational equilibrium between Earth's lithospheric crust blocks floating at depths determined by their thickness and density upon the denser mantle. Airy Model: h_root = h_mountain (ρ_crust / (ρ_mantle - ρ_crust))

Isothermal Process Thermodynamics & Statistical Physics J A thermodynamic change in state occurring at a constant temperature throughout the system. ΔT = 0 ⇒ ΔU = 0, Q = W = n R T ln(V2/V1)

Isotones Nuclear & Particle Physics Dimensionless Species Nuclides of different chemical elements that share the exact same number of neutrons N = A - Z while possessing different proton numbers Z. N = A - Z = Constant

Isotopes Nuclear & Particle Physics Dimensionless Species Variants of a particular chemical element that possess identical atomic numbers Z (same number of protons) but different mass numbers A (different number of neutrons). Nuclide Symbol: ᴬ_Z X (Same Z, Different A)

Jacobian Matrix Mathematical & Computational Physics Coordinate Transformation Factor A matrix of all first-order partial derivatives of a vector-valued multivariable function, representing local linear coordinate transformations. J_ij = ∂f_i / ∂x_j, dV' = |det J| dV

Josephson Effect Condensed Matter & Materials Physics mA (Current), μV (Voltage) The quantum tunneling of Cooper pairs across a thin insulating barrier separating two superconductors (a Josephson junction). I = I_c sin(Δφ), V = (ℏ / 2e) (dΔφ / dt)

Joule Heating Electromagnetism & Electronics J (Joules) The process in which the flow of electric current through a conductor generates thermal energy due to electrical resistance. H = I² R t = P t

Joule-Thomson Effect Thermodynamics & Statistical Physics K/Pa The temperature change experienced by a real gas when throttled through an insulated porous plug or valve at constant enthalpy. μ_JT = (∂T / ∂P)_H

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