Definitions

A combination of markscheme answers and textbook definitions.

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AS Level Physics Terms & Concepts

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Lv	Ch	Term	Definition
AS	1	Derived Units	Some combination of the base units. The base units may be multiplied together or divided by one another, but never added or subtracted.
AS	1	Homogeneous Units	When each term has the same base units, the equation is said to be homogeneous or 'balanced'.
AS	1	Accuracy	How close a reading is to its true value. When readings are accurate, the peak / average value moves towards the true value.
AS	1	Precision	Smallest change in value that can be measured by an instrument. OR Spread of values / measurements (scatter between each data is relatively small / lines are closer together / sharper peak).
AS	1	Random errors	Readings have positive and negative values around the peak value / values are scattered / wide range. To reduce errors, take several readings to get an average value.
AS	1	Systematic Error	The average / peak is not the true value / the readings are not centred around the true value. Look/check for zero error to avoid systematic error.
AS	1	Uncertainty	The range of value within which a measurement is likely to be in.
AS	1	Scalar	A quantity that has magnitude/size.
AS	1	Vector	A quantity that has magnitude/size and direction.
AS	2	Acceleration (vector)	Rate of change of velocity.
AS	2	Displacement (vector)	The straight line distance between start and finish points (in that direction) / minimum distance.
AS	2	Distance (scalar)	The actual path travelled.
AS	2	Speed (scalar)	Distance travelled per unit time taken.
AS	2	Velocity (vector)	Rate of change of displacement.
AS	2	Free Fall	The downward motion of an object under the influence of force of gravity with a constant acceleration (g = 9.81 ms⁻²).
AS	2	Air resistance	Effect due to a resistive / drag force that opposes the direction of motion. This drag force will affect acceleration of object.
AS	2	Drag force	A type of resistive force that is proportional to speed of the object, and opposes direction of motion. Drag due to friction of object with air particles can cause kinetic energy to be dissipated as heat.
AS	2	Projectile motion	Objects acted upon by a force with vector at perpendicular to its horizontal velocity. Assume zero frictional forces. Trajectory of the object will result in a parabola.
AS	2	Terminal velocity	Constant speed of object when resultant force is zero due to large air resistance.
AS	3	Force	Rate of change of momentum.
AS	3	Mass	A measure of inertia of a body, or the property of a body that resists change in motion.
AS	3	Newton's 1st Law	A body remains at rest or constant velocity unless acted on by a resultant (external) force.
AS	3	Newton's 2nd Law	The (resultant) force is proportional to the rate of change of momentum.
AS	3	Newton's 3rd Law	If one body exerts a force on another, it will experience a force by the other body, which is equal in magnitude & opposite in direction. Both forces are of the same kind.
AS	3	Conservation Of Momentum	Total momentum of (an isolated) system (of interacting bodies) remains constant, provided there are no resultant external forces (e.g. friction).
AS	3	Elastic Collisions	Total momentum and total Kinetic Energy of a system is conserved. Relative speed of approach is equal to the relative speed of separation.
AS	3	Impulse	The product of a force & the time during which the force is applied.
AS	3	Inelastic Collisions	Total momentum of a system is conserved, but the total Kinetic Energy is not conserved. Speed before impact is not equal to speed after.
AS	3	Linear Momentum	Product of an object's mass & velocity, with its direction always being the same as the direction of velocity.
AS	3	Weight	The gravitational force on a mass due to the gravitational field (W = mg).
AS	4	Centre Of Gravity	The point on an object at which the entire weight of the body seems to act.
AS	4	Moment / Torque	Product of the force and the perpendicular distance to the pivot.
AS	4	Principle Of Moments	The sum of the clockwise moments about a point equals the sum of the anticlockwise moments (about the same point).
AS	4	Torque Of A Couple	Product of one of the forces and perpendicular distance between forces. (The turning effect caused by two equal & opposite forces when their lines of action are different.)
AS	4	Equilibrium	Net / resultant force and moment is zero (OR sum of clockwise moments = sum of anticlockwise moments). If the triangle of forces is 'closed' then there is no resultant force and the object is in equilibrium.
AS	4	Density	Amount of mass per unit volume of a substance.
