G
g
Symbol representing the acceleration of an object in free fall due to the force of gravity; its magnitude is 9.80 m/sec^2 (32.0 ft/sec^2).
Galaxy
A grouping of about approximately 100 billion stars, held together by their own gravity. We live in the Milky Way galaxy.
Gamma (g) Radiation
g (gamma) radiation is high frequency electromagnetic radiation. It is a part of the electromagnetic spectrum. Gamma radiation carries no mass, no charge, travels at the speed of light. (It can be thought of as being made of particles called photons or as being a wave). It can be stopped by several centimetres of lead or a few metres of concrete. Gamma radiation is an ionising radiation, but is not as strongly ionising as a radiation. It is released during the disintegration of an unstable nucleus.
For the purposes of treating cancer, medical gamma rays from cobalt-60 are used.
Gamma Camera
This is a device used to detect the gamma radiation from a patient during an examination using a radioactive tracer. Since gamma radiation passes straight through most materials, it is not possible to build a camera with glass lenses or mirrors. Instead an array of holes drilled in a thick layer of lead is placed between the actual gamma detectors and the patient. This allows an image to be built up. Scintillation counters are used to detect the radiation after it has passed through the holes in the commutator.
Gases
A phase of matter composed of molecules that are relatively far apart moving freely in a constant, random motion and have weak cohesive forces acting between them, resulting in the characteristic indefinite shape and indefinite volume of a gas.
Gas Discharge Lamps
These lamps produce light by passing electrical current through a gas at low pressure. The electrical current excites ("energises") the electrons of the atoms in the gas. These electrons quickly give up their energy as light. Sodium is often used (in street lights) or an inert gas for colourful advertising lights.
Discharge lamps are more efficient than filament lamps as they waste less energy as heat.
Geiger-Muller tube
This is a device used to detect the presence of radiation. It relies on ionisation to produce tiny bursts of current which can be counted by a rate meter.
Geostationary
'Geostationary' refers to a satellite of planet Earth. Such a satellite will appear to hover above the surface of the Earth, since it is placed in an orbit with a period of 24 hours.
Geothermal Energy
This is a renewable energy resource, unlike fossil fuels. In some parts of the world, hot subsurface rock heats water which reaches the surface and can be used for heating or electrical power generation.
Generator
A machine which uses motion to produce electrical energy. Sometimes called an alternator. A generator is like a dynamo, but with the permanent rotor magnets replaced with (d.c) rotor coils (electromagnets). The stator coils have an a.c. induced within them.
Gold Grain
Radioactive gold grains are used in the treatment of cancer. Gold (which is unreactive) doped with radioactive (gamma emitting) nuclei is implanted in a patient, near or in a tumour. The radiation will destroy or damage cells near the grain. The radioactive material chosen will have a short half-life so that the grain's activity level will drop to background levels after several days.
Gram-atomic weight
The mass in grams of one mole of an element that is numerically equal to its atomic weight.
Gram-formula weight.
The mass in grams of one mole of a compound that is numerically equal to its formula weight.
Gram-molecular weight.
The gram-formula weight of a molecular compound.
Gravity
A general term for the phenomenon of attraction between things having mass. The attraction between our planet and a human-sized object causes the object to fall.
Gravitational constant G
The constant G which appears in the equation for Newton's law of gravitation is called the universal constant of gravitation or the gravitational constant. Numerically it is equal to the force of gravitation, which acts between two bodies of mass 1kg each separated by a distance of 1m. The value of G is 6.67×10-11 Nm²/kg².
Gravitational Field
The Earth generates a gravitational field which attracts all other masses. It is a called a force field - the word field means that the force changes as you move away from the surface of the Earth (in fact the force becomes steadily less as you move away from the surface of the Earth). It is the combined mass of all particles on Earth which cause the Earth's gravitational field. The strength of the field is measured in Nkg-1.
Gravitational Potential Energy
Gravitational potential energy is energy an object possesses because of its position in a gravitational field.
If a mass is lifted up then its potential energy is given as follows:
Ep = mgh
Ep is the potential energy
m is the mass in kilograms
g is the strength of the gravitational field (10ms-2 on Earth)
h is the height through which the object is lifted
Greenhouse effect
The process of increasing the temperature of the lower parts of the atmosphere through redirecting energy back toward the surface; the absorption and reemission of infrared radiation by carbon dioxide, water vapor, and a few other gases in the atmosphere.
Ground state
Energy state of an atom with electrons at the lowest energy state possible for that atom.