IGCSE PHYSICS DEFINITIONS

Define: Speed

distance travelled per unit time

Define: Velocity

speed in a given direction

Define: Acceleration

change in velocity per unit time (deceleration = negative acceleration)

Define: Vector quantity

has magnitude (size) AND direction (e.g. force, weight, velocity, acceleration, momentum, electric field strength)

Define: Scalar quantity

has magnitude (size) only (e.g. speed, distance, mass, time, energy)

Define: Mass

a measure of the amount of matter in an object (kg)

Define: Weight

the gravitational force acting on an object that has mass (N)

Define: Density

mass per unit volume

Define: Gravitational field strength

gravitational force per unit mass (N/kg)

Define: Resultant force

the single force that has the same effect as all the forces acting on the object

Define: Equilibrium

no resultant force AND no resultant moment (the object is balanced)

Define: Moment of a force

force × perpendicular distance from the pivot

Define: Centre of gravity

the point where the weight of an object appears to act

Define: Hooke’s law / limit of proportionality

extension is directly proportional to the force applied, up to the limit of proportionality

Define: Pressure

force per unit area (acting normally to the surface)

Define: Momentum

mass × velocity

Define: Impulse

force × time for which the force acts (= change in momentum)

Define: Work done

energy transferred when a force moves an object: force × distance moved in the direction of the force

Define: Power

work done (energy transferred) per unit time

Define: Efficiency

(useful energy output ÷ total energy input) × 100%

Define: Specific heat capacity

the energy required per unit mass per unit temperature increase

Define: Evaporation

the escape of the more energetic particles from the surface of a liquid, at any temperature, which leaves the liquid cooler

Define: Absolute zero

the lowest possible temperature: 0 K = −273 °C

Define: Brownian motion

the random motion of small visible particles (e.g. smoke or dust) caused by collisions with fast-moving, invisible air molecules

Define: Conduction

transfer of thermal energy through a solid by vibrating particles passing on energy (in metals also by free electrons)

Define: Convection

transfer of thermal energy in liquids and gases: warmer fluid is less dense so it rises and cooler fluid sinks

Define: Thermal radiation

transfer of thermal energy by infrared waves; no medium needed

Define: Wavelength

the distance between two successive identical points on a wave (e.g. crest to crest)

Define: Frequency

the number of waves passing a point (or oscillations) per second, measured in hertz (Hz)

Define: Amplitude

the maximum displacement of a point on a wave from its rest (equilibrium) position

Define: Wavefront

a line joining points on a wave that are in phase, e.g. all the crests

Define: Transverse wave

the direction of vibration is at right angles (perpendicular) to the direction of propagation (e.g. light, water waves)

Define: Longitudinal wave

the direction of vibration is parallel to the direction of propagation (e.g. sound)

Define: Diffraction

the spreading out of waves when they pass through a gap or round an edge

Define: Refraction

the change in speed (and usually direction) of a wave when it passes from one medium into another

Define: Refractive index

n = sin i ÷ sin r = speed of light in vacuum ÷ speed of light in the material

Define: Critical angle

the angle of incidence in the denser medium for which the angle of refraction is 90°

Define: Total internal reflection

all the light is reflected back inside the denser medium when the angle of incidence is greater than the critical angle

Define: Principal focus of a converging lens

the point on the principal axis where rays parallel to the principal axis converge (meet)

Define: Focal length

the distance from the centre of the lens to the principal focus

Define: Real image

an image formed where light rays actually meet; it can be shown on a screen

Define: Virtual image

an image where the rays only appear to come from; it cannot be shown on a screen

Define: Dispersion

the splitting of white light into its colours (a spectrum) by refraction, e.g. in a prism

Define: Monochromatic light

light of a single frequency (one colour)

Define: Ultrasound

sound with a frequency above 20 000 Hz (20 kHz), above the range of human hearing

Define: Echo

sound that has been reflected from a surface

Define: Electric current

the rate of flow of charge (I = Q ÷ t); in a metal the charge carriers are electrons

Define: Potential difference (p.d.)

the work done (energy transferred) per unit charge between two points in a circuit

Define: Electromotive force (e.m.f.)

the work done (energy transferred) per unit charge by the source in driving charge round a complete circuit