AS	4	Pressure	The perpendicular/normal force applied per unit area.
AS	4	Upthrust	The resultant force on a submerged object due to pressure difference between the higher pressure at the bottom of the object and the lower pressure at the top of the object in a fluid.
AS	5	Energy	The stored ability to do work.
AS	5	Work Done	Product of a force & the distance moved in the direction of the force.
AS	5	Elastic Potential Energy	Energy stored due to deformation or change in shape of an object.
AS	5	Electric Potential Energy	Potential energy (stored) when charge is moved due to work done in an electric field.
AS	5	Gravitational Potential Energy	Energy stored due to height/position of mass.
AS	5	Kinetic Energy	Energy of an object due to its motion.
AS	5	Potential Energy	Energy stored by an object (at a position) to do work.
AS	5	Power	Rate of work done OR work done over time taken.
AS	5	Efficiency	The fraction of the useful power output obtained from the total power input.
AS	6	Force-Extension Graph	The area under such a graph is the work done in stretching a material. For the straight-line portion of the graph, it is a measure of the elastic potential energy stored by the material, provided that the graph for decreasing loads is the same as that for increasing loads. Also known as strain energy.
AS	6	Hooke's Law	Force/load is proportional to extension/compression if the proportionality limit is not exceeded.
AS	6	Elastic Deformation	Object returns to its original length (zero extension) when load is removed.
AS	6	Plastic Deformation	Wire/body does not return to its original shape / length when load is removed.
AS	6	Strain	Extension over original length (ratio). Stress is the cause & strain is the effect.
AS	6	Stress	The force per unit cross-sectional area required to stretch a material.
AS	6	Young's Modulus	Ratio of stress to strain.
AS	7	Longitudinal Waves	A wave in which displacement of particles is parallel to the direction of wave propagation. Vibrations are parallel to the direction of energy travel.
AS	7	Progressive wave	The transfer or propagation of energy as a result of oscillations / vibrations.
AS	7	Transfer of Energy	The transfer of energy is due to a progressive wave, NOT a standing/stationary wave.
AS	7	Transverse Waves	A wave in which displacement of particles is perpendicular to the direction of wave propagation, resulting in crests & troughs. Vibrations are perpendicular / normal to the direction of energy travel.
AS	7	(Wave) Intensity	The energy passing through unit area per unit time.
AS	7	(Wave) Speed	Speed at which energy is transferred / speed of wavefront. It is NOT the speed with which particles in the wave move.
AS	7	Amplitude	Maximum displacement of a particle from its equilibrium position.
AS	7	Displacement	Distance (of point on wave) from rest / equilibrium position.
AS	7	Frequency (Hz)	Number of oscillations per unit time.
AS	7	Period	The time taken to complete one oscillation/cycle. Or time between adjacent wavefronts.
AS	7	Phase Difference	The difference in the relative positions of the crests or troughs of two waves of the same frequency, expressed in radians or degrees.
AS	7	Wavelength	Distance moved by wave energy / wavefront during one cycle of the source, or minimum distance between two points with the same phase, or between adjacent crests or troughs.
AS	7	(CRO) Timebase setting	Adjusts horizontal distance of one cycle on the oscilloscope screen. Period (s) = Horizontal divisions × timebase.
AS	7	(CRO) Y-gain setting	Adjusts vertical height of signal on the screen. Amplitude (V) = Vertical divisions × y-gain.
AS	7	Doppler Effect	Change in observed frequency when source moves relative to the observer.
AS	7	Electromagnetic Waves	Transverse waves that can travel through a vacuum / free space. The displacement is a variation in electric & magnetic fields, perpendicular to each other.
AS	7	Polarisation	Oscillations or vibrations are in one direction, perpendicular to direction of propagation.
AS	8	Coherent	Two waves with a constant phase difference are said to be coherent.
AS	8	Constructive Interference	Two waves' path difference is either λ or nλ, OR phase difference is 360°.
AS	8	Destructive Interference	Two waves' path difference is either λ/2 or (n + ½)λ, OR phase difference is an odd multiple of 180°.
AS	8	Principle of Superposition	When two waves of the same type with similar frequency & speed superpose/meet/overlap, the resultant / total displacement is the sum of their individual displacements.
