Browse All 842 Physics Terms A–Z

Kelvin Foundations of Physics K The SI base unit of thermodynamic temperature, defined by fixing the Boltzmann constant to k = 1.380649 × 10⁻²³ J/K. K = °C + 273.15

Kinetic Energy Classical Mechanics J The scalar energy an object possesses by virtue of its motion, proportional to its mass and the square of its speed. KE = ½ m v²

Kink & Sausage Instabilities Plasma Physics MHD Instability Modes Ideal magnetohydrodynamic (MHD) macro-instabilities occurring in current-carrying pinched plasma columns. m = 0 (Sausage Mode), m = 1 (Kink Mode)

Kirchhoff's Laws Electromagnetism & Electronics A (Current), V (Voltage) Two fundamental circuit analysis laws expressing conservation of electric charge (Current Law) and conservation of energy (Voltage Law). KCL: ∑ I_in = ∑ I_out; KVL: ∑ V = 0

La Niña Geophysics & Environmental Physics Sea Surface Temperature Anomaly (°C) The cool climate phase of the El Niño-Southern Oscillation (ENSO) characterized by abnormally cold sea surface temperatures in the central and eastern tropical Pacific. ENSO Index: Southern Oscillation Index (SOI) > 0, Sea Surface Temp Anomaly < -0.5 °C

Lagrangian Formalism Mathematical & Computational Physics Joules (Action S in J·s) A formulation of classical mechanics where system dynamics are derived from a single scalar function, the Lagrangian L(q, q̇, t). L = T - V (Kinetic Energy minus Potential Energy)

Lamb Shift Quantum & Atomic Physics MHz or eV A small energy difference between the 2s1/2 and 2p1/2 states of hydrogen caused by quantum vacuum fluctuations. ΔE_Lamb ≈ 1057 MHz (for Hydrogen 2s1/2 and 2p1/2)

Landau Damping Plasma Physics s⁻¹ (Damping Rate γ) The collisionless damping of electrostatic plasma waves caused by energy exchange between wave electric fields and particles moving near wave phase velocity. γ_L = (π ω_pe³ / (2 k²)) (df₀ / dv)|_(v = ω/k)

Langmuir Probe Plasma Physics A (Measured Current) & V (Bias Voltage) A diagnostic electrode inserted directly into a plasma to measure local electron temperature, plasma density, and floating potential from its current-voltage (I-V) curve. I(V) = I_i,sat - I_e,sat exp(e (V - V_plasma) / (k_B T_e))

Laplace Transform Mathematical & Computational Physics s = σ + i ω (Complex Frequency Domain) An integral transform that maps a real-valued time-domain function f(t) into a complex frequency-domain function F(s). L{f(t)} = F(s) = ∫₀^(∞) f(t) e^(-s t) dt

Large Hadron Collider (LHC) Nuclear & Particle Physics TeV (Tera-electronvolts) The world's largest and highest-energy particle accelerator, located in a 27-kilometer underground circular tunnel at CERN near Geneva. Center-of-Mass Energy √s = 13.6 TeV

Laser Optics W (or J) Light Amplification by Stimulated Emission of Radiation—a device producing coherent, monochromatic, highly directional, high-intensity light. E = h ν, Population Inversion N2 > N1

Laser Cooling Quantum & Atomic Physics μK or nK (Temperature) A technique using photon momentum from red-detuned laser beams to slow down moving gas atoms, cooling them to microkelvin temperatures. F_force = ℏ k γ (I / I_sat) / (1 + 4 (Δ ± k v)² / γ²)

Laser Wakefield Acceleration Plasma Physics GeV/m (Accelerating Electric Field Gradient) An advanced particle acceleration technique where ultra-intense laser pulses launched through plasma create giant relativistic plasma wave wakes that accelerate electrons over short distances. E_wake ≈ (m_e c ω_p) / e ~ 100 GeV/m (Gradient ~ 1000× conventional LINACs)

Latent Heat of Fusion Thermodynamics & Statistical Physics J/kg The amount of thermal energy required to change a unit mass of a substance from solid to liquid at constant melting temperature. Q = m L_f

Latent Heat of Vaporization Thermodynamics & Statistical Physics J/kg The thermal energy required to convert a unit mass of liquid into gas at constant boiling temperature. Q = m L_v

Lawson Criterion Plasma Physics keV·s/m³ (Fusion Triple Product) A fundamental condition for net thermonuclear fusion power gain, specifying minimum values for plasma density n, energy confinement time τ_E, and temperature T. n τ_E T ≥ 3 × 10²¹ keV·s/m³ (Triple Product for D-T Ignition)

LC Circuit & Oscillations Electromagnetism & Electronics Hz (Frequency), J (Energy) An electrical circuit containing an inductor and capacitor that exchanges energy periodically between the electric field of the capacitor and magnetic field of the inductor. f₀ = 1 / (2π √(L C)), E_total = ½ C V² + ½ L I²

Least Count Foundations of Physics Various The smallest scale division or magnitude that can be measured directly using a given instrument. Least Count = Main Scale Division / Total Sub-divisions

Least Squares Data Fitting Mathematical & Computational Physics Statistical Regression Method A standard mathematical regression technique for determining the best-fitting curve or line to a set of data points by minimizing the sum of squared residual deviations. Minimizing S = ∑_(i=1)^(N) (y_i - f(x_i, β))²

Least Squares Method Foundations of Physics Dimensionless A statistical technique used to determine the line or curve of best fit through experimental data points by minimizing the sum of squared residual deviations. Minimizing S = ∑ (y_i - (m x_i + c))²

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

Lehmann Discontinuity Geophysics & Environmental Physics km (Boundary Depth) The seismic boundary at a depth of ~5,150 km marking the transition between Earth's liquid outer core and solid inner core. Depth z ≈ 5150 km (Outer Core - Inner Core Boundary)

Length Foundations of Physics m The spatial extent or distance between two points measured along a specified path.

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