Q
Quanta
Fixed amounts; usually referring to fixed amounts of energy absorbed or emitted by matter.
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Quantum number
A numerical label used to classify a quantum state.
Quantum mechanics
Model of the atom based on the wave nature of subatomic particles, the mechanics of electron waves; also called wave mechanics.
Quality factor
The number of oscillations required for a system's energy to fall off by a factor of 535 due to damping.
Quantized
Describes quantity such as money or electrical charge that can only exist in certain amounts.
Quasar
Quasar means 'quasi-stellar radar source'. These are objects which look like stars but are not. (They are only visible in big telescopes, because of their great distance from us. A typical quasar differs most noticeably from a star because it gives off the same intensity of light as would one million suns).
R
Rad
A measure of radiation received by a material (radiation absorbed dose).
Radar Gun
A hand-held device used by police to measure the speed of passing cars.
Radial
Parallel to the radius of a circle; the in-out direction.
Radiant energy
The form of energy that can travel through space; for example, visible light and other parts of the electromagnetic spectrum.
Radiation
The word 'radiation' can refer to electromagnetic radiation (including gamma radiation) or to alpha and beta particles released from radioactive decay. Radiation can cause ionisation of neutral atoms, by knocking away electrons. This can damage or kill cells. In this way, radiation can cause cancer and be used to treat cancer.
Radiation (heat)
Heat radiation is a part of the electromagnetic spectrum, conventionally called 'infra-red'. Heat can be transferred from a hot object to a cold one by radiation, convection or conduction. Transfer by radiation is possible in a vacuum (or in presence of matter).
Radioactive decay constant
A specific constant for a particular isotope that is the ratio of the rate of nuclear disintegration per unit of time to the total number of radioactive nuclei.
Radioactive decay series
Series of decay reactions that begins with one radioactive nucleus that decays to a second nucleus that decays to a third nucleus and so on until a stable nucleus is reached.
Radioactive decay
The natural spontaneous disintegration or decomposition of a nucleus.
Radioactivity
Spontaneous emission of particles or energy from an atomic nucleus as it disintegrates.
Radio
A radio is an electronic system which allows a listener to hear programmers sent from a transmitter. The main components of a radio (in order) are: the aerial, tuner, decoder, amplifier and loudspeaker.
Radioactive
A material is said to be radioactive if it emits radiation. Radioactive materials have unstable nuclei which have a tendency to decay. It is when a decay occurs that radiation is released from the nucleus.
Radiometric Dating
This is the name given to the general technique of using half-life to find out the age of samples of rock, fossil or material which is organic in nature. Carbon dating is used to date organic material up to about 30 000 years.
Radio Telescope
Astronomers use radio telescopes to help build up a picture of the Universe. Since radio waves can penetrate Earth's atmosphere, we can detect them by building large antennae or dishes. The radio part of the spectrum provides an alternative and complimentary view of the Universe - importantly radio waves convey information about much lower energy events than visible light does, much of what a radio telescope 'sees' will be not appear in visible light images.
Radio Transmitter
A source of radio waves. Radio transmitters are usually large vertical conducting masts which emit radio waves. Moving electrons in the metal of the mast radiate the radio waves.
Rem
A unit for measuring a person's exposure to radioactivity.
Red shift
This refers to the fact that the spectrum of distant galaxies is shifted towards the long wavelength 'red' end of the electromagnetic spectrum.
Rarefaction
A part of a longitudinal wave in which the density of the particles of the medium is less than the normal density is called a rarefaction.
Real image
An image generated by a lens or mirror that can be projected onto a screen.
Rectilinear Motion
The motion of a body in a straight line is called rectilinear motion.
Reed Relay
Relays can be used for remote control of electrical circuits in dangerous places. For example, a circuit within a nuclear reactor can be switched on or off with a reed relay. In general, relays can be used to switch high current circuits using low current circuits.
Reed Switch
A simple switch, controlled by a magnet. The reed switch can be designed either to make or break contact when a magnetic field is present.
Reflected ray
A line representing direction of motion of light reflected from a boundary.
Reflection
The change when light, sound, or other waves bounce backwards off a boundary. All waves can be reflected from an obstacle - for example, water waves can reflect off a harbour wall and light waves can reflect off a mirror.
The law of reflection is very simple:
The angle of incidence is equal to the angle of reflection.
Refraction
All waves can be refracted. Refraction means bending. Light rays refract when they pass through a medium of different density (e.g. when light travels from air into glass). When light passes into a region of increased density (e.g. air to glass) it bends towards the normal. When light passes into a region of reduced density, it bends away from the normal.
Relative humidity
The percentage of the amount of water vapor actually present in a certain volume of the air to the amount of water vapor needed to saturate it is called the relative humidity of the air.
Renewable Energy Sources
Solar, wind, geothermal, tidal and wave power plants are examples of renewable energy resources.
Repulsive
Describes a force that tends to push the two participating objects apart.
Resistance
The ratio of the voltage difference to the current in an object made of an ohmic substance.The SI unit of resistance is the ohm (W).
Resistor
A resistor is an electronic component which opposes the flow of current.
Resonance
The tendency of a vibrating system to respond most strongly to a driving force whose frequency is close to its own natural frequency of vibration.
Restoring force
The force which tends to bring an oscillating body towards its mean position whenever it is displaced from the mean position is called the restoring force.
Rest mass
The rest mass of an object is the inertial mass that an object has when it is at rest.
Resultant Force
A single force, which acts on a body to produce the same effect in it as, done by all other forces collectively, is called the resultant force.
Retardation
Negative acceleration is called retardation. In retardation the velocity of a body decreases with time.
Reverberation
Apparent increase in volume caused by reflections, usually arriving within 0.1 second after the original sound.
Rheostat
A variable resistor, when used with just two of its terminals, it is called a rheostat. This arrangement is for controlling current.
Ring Circuit
This is a special type of parallel circuit, used in household wiring. The advantage of the ring circuit is that thinner, cheaper cable can be used. This is because there are two separate paths to any socket. Therefore, if each cable is capable of carrying up to 18 A, then the maximum current the ring can draw from the consumer unit is 36 A.
R.M.S. Voltage
This is a kind of average voltage of an a.c. source (averaged over a complete cycle). It is the effective voltage delivered by the source and can be used in power calculations.
Rotor Coil
In an a.c. generator, the rotor coil is a rotating electromagnet, with a d.c. power supply. It replaces the permanent rotating magnets of a dynamo.