AS	8	Fundamental frequency	The lowest frequency stationary wave (1st harmonic) for a particular system.
AS	8	Maxima / Antinode	Position with maximum amplitude.
AS	8	Minima / Node	Position with zero amplitude.
AS	8	Stationary Waves	Incident wave is reflected at the end. Incident and reflected waves travelling in opposite directions with the same frequency/wavelength/speed overlap and superpose. The resultant displacement is the sum of displacements of each wave, producing nodes and antinodes.
AS	8	Fringe Width/Separation	The separation between one bright fringe & the next bright fringe.
AS	8	Interference	When two waves superpose/overlap, the resultant displacement is the sum of the displacement of each wave. When crests of both waves coincide, constructive interference gives maximum displacement. When a crest meets a trough, destructive interference gives minimum (or zero) displacement.
AS	8	Diffraction	When a wave (front) passes by/incident on an edge/slit, the wave spreads into the geometrical shadow.
AS	8	Diffraction Grating	When waves pass through the elements / gaps / slits in the grating, the wave bends/spreads into the geometrical shadow.
AS	9	Charge	Charge = current × time.
AS	9	Coulomb	A charge of 1 C passes a point when a current of 1 A flows for 1 s. (SI unit)
AS	9	Quantised	Charge only exists in discrete amounts. Charge on carriers is quantised.
AS	9	Electric Current	Amount of charge flowing past a point per unit time.
AS	9	Ampere	Amount of coulombs flowing past a point per unit second.
AS	9	Potential Difference (p.d.)	Energy converted from electrical to other forms of energy (work done) per unit charge that passes through.
AS	9	Volt	Joule converted per coulomb when charge passes from one point to another in a circuit
AS	9	Ohm (Ω)	volts per unit ampere.
AS	9	Ohm's Law	The current through a metallic conductor is proportional to the p.d. across it, provided that its temperature remains constant. (V=IR)
AS	9	Resistance	The ratio of p.d. over the current for an electrical component.
AS	9	Resistivity, ρ	The resistivity of a wire of a particular material is its resistance per unit length.
AS	9	Thermistor	A specific type of resistor in which, as temperature increases, the resistance decreases, & vice versa.
AS	10	Electromotive Force (e.m.f.)	Energy converted from chemical into electrical energy per unit charge supplied.
AS	10	Internal Resistance, r	Resistance of the cell, causing loss of voltage / energy loss within the cell.
AS	10	Lost volts	Energy lost in the battery (as heat) due to internal resistance when current flows through the battery.
AS	10	Output Power (Circuit)	A battery delivers maximum power to a circuit when the load resistance equals the internal resistance. When load resistance is zero, power dissipated by load is zero (P = I²R). When load resistance is very large, power dissipated becomes very small as current is reduced significantly.
AS	10	Terminal p.d.	p.d. across the battery's terminals after accounting for lost volts due to internal resistance (terminal p.d. = e.m.f. − lost volts).
AS	10	Kirchhoff's 1st Law	The sum of currents into a junction = sum of currents out of a junction. (Conservation of charge — charge cannot be created or destroyed.)
AS	10	Kirchhoff's 2nd Law	Sum of e.m.f.s = sum of p.d.s around a loop/circuit. (Conservation of energy — any gains in electrical energy must be balanced by corresponding losses.)
AS	10	Potentiometer	When a potential divider arrangement is used to compare e.m.f.s of two sources, it is known as a potentiometer.
AS	10	Null method	A method where the reading on the galvanometer is zero.
AS	11	Isotopes	Atoms (of the same element) which have the same proton number, but a different nucleon number / number of neutrons.
AS	11	Nucleon Number, A	The total number of protons and neutrons in the nucleus (also called mass number).
AS	11	Proton Number, Z	The number of protons in the nucleus of an atom (also called atomic number).
AS	11	Alpha-particle scattering experiment	Alpha particles were bombarded into a thin gold foil to study the structure of the atom. Most α-particles passed through or were deflected at small angles. A few were deviated by large angles.
AS	11	Electron (β⁻)	A negatively charged (−1e) lepton particle that is part of the atom.
