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Cambridge IGCSE
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Note: This glossary is intended to aid students studying GCSE physics by explaining physics terms.
It is not intended to offer strict concise definitions.


absolute magnitude A measure of the brightness of a star when viewed from a standard distance away.
acceleration The rate of change of velocity. (When a body 'speeds up').
alpha particles/radiation Nuclear radiation consisting of 2 protons and 2 neutrons.
alternating current Current that continuously changes direction.
ammeter Used to measure current in circuits.
amplitude The height of a wave from the centre line.

background radiation Radiation that is all around us, caused by natural sources and man-made sources.
balanced (forces) Forces that cancel out to leave no resultant force.
battery Two or more cells joined together.
beta particles/radiation Nuclear radiation consisting of fast moving electrons.
big bang theory The idea that the universe had a beginning. It was formed approximately 15 billion years ago.
black hole A star that has collapsed down into a point, but retaining a very strong gravitational field.
braking distance The distance travelled by a vehicle whilst the brakes are applied.

carbon dating The use of the isotope carbon-14 to find the age of a sample.
chain reaction A self sustaining nuclear reaction.
circuit breaker A device that automatically stops current flowing in a circuit if faults are detected.
commutator A device used to switch the current flow in the coil of a motor or generator.
conservation of momentum The total momentum before and after a collision remains the same, unless external forces are applied.
contamination (of radioactivity) A radioactive isotope is present within an object.
control rods Rods that absorb neutrons in a nuclear fission reaction, to slow it down.
convection The transfer of heat by a moving fluid. (for example - hot air rising).
conventional current Current in a circuit defined as flowing from positive to negative.
cosmic microwave background Left over microwave radiation from the big bang. Found throughout the universe.
critical angle The smallest angle of incidence where light is reflected internally at a boundary, instead of being refracted.

density The mass per unit volume of a substance. Measured in kg/m3 or g/cm3.
diodes Electrical devices that only allow current to flow in one direction.
direct current Current that flows in one direction only, at a constant value.
displacement The distance travelled in a specific direction. (A 'vector' quantity).
dissipated energy Energy lost to the environment.
doppler effect The change in frequency caused by relative movement between source and observer.

elastic behavior Found when an elastic material can be stretched and then will return to its original length or shape
elastic energy Energy stored by stretching (or compressing) an elastic material
electromagnetic spectrum The family of electromagnetic waves of varying frequencies, such as light and radio waves.
electrostatic energy Energy stored by the attraction or repulsion between charged objects.
energy conservation The idea that energy cannot be created or destroyed, only converted from one form or store to another.

fission (nuclear) The process of releasing energy by splitting atoms (e.g. uranium).
fluorescence Found when a substance absorbs ultraviolet light and re-emits visible light.
fossil fuels Fuels formed over millions of years from fossilized plants and animals
free fall When a body falls freely under gravity.
frequency The number of waves per second (measured in hertz, Hz)
fuses Thin wire that melts and breaks a circuit when the current is too high
fusion (nuclear) The process of releasing energy by atoms fusing (joining) together (e.g. hydrogen in stars).

galaxy A group of billions of stars.
geothermal power Power produced from heat from hot rocks and volcanic activity underground.
gravitational potential energy Energy stored by an object raised upwards.
gravitational field strength, 'g' The force of gravity pulling downwards per kilogram. (On Earth, g = 9.8 N/kg)

half-life The time taken for half of the particles in a radioactive sample to decay. Is is also the time taken for the activity to halve.
Hooke's Law The idea that for some elastic materials, the extension is proportional to the force applied.

induction The production of a voltage in a conductor by movement relative to a magnetic field.
insulation Materials such as wool used to prevent heat loss - poor heat conductors.
ionising radiation Nuclear radiation that can affect atoms by 'knocking' an electron out of orbit.
irradiation When a substance is exposed to nuclear radiation.
isotopes Two nuclear isotope atoms will have the same number of protons but a different number of neutrons.

kinetic energy Energy stored in a moving object.

