G

gain - of an amplifier output voltage/input voltage
galaxy - a collection of around 100 000 000 000 stars.
galvanometer - instrument for measuring electric current (usually small currents)
gamma - very short wave electromagnetic radiation emitted in some radioactive decays
Ganymede - largest satellite of Jupiter
gas - state of matter where there is 'no' attraction between the molecules. Fills container
gas constant - symbol R. R = PV/T
gate - electronic switch
geiger counter - instrument for measuring the activity of a radioactive sample
Gemini - two man spacecraft launched by the USA during the 1960s
generator - device for turning mechanical energy into electrical energy
geomagnetism - the study of the Earth's magnetic field
geostationary - a satellite that stays above the same point on the Earth's surface (the orbit is above the equator)
geosynchronous – a satellite that passes over a fixed point on the Earth at the same time each day
geothermal - energy produced from hot rocks beneath the surface of the Earth
gibbous - phase of Moon or a planet when between half and full
giga - 1000 000 000 (10⁹) as in gigahertz (1 GHz) = 10⁹ Hz
Giotto - a European Space Agency probe that rendezvoused with Halley's comet in 1986
globular cluster - huge spheres of 100 000 to two million stars up to a few hundred light years in diameter
gluon - elementary particle exchanged in actions involving the strong nuclear force
GMT - Greenwich Mean time, mean solar time at the longitude of Greenwich
gnomon - the pointer in a sundial
graded index fibre - an optical fibre with a refractive index that varies steadily along a rdius
gradient - slope, as in 'gradient of a graph'
gravitation - force between objects due to their mass
gravitational lensing - the formation of an image due to the deflection of light by gravity
gravity - force of gravity, acceleration of an object due to its attraction to a planet
gravity assist - the use of the gravitational pull of a planet to accelerate a space probe
graviton - a hypothetical particle proposed as the 'carrier' of the gravitational field
gray - unit of absorbed dose of ionising radiation (= 1 Jkg^-1 of irradiated material)
greenhouse effect – increase in temperature of the atmosphere
ground state - lowest possible energy level for an electron within an atom