AS	11	Neutron	A neutral baryon with three quarks (udd).
AS	11	Proton	A positively charged (+1e) baryon with three quarks (uud).
AS	11	The Atom	Made up of three sub-atomic particles: the proton (positively charged), the neutron (uncharged, equal mass to proton), & the electron (negatively charged, equal charge magnitude to proton, but much smaller in size & mass).
AS	11	Alpha particle	A helium nucleus — two protons and two neutrons, mass 4u. Can be deflected by electric/magnetic fields, absorbed by thin paper or a few cm of air. Highly ionising.
AS	11	Antineutrino / neutrino	A very small and light lepton particle produced during beta decay.
AS	11	Beta⁻ Decay	A down quark in a neutron (udd) becomes an up quark, converting it to a proton (uud). An electron (particle) and antineutrino (antiparticle) are produced.
AS	11	Beta Particle	Produced due to weak nuclear force/interaction. β-particles are fast-moving electrons with speeds up to 0.99c. Deflected by electric and magnetic fields. Absorbed by 1–4 mm of aluminium; range in air 0.5–2 m.
AS	11	Beta⁺ Decay	An up quark in a proton (uud) becomes a down quark, converting it to a neutron (udd). A positron (antiparticle) and neutrino (particle) are produced.
AS	11	Gamma Radiation	γ-radiation is part of the electromagnetic spectrum with wavelengths between 10⁻¹¹ m and 10⁻¹³ m.
AS	11	Baryon	A type of hadron made up of three quarks (e.g. proton and neutron).
AS	11	Hadron	Class of particles made up of quarks held together by the strong nuclear force. Not a fundamental particle.
AS	11	Lepton	Class of very small and light fundamental particles (e.g. electron and neutrinos).
AS	11	Meson	A type of hadron made up of two quarks.
AS	11	Quark	Fundamental particles that make up baryons and mesons.
AS	11	Down / Strange / Bottom	A quark particle with charge −⅓ e.
AS	11	Up / Charm / Top	A quark particle with charge +⅔ e.

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A2 Level Physics Terms & Concepts

Useful set of flashcards for A2 revision

• https://quizlet.com/392317817/cie-physics-a-level-definitions-flash-cards/ 

 

Lv	Ch	Term	Definition
A2	12	Period, T	Time taken to complete one complete cycle.
A2	12	Radian	Angle subtended at the centre of a circle by an arc of length equal to the radius of the circle.
A2	12	Angular acceleration	Rate of change of angular velocity.
A2	12	Angular displacement, θ	The angle through which the object has moved.
A2	12	Angular velocity, ω	Rate of change of angular displacement (ω = 2πf = 2π/T).
A2	12	Tangential speed, v	Speed of object that is tangent to the circular path (v = rω).
A2	13	Gravitational field	A region of space where a force per unit mass acts on a particle with mass.
A2	13	Gravitational field lines	Lines with arrows that show the direction of gravitational force on a mass.
A2	13	Newton's Law of Gravitation	Gravitational force is directly proportional to the product of masses and inversely proportional to the square of the separation.
A2	13	Gravitational field strength, g	Gravitational force per unit mass on the object.
A2	13	Gravitational potential, Φ	Work done per unit mass in bringing the mass from infinity to a point.
A2	13	Gravitational potential energy	Work done in bringing a mass from infinity to a point.
A2	13	Geostationary orbit	Orbit in which a satellite is positioned so that it orbits the Earth at the same rate as the Earth's rotation. The satellite remains above a fixed point on the Earth's surface.
A2	14	Absolute zero	Temperature at which atoms have minimum or zero energy.
A2	14	Heat / Thermal Energy, Q	Energy transferred from one object to another because of a temperature difference. Increases internal energy.
A2	14	Temperature	A measure of the average kinetic energy of particles; shows the direction of net heat flow between two bodies in contact.
A2	14	Thermal Equilibrium	When two or more objects in contact have the same temperature so that there is no net flow of thermal energy.
A2	14	Calibration	Uses fixed points as upper/lower reference points and assumes a linear change of property with temperature.
A2	14	Fixed points	Standard reference temperatures used when calibrating thermometers.
