THE VOCABULARY
Instruments, concepts, and phenomena — the shared vocabulary of the site.
Heat
Energy transferred between bodies because of a temperature difference — an amount, measured in joules, not a substance.
Heat capacity at constant volume and pressure (C_v, C_p)
A gas has two heat capacities: one at fixed volume and one at fixed pressure, differing by R because expansion does work.
Heat death of the universe
The hypothetical final state in which entropy reaches its maximum and no usable energy gradients remain.
Heat engine
A cyclic device that takes heat from a hot reservoir, turns part of it into work, and rejects the rest to a cold reservoir.
Heat pump
The same machine as a refrigerator, read from the warm side — and the reason it beats a resistive heater threefold.
Heat reservoir
An idealised body so large that it can absorb or supply heat without its own temperature changing — the constant-temperature partner of an isothermal process.
Helmholtz free energy
F = U − TS, the work obtainable from a system held at constant temperature and volume.
Henry
The SI unit of inductance. One henry is the inductance of a coil in which a current changing at one ampere per second induces an EMF of one volt. Symbol: H. 1 H = 1 V·s/A.
Hertzian dipole
The idealised point-dipole antenna — an infinitesimally short conductor of length L ≪ λ carrying a uniform oscillating current I(t) = I₀ cos(ωt). Used as the basic radiating element from which the fields of all more complex antennas are built by superposition.
Hohmann transfer
The most fuel-efficient two-burn manoeuvre for moving between two circular orbits; uses a half-ellipse as the transfer path.
Holonomy
The rotation a vector picks up when parallel-transported around a closed loop on a manifold. Vanishes if and only if the curvature is zero. On a 2-sphere of radius R, transporting around a loop enclosing area A rotates the vector by A/R² — the spherical-triangle holonomy reveal that motivates the Riemann tensor.
Hubble constant
The present-day rate of cosmic expansion: the proportionality between a galaxy's distance and its recession velocity.
Huygens's principle
Every point on a wavefront acts as a source of secondary spherical wavelets; the envelope of all the wavelets gives the wavefront at the next instant. Christiaan Huygens, 1678; generalised by Fresnel and Kirchhoff.
Hydrostatic
Relating to fluids at rest. In the hydrostatic limit, pressure varies only with depth: dp/dz = −ρg.
Hysteresis loop
The closed curve traced by B (or M) versus H in a ferromagnet under a cycled applied field. Its enclosed area equals the energy dissipated per unit volume per cycle.
Ideal gas law
PV = nRT — the single equation onto which Boyle, Charles and Avogadro collapse, valid for any gas dilute enough that its molecules barely notice one another.
Image charge
A fictitious charge placed outside the region of interest whose field, together with the real charge's field, satisfies the conductor's boundary conditions.
Impact Parameter
The perpendicular distance from a mass to the undeflected path of a passing ray or particle — the single number that sets how strongly it is bent.
Impedance
Z = V/I for a component or network driven at a single frequency, generalising resistance to the complex plane. Z = R + jX, where R is the resistance (dissipative) and X is the reactance (energy-storing).
impulse
The change in momentum delivered by a force acting over a time interval: J = ∫F dt = Δp.
Incompressible flow
Fluid motion in which density is effectively constant. Liquids, and gases at Mach ≪ 1.
Induced charge
The surface charge that appears on a conductor in response to a nearby external charge, redistributed until the conductor's interior field is zero.
inelastic collision
A collision in which kinetic energy is not conserved; the missing energy goes into heat, sound, or deformation.
inertia
The tendency of a body to resist changes in its motion; the first of Newton's three laws.
inertial frame
A reference frame in which Newton's laws hold in their simple form; one that is not itself accelerating.
Inertial navigation
Navigation by integrating a vehicle's own rotations and accelerations — no external reference needed.
Intensive quantity
A property independent of system size — temperature and pressure are the same whether you take half the gas or all of it.
Interferometer
An instrument that measures tiny length or phase changes by recombining split beams of light and reading their interference.
Internal energy
The total microscopic energy stored in a system — the sum of its molecules' kinetic and potential energies — and the only quantity a body genuinely 'contains'.
invariance
The property of a physical quantity or law of remaining unchanged under a specified transformation.