Define: Resistance

potential difference per unit current (R = V ÷ I); it opposes the current

Define: Electric field

a region in which an electric charge experiences a force

Define: Kilowatt-hour (kW h)

the energy transferred by a 1 kW device in 1 hour (1 kW h = 3.6 × 10⁶ J)

Define: Direct current (d.c.)

current that flows in one direction only

Define: Alternating current (a.c.)

current that repeatedly reverses its direction

Define: Magnetic field

a region in which a magnetic pole (or magnetic material) experiences a force

Define: Electromagnetic induction

the production of an e.m.f. in a conductor when it is in a changing magnetic field (or cuts magnetic field lines)

Define: Isotopes

atoms of the same element with the same number of protons but a different number of neutrons

Define: Nucleon number (A)

the total number of protons and neutrons in the nucleus

Define: Proton number (Z)

the number of protons in the nucleus

Define: Half-life

the time taken for half of the nuclei in a sample of a radioactive isotope to decay

Define: Radioactive decay

the random and spontaneous emission of radiation from an unstable nucleus

Define: Ionising radiation

radiation that removes electrons from atoms (turns them into ions)

Define: Background radiation

ionising radiation that is always present in the environment

Define: Alpha particle

a helium nucleus (2 protons + 2 neutrons); most ionising, least penetrating

Define: Beta particle

a fast-moving electron emitted from the nucleus (a neutron changes to a proton)

Define: Gamma radiation

electromagnetic radiation from the nucleus; least ionising, most penetrating

Define: Nuclear fission

the splitting of a large unstable nucleus into smaller nuclei, releasing energy

Define: Nuclear fusion

the joining of small nuclei (e.g. hydrogen) to form a larger nucleus, releasing energy; this powers the Sun

Define: Light-year

the distance travelled by light in one year (≈ 9.5 × 10¹⁵ m)

Define: Redshift

light from a distant galaxy is observed at a longer wavelength than it was emitted; the galaxy is moving away

Define: Hubble constant (H₀)

the ratio of a galaxy’s recession speed to its distance from Earth (H₀ = v ÷ d)

Define: Big Bang theory

the Universe started from a single very small, hot, dense point about 13.8 billion years ago and has been expanding since

Define: Cosmic microwave background radiation (CMBR)

microwave radiation arriving from all directions in space, left over from the Big Bang

Define: Orbital period

the time taken for an object to complete one orbit

Define: Stable star

a star in which the inward gravitational force is balanced by the outward pressure from nuclear fusion

Name: its resistance falls as light intensity increases

Light-dependent resistor (LDR)

Name: its resistance falls as temperature increases

Thermistor

Name: lets current flow in one direction only

Diode

Name: a diode that gives out light

LED (light-emitting diode)

Name: its resistance can be changed to vary the current

Variable resistor

Name: a thin wire that melts if the current is too high

Fuse

Name: a switch operated by an electromagnet so a small current controls a larger one

Relay

Name: a coil on a soft-iron core that is magnetic only while current flows

Electromagnet

Name: measures current; connected in series

Ammeter

Name: measures p.d.; connected in parallel across the component

Voltmeter

Name: two coils on a soft-iron core that change an a.c. voltage

Transformer (primary coil in, secondary coil out)

Name: the part of a d.c. motor that reverses the current every half turn

Split-ring commutator

Name: carbon contacts that touch the commutator or slip rings and carry current

Brushes

Name: the parts of an a.c. generator that keep the coil connected as it rotates

Slip rings (with brushes)

Name: coil, magnet and cone that turn a changing current into sound

Loudspeaker

Name: the material used for the core of a transformer or electromagnet

Soft iron (easily magnetised and demagnetised)

Name: a lens that makes parallel rays meet at a point

Converging (convex) lens

Name: the line through the centre of a lens at right angles to it

Principal axis

Name: a small compass used to show the direction of a magnetic field

Plotting compass

Name: a glass block that splits white light into a spectrum

Prism

Name: a tank of water used to show reflection, refraction and diffraction of waves

Ripple tank

Name: used to measure the volume of a liquid, or of an irregular solid by displacement

Measuring cylinder

Name: the regions of the EM spectrum in order of increasing frequency

Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma

Name: the colours of visible light in order (red to violet)

Red, orange, yellow, green, blue, indigo, violet (red refracts least, violet most)

Name: the energy stores

Kinetic, gravitational potential, chemical, elastic (strain), nuclear, thermal (internal), electrostatic, magnetic