A2	14	Thermocouple	Device consisting of wires of two different metals across which an e.m.f. is produced when the two junctions are at different temperatures.
A2	14	Thermodynamic / Absolute Scale	A temperature scale that does not depend on the property of any particular substance.
A2	14	Boiling	The process by which a liquid changes into its gaseous state at a constant specific temperature (boiling point). Heat energy goes towards overcoming intermolecular forces to move atoms far enough so that interatomic forces and potential energy are negligible.
A2	14	Evaporation	The process by which molecules on the surface of a liquid with sufficient kinetic energy break free from attractive intermolecular forces & escape as gas particles. This occurs below the boiling point of a liquid.
A2	14	Melting	The process by which a solid changes into its liquid state at a constant specific temperature (melting point). Heat energy is used to overcome rigid forces between atoms. Potential energy increases, but kinetic energy remains constant.
A2	14	Specific heat capacity, c	Energy required per unit mass per unit temperature rise of 1 K or 1°C.
A2	14	Specific latent heat of fusion	Energy required per unit mass of a substance to change it from solid to liquid without a change in temperature.
A2	14	Specific latent heat of vaporisation	Energy required per unit mass of a substance to change it from liquid to gas without a change in temperature.
A2	14	Specific latent heat, L	Energy required per unit mass of a substance to change its state without any change in temperature.
A2	15	Ideal gas	A gas that obeys the ideal gas law PV = NkT and has no intermolecular forces.
A2	15	Mole	The amount of a substance which contains the same number of particles as there are atoms in 0.012 kg of carbon-12.
A2	15	Brownian Motion	The random movement of tiny suspended particles (such as smoke or pollen) in a fluid, caused by collisions with surrounding molecules.
A2	15	Kinetic Model of Gas Pressure	Molecules collide with the walls of the container, resulting in a change in momentum of each particle and hence a force on the wall (F = Δp/Δt). Many collisions over the area of the container produce pressure (P = F/A).
A2	15	Kinetic Theory	Molecules are in rapid, random motion. Collisions between gas molecules and container walls are elastic. Intermolecular forces of repulsion act only during collisions. The duration of collisions is negligible compared to the time between collisions. The volume of the molecules themselves is negligible compared to the volume of the container.
A2	15	Internal energy, U	Sum of the random distribution of kinetic and potential energies of all particles / molecules in a system.
A2	16	First Law of Thermodynamics	The increase in internal energy of a body equals the thermal energy transferred to it by heating plus the mechanical work done on it (ΔU = q + w).
A2	17	Amplitude, a	Maximum displacement from the equilibrium position.
A2	17	Period, T	Time taken to complete one complete cycle.
A2	17	Simple Harmonic Motion	Acceleration is directly proportional to displacement from the equilibrium position, and always directed towards it. Acceleration and displacement are in opposite directions (a = −ω²x).
A2	17	Damping	Oscillations / amplitude / energy decrease over time due to friction or resistive forces from the surroundings.
A2	17	Forced oscillation	Oscillation caused by an external driving force; the frequency equals that of the driving force.
A2	17	Free oscillation	Oscillation whose frequency is the natural frequency of the oscillator.
A2	17	Natural frequency	The unforced frequency of oscillation of a freely oscillating object.
A2	17	Resonance	When a system is forced to vibrate at or near its natural frequency, the amplitude of vibration increases rapidly. Maximum amplitude occurs when the driving frequency equals the natural frequency.
A2	18	Coulomb	A charge of 1 C passes a point when a current of 1 A flows for 1 s.
A2	18	Electric field	A region of space where a force per unit charge acts on a particle with charge.
A2	18	Electric field lines	Line spacing represents electric field strength. Arrows show the direction of force on a positive test charge.
A2	18	Electronvolt	The energy gained by an electron travelling through a p.d. of 1 V.
A2	18	Elementary charge	The smallest unit of charge that a particle can have.
A2	18	Coulomb's Law	Electric force is proportional to the product of the charges and inversely proportional to the square of the separation.
A2	18	Electric field strength, E	Electric force per unit positive test charge.
A2	18	Electric potential, V	Work done per unit charge (by an external force) in bringing a unit positive charge from infinity to a point.
