Physics terms: G

G

Galilean relativityHL only · also principle of Galilean relativity

The principle that the laws of mechanics (Newton's laws) are the same in all inertial reference frames, with time the same for all observers.

Introduced in A.5 Galilean and special relativity

Galilean transformationHL only · also Galilean transformations

The equations $x' = x - vt$ and $t' = t$ linking the coordinates of an event in two inertial frames in Newtonian physics, with velocities related by $u' = u - v$.

Introduced in A.5 Galilean and special relativity

GalvanometerHL only · also galvanometers

A sensitive ammeter, usually with its zero in the centre of the scale, used to detect small currents in either direction.

Introduced in D.4 Induction

Gamma radiationalso gamma ray, gamma rays, γ ray, gamma photon, gamma decay

High-energy photons emitted when a nucleus falls from an excited state to a lower energy state; the numbers of protons and neutrons do not change.

Introduced in C.2 Wave model

Gauge pressure

The pressure of a fluid above atmospheric pressure, as read by a tyre gauge; absolute pressure = gauge pressure + atmospheric pressure.

Introduced in B.3 Gas laws

Geiger–Müller tubealso GM tube, Geiger counter, Geiger-Muller tube

A detector of ionising radiation: each particle or photon that ionises the gas inside produces a pulse of current, which a counter records.

Introduced in E.3 Radioactive decay

GeophoneHL only · also geophones

A sensor that detects ground vibrations: relative motion between a coil and a magnet, one of them held by a spring, induces an emf proportional to the speed of the ground.

Introduced in D.4 Induction

Geostationary orbitalso geostationary satellite, geosynchronous equatorial orbit

A circular equatorial orbit with a period of one day, travelling west to east, so the satellite stays above the same point on the Earth; it is about 36 000 km above the surface.

Introduced in D.1 Gravitational fields

Grating spacingHL only · also line spacing

The distance $d$ between neighbouring slits of a diffraction grating: $d = 1/N$ for $N$ lines per metre.

Introduced in C.3 Wave phenomena

Gravitational constantalso universal gravitational constant

The constant $G$ in Newton's law of gravitation, $6.67\times10^{-11}\ \text{N m}^2\ \text{kg}^{-2}$

Gravitational fieldalso gravitational fields

A region of space in which a mass experiences a force because of its mass.

Introduced in D.1 Gravitational fields

Gravitational field strengthalso g, field strength

The gravitational force per unit mass at a point, in $\text{N kg}^{-1}$; $9.8\ \text{N kg}^{-1}$ near the Earth's surface.

Introduced in A.2 Forces and momentum

Gravitational potentialHL only · also potential

The work done per unit mass to bring a small test mass from infinity to a point, $V_\mathrm{g} = -GM/r$; a scalar measured in $\text{J kg}^{-1}$, zero at infinity.

Introduced in D.1 Gravitational fields

Gravitational potential energyalso GPE, E_p

The energy stored by a mass because of its position in a gravitational field. Near the Earth's surface its change is $\Delta E_\mathrm{p} = mg\Delta h$.

Introduced in A.3 Work, energy and power

Greenhouse effectalso natural greenhouse effect

The warming of a planet's surface by atmospheric gases that let through short-wavelength sunlight but absorb the infrared emitted by the surface and re-emit it in all directions, partly back to the surface.

Introduced in B.2 Greenhouse effect

Greenhouse gasalso greenhouse gases

A gas in the atmosphere, such as water vapour, carbon dioxide, methane or nitrous oxide, that absorbs and emits infrared radiation.

Introduced in B.2 Greenhouse effect

Grey bodyalso grey bodies, gray body

A body whose emissivity is less than 1 but about the same at all wavelengths, so it radiates a fixed fraction of the black-body power at its temperature.

Introduced in B.2 Greenhouse effect

Ground statealso ground states

The lowest energy level of an atom, the state in which it is most stable.

Introduced in E.1 Structure of the atom