LDRs Light dependent resistors, that typically have a lower resistance when the light level increases.
LEDs Light emitting diodes.
limit of proportionality The point where an elastic substance no longer follows Hooke's law and the extension is not proportional to the force applied.
longitudinal waves Waves where the particle movement (or 'oscillation') is in the same direction as the wave motion.

main sequence stars Stars that are in their main hydrogen burning phase, and are stable. This is the middle phase in the life-cycle of a star.
mains electricity Electricity that is found in homes and offices, typically alternating current of 240V.
mass A measure of the amount of matter present in an object. It is measured in kilograms (kg).
medical tracers A radioactive isotope that is injected into a patient, and can be used to trace where it travels to by the radiation emitted.
moderator (nuclear) A substance used in a nuclear fission reactor to slow down neutrons to a suitable speed for fission to occur.
moment A 'turning force' - calculated by finding the force x the perpendicular distance to the pivot of the rotating system.
momentum A measure of how difficult it is to stop a body from moving. It is defined as the mass x the velocity of a moving object.
motor effect The effect of producing a force on a wire by passing a current through it whilst in a magnetic field.
motors A device for turning electrical energy into rotational kinetic energy.

national grid A network of high voltage cables all around a country providing electrical energy to all.
nebula An enormous cloud of gas and dust in space.
neutron star An extremely dense star where electrons and protons have been forced together under very strong gravity to produce a star made entirely of neutrons.
Newton's third law 'Action and reaction are equal and opposite'. (If object A pushes on B, then B pushes back on A with an equal but opposite force).
normal line A line drawn perpendicular to a mirror or surface to allow angles of incidence/reflection/refraction to be measured.
nuclear waste The dangerous radioactive isotopes left as a result of fission in a nuclear power station.

optical fibres Thin glass wires that allow light to be transmitted along them by total internal reflection.
orbits One object travelling around another in space.
oscilloscope A device for displaying waves on a screen.

period Also called the time period - the time taken for one complete wave or oscillation.
pitch A description of frequency. High pitched = high frequency.
pivot The centre of rotation of a system.
potential energy See gravitational potential energy.
power The work done per second. Measured in watts (W).
pressure The force acting on a unit area. Measured in pascals (Pa) or N/cm2
prism A glass or plastic triangular block used to refract light.

radioactivity A substance is radioactive if the atoms are unstable and decay by emitting particles or rays.
radiotherapy Nuclear radiation used to kill cancer cells.
red giant

A star that runs out of hydrogen in its core and swells up into a large but cooler red star.

red shift The lengthening of the wavelength of light emitted from galaxies travelling at high speeds away from us. Evidence for an expanding universe.
refraction The bending of light by glass/water/plastic or similar substance.
resistance A high resistance in a circuit means there will be a low current flowing. Resistance is measured in ohms (Ω).
resultant (force) The result of adding two or more forces together.
right-hand grip rule used to predict the magnetic field direction around a current-carrying conductor.

scalar A measurement with size but no direction. (measurements that are not vectors).
solar system A star with planets (or sometimes other stars) in orbit around it.
solenoid A coil of wire used as an electromagnet.
specific heat capacity The energy needed to heat 1 kg by 10C. Different substances have different S.H.C.'s, measured in J/kg/0C
speed The distance travelled per unit time, typically measured in m/s or km/h.
stars A large body in space that gives out heat and light through nuclear fusion reactions.
states of matter A phrase used to describe solids, liquids and gases.
static electricity Electricity produced by friction between surfaces. It typically collects on a surface and remains there.
stopping distance The total distance required to stop a vehicle. It is equal to the thinking distance + the braking distance.
supernova A giant explosion caused when a huge star runs out of nuclear fuel in the core.

terminal velocity The top speed of an object, reached when friction/air resistance balances the driving force.
thermal conductivity A measure of how easily heat conducts through a material. Metals have a high thermal conductivity.
thermal energy Energy stored as heat.
thermistors A resistor that changes value as the temperature changes. (Typically, they have lower resistance at high temperatures).
thinking distance The time taken by a driver of a vehicle to react before applying the brake.
tidal power Power produced by the movement of the sea caused by tides.
time period (wave) See period.
total internal reflection (TIR) Happens when light is reflected from the inside surface of a material. Only occurs above the critical angle.
transformers A device used to increase or decrease the voltage. Only works with a.c.
transverse waves Waves where the particle movement (or 'oscillation') is in the same direction as the wave motion.

unbalanced (forces) Forces that - when added together - do not cancel each other out and leave a resultant force.

vector A measurement with size and direction.
velocity The speed of an object in a particular direction. Velocity is a vector measurement.
voltage The 'kick' or 'push' that makes a current flow in a circuit.

wasted energy Energy lost (often as heat) when a device tries to convert energy from one form to another.
wavefront An imaginary line joining neighbouring peaks on a wave. (Surfers ride 'wavefronts' towards the shore).
weight The pull of gravity on an object. It is a force so is measured in newtons (N).
white dwarf A small, white hot star that has collapsed and run out of fuel for fusion. It is the end stage of typical stars like the Sun.
work Work is done when energy is transferred from one store or form to another.





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