A2	18	Electric potential energy	Work done (by an external force) in bringing a unit positive charge from infinity to a point.
A2	18	Potential gradient	Electric field strength is the negative potential gradient.
A2	19	Capacitance, C	Charge (on one plate) per unit potential difference across the plates.
A2	19	Exponential decrease (graph)	A graph where the rate of decrease (gradient) is proportional to the quantity itself.
A2	19	Time constant	The time taken for the charge on a capacitor to decrease to 37% of its original value. Equal to the product of R and C.
A2	20	Magnetic field	A region of space where a moving charge experiences a magnetic force.
A2	20	Magnetic field lines	Smooth curves that point from North to South pole outside a magnet.
A2	20	Magnetic flux density, B	Force per unit length per unit current in a straight conductor placed at right angles to the field.
A2	20	Magnetic flux linkage, NΦ	Product of magnetic flux and the number of turns.
A2	20	Magnetic flux, Φ	Product of the magnetic flux density normal to a circuit and the cross-sectional area of the circuit.
A2	20	Tesla	The unit of magnetic flux density; one newton per ampere per metre for a current-carrying wire at right angles to the field.
A2	20	Right-hand grip rule	Direction of magnetic field (curl fingers) is perpendicular to current (thumb). B is proportional to current.
A2	20	Fleming's Left-hand rule	When current / charge moves perpendicular to a magnetic field, the magnetic force on the current / charge is perpendicular to both the magnetic field (B) and the current / velocity of the charge.
A2	20	Velocity selector	A setup where electric and magnetic fields are perpendicular to each other so that particles of a specific speed pass through undeviated, as the electric and magnetic forces on them are equal in magnitude and opposite in direction.
A2	20	Hall voltage	p.d. between opposing sides of a conductor/semiconductor due to a current in a magnetic field, where electric and magnetic forces on moving charges are equal.
A2	20	Semiconductor	A material with fewer free electrons per unit volume compared to conductors, resulting in a larger Hall voltage.
A2	20	Faraday's Law	The induced e.m.f. is proportional to the rate of change of magnetic flux linkage.
A2	20	Lenz's Law	The induced current is in a direction so as to produce effects which oppose the change producing it.
A2	20	Eddy current	Induced currents in large conductors (e.g. metal plates and iron cores) that dissipate electrical energy as thermal energy (heat).
A2	20	Soft iron core	Easily magnetised and demagnetised material used to concentrate magnetic flux and increase flux linkage. Can be laminated to reduce energy loss due to eddy currents.
A2	21	r.m.s. current	Value of direct current that produces the same mean power or heating effect as the alternating current in a resistor.
A2	21	Rectification	Conversion from alternating to direct current using diodes.
A2	21	Rectification (Full wave)	Output produced by a bridge rectifier, giving a higher mean power.
A2	21	Rectification (Half wave)	Output produced by diodes in a circuit to ensure current only flows in one direction.
A2	21	Smoothing	Reduction in the variation of output voltage / current.
A2	22	Photon	A quantised packet of electromagnetic energy (E = hf).
A2	22	Photoelectric effect	Interaction between a photon and an electron in which the electron is removed from the atom.
A2	22	Stopping potential	The potential difference required to bring a moving electron (with kinetic energy) to rest.
A2	22	Threshold frequency	Minimum frequency required to release electrons from the surface of a metal.
A2	22	Work function, Φ	Minimum amount of energy required by a surface electron to escape the metal.
A2	22	de Broglie wavelength	Wavelength associated with a moving particle (λ = h/p).
A2	22	Absorption line spectrum	A dark line of a unique wavelength seen in a continuous spectrum.
A2	22	Emission line spectrum	A sharp and bright line of a unique wavelength seen in a spectrum.
A2	22	Discrete Energy Levels	A change in electron energy level emits or absorbs a single photon, where the difference in energy levels equals the energy of the photon at its corresponding frequency. Discrete frequencies imply discrete energy gaps, and therefore discrete energy levels.
A2	23	Isotopes	Same atomic number but different nucleon number. Same number of protons but different number of neutrons.
A2	23	Nucleus	The tiny central region of an atom that contains most of the mass and all of its positive charge.
