אוצר המילים
מכשירים, מושגים ותופעות — אוצר המילים המשותף של האתר.
Joule
The SI unit of energy — one newton acting through one metre — named for James Prescott Joule.
Joule–Thomson effect
Squeeze a gas through a plug and it changes temperature — but only cools if it is already cold enough.
KAM theorem
Most invariant tori of a near-integrable Hamiltonian system survive small perturbations. Proves the solar system is mostly stable.
Kelvin scale
The absolute temperature scale whose zero is the floor of thermal motion, making it the only scale on which temperature is a true ratio.
Kelvin–Planck statement
No cyclic engine can turn heat from a single reservoir entirely into work with no other effect.
Kinetic theory of gases
The picture of a gas as countless tiny molecules in ceaseless motion, from which pressure and temperature emerge as statistical averages.
Kirchhoff's current law (KCL)
The sum of currents flowing into any node in a circuit equals the sum of currents flowing out. Equivalently: charge conservation applied to circuit junctions, ∇·J = 0 in steady state.
Kirchhoff's voltage law (KVL)
The sum of voltage drops around any closed loop in a circuit equals zero. Equivalently: the electrostatic field is conservative, ∮E·dℓ = 0 in the quasi-static limit.
Kolmogorov spectrum
E(k) ∝ ε^(2/3) k^(−5/3) — the universal inertial-range energy spectrum of fully developed turbulence.
Kramers formula
Hendrik Kramers's 1923 classical thick-target bremsstrahlung spectrum, dN/dE ∝ (E_max − E)/E, giving the continuous-continuum shape of an X-ray tube running at accelerating voltage U with E_max = eU. Full treatment in a later branch.
Laminar flow
Orderly flow in parallel layers, without mixing across them. Characteristic of low Reynolds number.
Landauer's principle
Erasing one bit of information dissipates at least k_BT ln 2 of heat.
Larmor formula
The classical expression P = q²a²/(6πε₀c³) for the total electromagnetic power radiated by a non-relativistic point charge of acceleration a. Derived by Joseph Larmor in 1897; the foundational result of classical radiation theory.
Larmor power
The instantaneous total radiated power P = q²a²/(6πε₀c³) of an accelerating point charge, integrated over all solid angle. Synonym of the Larmor formula result; the quantity that appears as the rate of energy loss in radiation-reaction problems.
Laser cooling
Cooling atoms by shining light on them — down to a millionth of a degree.
Latent heat
The heat absorbed or released during a phase change at constant temperature — energy spent on bonds, not on warming.
Latent heat of fusion
The heat needed to melt a unit mass of a solid at its melting point — 334 kJ/kg for water — released again on freezing.
Latent heat of vaporisation
The heat needed to boil a unit mass of a liquid at its boiling point — 2260 kJ/kg for water — far larger than the heat of fusion.
Legendre transform
The operation that swaps a variable for its conjugate slope, relating the four thermodynamic potentials.
Length contraction
The relativistic effect that an object of proper length L₀ measured in its rest frame appears contracted to L = L₀/γ along the direction of motion when measured by an observer in any inertial frame moving relative to the object. Symmetric; not a material compression; perpendicular dimensions unchanged.
Length contraction of current
The relativistic effect that a current-carrying lattice, viewed in the rest frame of its drift electrons, has its inter-ion spacing contracted by the Lorentz factor γ. Produces the net + charge density that explains magnetic attraction as relativistic electrostatics.
Lenz's law
The direction of an induced current is always such that its own magnetic field opposes the change in flux that caused the induction. Equivalently: the minus sign in Faraday's law is a consequence of energy conservation.
Light-cone
The locus of events null-separated from a chosen origin event in spacetime; geometrically a 4D double cone with apex at the origin. The boundary between causally accessible (timelike-interior) and causally inaccessible (spacelike-exterior) regions; the structure that encodes relativistic causality.
Line integral
The integral of a vector field along a curve, measuring the accumulated effect of the field's component tangent to the path.
Linear polarization
An EM wave in which E oscillates along a single fixed line perpendicular to k. Equivalent to a superposition of two circularly polarised waves of equal amplitude with opposite handedness.
Liouville's theorem
Under Hamiltonian flow, phase-space volume is exactly conserved. The foundation of classical statistical mechanics.
Lorentz force
The total electromagnetic force on a point charge: F = q(E + v×B). Bridges Maxwell's macroscopic fields to the motion of individual particles.
Lorentz invariants of the EM field
The two scalar quantities — E·B and |E|² − c²|B|² — that every observer agrees on, regardless of their motion. Constructed as tensor traces from F^{μν}; classify the field as 'electric-like', 'magnetic-like', or 'null'.
Lorentz transformation
The linear coordinate transformation between two inertial frames in special relativity. For a boost along +x at velocity v: t' = γ(t − vx/c²), x' = γ(x − vt), y' = y, z' = z. Replaces the Galilean transformation; reduces to it in the limit β → 0.
Lorentz transformation of fields
The closed-form rule by which the components of E and B transform under a Lorentz boost. Parallel components are unchanged; perpendicular components mix linearly with the perpendicular components of the OTHER field. Derived by Hendrik Lorentz in 1895.