THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Magnetic susceptibility
The dimensionless ratio χ = M/H measuring how strongly a linear magnetic material is magnetised by an applied H-field. Negative for diamagnets, positive and small for paramagnets, large for ferromagnets.
Magnetization
The vector M = (magnetic dipole moment)/(volume) inside a magnetised material, measured in amperes per metre. The magnetic analogue of polarization density.
Malus's law
I = I₀ cos²θ. The transmitted intensity of linearly polarised light through an ideal polariser depends on the cosine-squared of the angle between the incident polarisation and the polariser's transmission axis.
mass
The quantitative measure of a body's inertia; the m in F = ma.
Mass-energy equivalence
Einstein's 1905 result E = mc² — that mass and energy are the same physical quantity expressed in different units, with c² as the conversion factor. Binding energies and kinetic energies register on a balance as mass; the foundation of nuclear physics, fusion, fission, and particle production.
Maxwell stress tensor
T_ij = ε₀(E_iE_j − ½δ_ij E²) + (1/μ₀)(B_iB_j − ½δ_ij B²). The 3×3 symmetric tensor whose divergence gives the mechanical force per unit volume the electromagnetic field exerts on charges and currents.
Maxwell's equations
The four coupled partial differential equations (∇·E = ρ/ε₀, ∇·B = 0, ∇×E = −∂B/∂t, ∇×B = μ₀J + μ₀ε₀ ∂E/∂t) that fully describe classical electromagnetism. Every electromagnetic phenomenon below the quantum scale follows from them.
Meißner effect
The active expulsion of magnetic flux from the interior of a superconductor on cooling below T_c in an applied field. The signature that superconductivity is a distinct thermodynamic phase, not mere zero resistance.
Milankovitch cycles
The three astronomical cycles — eccentricity, obliquity, precession — that drive Earth's ice ages on 10,000- to 100,000-year timescales.
Mode
An allowed standing-wave pattern of a bounded system, labelled by an integer.
Moment arm
The perpendicular distance from a rotation axis to the line of action of a force. Torque = force × moment arm.
moment of inertia
The rotational analogue of mass: I = Σ m_i · r_i² (or ∫ r² dm for a continuous body).
momentum
Mass times velocity (a vector, p = m·v); conserved in any closed system.
monkey and the hunter
Classic thought experiment: a dart fired straight at a monkey that falls at the trigger still hits the monkey, because both are in the same free fall.
Motional EMF
The EMF induced in a conductor moving through a magnetic field, driven by the magnetic part of the Lorentz force on free charges inside the conductor. Equals (v × B) · ℓ for a straight rod of length ℓ moving with velocity v in field B.
Mutual inductance
The coupling between two separated coils: M = Φ₁₂/I₂, the flux that coil 2's current produces through coil 1 per unit coil-2 current. The operating principle of every transformer.
Navier-Stokes equations
The PDEs governing viscous fluid flow. Nonlinear, exact, and generally unsolved — smoothness in 3D is a Millennium Problem.
Near-field zone
The region r ≪ λ surrounding an oscillating source where the field resembles a time-varying instantaneous quasi-static field (amplitude ∝ 1/r³ for a dipole) that stores and returns energy rather than radiating it. Also called the induction zone or reactive zone.
Newton's laws of motion
The three laws — inertia, F = ma, and equal-and-opposite reaction — that launched classical mechanics in 1687.
Newtonian fluid
A fluid whose shear stress is strictly proportional to its velocity gradient, with viscosity independent of shear rate.
Node
A point on a standing wave that never moves.
Noether's theorem
Every continuous symmetry of a physical system's action gives rise to a conserved quantity.
Non-abelian gauge theory
A gauge theory whose gauge group is non-commutative, so the gauge fields themselves carry charge under the group and the field strength tensor acquires a self-interaction term. Yang-Mills 1954 introduced the construction; QCD and the weak force are non-abelian; QED is the abelian exception.
nonlinear dynamics
Study of systems where output is not proportional to input; chaos, solitons, turbulence.
normal modes
Independent oscillation patterns of a coupled system; any motion is their superposition.
Null interval
A separation between two events with invariant interval s² = 0 — meaning a light signal exactly connects them. The world-lines of photons are null curves; the light-cone of any event is the locus of null-separated points; null separations sit on the boundary between timelike and spacelike.
Numerical aperture
NA = n sin θ_max. For a fibre, NA = √(n_core² − n_cladding²) gives the sine of the maximum acceptance half-angle. For a microscope objective, NA determines the diffraction-limited resolution λ/(2·NA).
nutation
Small oscillations of a spinning body's axis superimposed on its steady precession.
Obliquity
The tilt angle between a planet's rotation axis and the perpendicular to its orbital plane.
Ohm's law
V = IR. For a metallic conductor at fixed temperature, the current through it is proportional to the voltage across it, with the proportionality constant R being the resistance.