A2	23	Nuclide	One type of nucleus characterised by a particular nucleon number and a particular proton number.
A2	23	Binding energy	Minimum energy needed to separate the nucleons in a nucleus to infinity.
A2	23	Einstein's Equation	The mass of a system increases when energy is supplied to it (E = mc²).
A2	23	Mass defect	Difference between the total mass of the individual separate nucleons and the mass of the nucleus. The difference in mass is converted to energy.
A2	23	Rest mass	The mass of a particle when it is stationary.
A2	23	Nuclear fission	The splitting of a heavy nucleus into two large fragments and a small number of neutrons.
A2	23	Nuclear fusion	A nuclear reaction where two light nuclei join together to form a heavier but more stable nucleus.
A2	23	Decay constant, λ	Probability of a nucleus decaying per unit time interval.
A2	23	Half-life	Mean time taken for half the number of active nuclei in a radioactive sample to decay.
A2	23	Radioactive Decay	A nucleus emits α-particles or β-particles and/or γ-radiation to form a more stable nucleus. The process is random and spontaneous — there is no way to predict which nucleus will decay next, as shown by fluctuations in count rate.
A2	23	Random	Cannot predict when or which nucleus will decay next, though the probability of decay of any given nucleus is constant.
A2	23	Spontaneous	Nuclear decay is not affected by any external / environmental factors such as temperature and pressure.
A2	24	Attenuation	Exponential decrease of wave intensity / power / amplitude as it travels through a medium, due to energy absorption by the medium.
A2	24	X-Ray Scan	X-rays pass through a structure and the transmitted waves are detected. Differences in transmitted intensities are used to form an image.
A2	24	Braking radiation	X-rays produced when electrons are rapidly decelerated.
A2	24	Contrast	Degree of difference in blackening between regions of an image due to different transmitted intensities. Good contrast means the transmitted intensities of two adjacent media are very different.
A2	24	CT Scan	X-rays of a slice are taken at different angles. A computer combines the images into a 2D cross-section. This is repeated for successive slices along the body to create a 3D image.
A2	24	Ultrasound scan	An alternating p.d. makes a piezoelectric crystal in the transducer vibrate, generating an ultrasound pulse. Ultrasound is reflected at boundaries and detected by the same crystal. The time delay between emission and detection gives depth information; the intensity of the reflected wave gives information about the nature of the boundary.
A2	24	Specific Acoustic impedance, Z	Product of the density of a substance and the speed of sound in that substance (Z = ρc).
A2	24	Reflection coefficient	Fraction / percentage of wave intensity reflected at a boundary between two media of different acoustic impedance. A lower reflection ratio means higher transmitted intensity.
A2	24	Transducer	A device that converts one form of energy into another (e.g. electrical to mechanical).
A2	24	PET Scan	A tracer containing a β⁺ emitter is injected into the body. Emitted positrons interact with electrons and annihilation occurs, converting mass into gamma photons. The pair of gamma photons travel in opposite directions and are detected at a ring of detectors at different times. A computer determines the location of gamma production and produces an image of tracer concentration in the tissue.
A2	24	Annihilation	Event where an electron (particle) and a positron (antiparticle) interact to produce two gamma ray photons.
A2	24	Tracer	A substance with radioactive nuclei (β⁺ emitter) that is injected into the body and absorbed by the tissue being studied.
A2	25	Luminosity, L	Total power of electromagnetic radiation emitted by a star.
A2	25	Radiant Flux Intensity, F	Apparent / perceived brightness of an object at a distance.
A2	25	Standard Candle	An astronomical object with known luminosity, used to calculate distances.
A2	25	Wien's Law	Wavelength of maximum intensity is inversely proportional to thermodynamic temperature.
A2	25	Doppler redshift	Observed / apparent wavelength is greater than the known value. Lines in the spectrum of light from a star are compared with a known reference spectrum.
A2	25	Hubble's Law	Recession speed of a galaxy is proportional to its distance from the observer (v = Hd).
A2	25	Big Bang theory of origin	Redshift shows all parts of the Universe moving away from each other, with more distant objects moving away faster. This implies that matter must have been very close together / a singularity / very dense in the past.

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Updated 2026.