AS&A Physics CD-ROM Glossary.Indd

www.igcse.at.ua

Glossary

absoluteolute scale of temtemperatureperature seseee thermodynamic astronomical uunitnit Th e averageaverage distance of the Earth scale from the Sun. absolute zero Th e temperature at which a attenuation Th e gradual loss in strength or intensity system has minimum internal energy; equivalent of a signal. to −273.15 °C. average speed Th e total distance travelled by an absorption line spectrum A dark line of a unique object divided by the total time taken. wavelength seen in a continuous spectrum. Avogadro constant Th e number of particles in one acceleration Th e rate of change of an object’s mole 23 −1 velocity: of any substance (6.02 × 10 mol ), denoted NA. bandwidth (communications) A measure of a = ∆v ∆t the width of a range of frequencies being transmitted. −2 Unit: m s . base station A receiver and transmitter used to accuracy An accurate value of a measured quantity is maintain contact with a number of mobile one which is close to the true value of the quantity. phones (cell phones) in a local area. acoustic impedance Acoustic impedance Z is best fi t line A line drawn through the points plotted the product of the density ρ of a substance and ρ on a graph so that it passes through as many points the speed c of sound in that substance (Z = c). as possible, taking into consideration a balance −2 −1 Unit: kg m s . between the number of points above and below activity Th e rate of decay or disintegration of the line. nuclei in a radioactive sample. binding energy Th e minimum external energy amorphous Describes a material whose particles are required to separate all the neutrons and protons arranged in a disordered way. of a nucleus. ampere Th e SI unit of electric current. bit A basic unit of information storage. Th e amount amplitude modulation A form of modulation where of information stored by a device that exists in the signal causes variations in the amplitude of a only two distinct states, usually given as the carrier wave. binary digits 0 and 1. amplitude Th e maximum displacement of a particle Boyle’s law Th e pressure exerted by a fi xed mass of from its equilibrium position. gas is inversely proportional to its volume, provided analogue signal A signal that is continuously the temperature of the gas remains constant. variable, having a continuum of possible values. braking radiation X-rays produced when electrons analogue-to-digital conversion (ADC) Conversion are decelerated (also called Bremsstrahlung of a continuous analogue signal to discrete radiation). digital numbers. brittle Describes a material that shows no plastic angular displacement Th e angle through which an deformation and breaks just beyond its elastic limit. object moves in a circle. Brownian motion Th e random movement of angular frequency Th e rate of change of angle small particles caused by bombardment of invisible expressed in radian per second: molecules. π angular frequency ω = 2 capacitance Th e ratio of charge stored by a capacitor T to the potential diff erence across it. angular velocity Th e rate of change of the angular carbon-dating A technique used to date relics using position of an object as it moves along a curved path. the carbon-14 isotope. antinode A point on a stationary wave with carrier wave a waveform (usually sinusoidal) that is maximum amplitude. modulated with an input signal to carry information. antiphase Describes two waves or oscillations that are cellular exchange A switching centre connecting all 180° out of phase. the base stations in an area.

AS and A Level Physics © Cambridge University Press Glossary 1 www.igcse.at.ua www.igcse.at.ua

centre of gravity Th e point where the entire weight control rods Rods of a neutron-absorbing material of an object appears to act. used to reduce the rate of a nuclear chain reaction. centripetal force Th e net force acting on an object coolant A substance used to transfer thermal energy moving in a circle; it is always directed towards the from the core of a nuclear reactor. centre of the circle. coulomb Th e SI unit of electrical charge. A charge of chain reaction An exponential growth of a fi ssion 1 C passes a point when a current of 1 A fl ows for reaction caused by the increasing fl ux of neutrons 1 s. 1 C = 1 A s causing fi ssion. Coulomb’s law Any two point charges exert an characteristic radiation Very intense X-rays electrical force on each other that is proportional produced in an X-ray tube having specifi c to the product of their charges and inversely wavelengths that depend on the target metal. proportional to the square of the distance between charge carrier Any charged particles, such as them. electrons, responsible for a current. count rate Th e number of particles (beta or alpha) Charles’s law Th e volume occupied by a gas at or gamma-ray photons detected per unit time by a constant pressure is directly proportional to its Geiger–Müller tube. Count rate is always a fraction thermodynamic (absolute) temperature. of the activity of a sample. closed system A system of interacting objects where cross-linking (cross-talk) A signal transmitted in there are no external forces. one circuit or channel picked up, undesirably, in coaxial cable An electrical cable with an inner another circuit or channel. conductor surrounded by a tubular insulating layer crystalline Describes a material whose particles are and an outside conducting layer. arranged in an ordered way. coherent Two sources are coherent when they emit damped Describes an oscillatory motion where the waves with a constant phase diff erence. amplitude decreases with time due to energy losses. collimated beam A parallel-sided beam of radiation. de Broglie wavelength Th e wavelength associated collimator A device for producing a parallel beam with a moving electron given by the equation: of radiation. λ h components (of a vector) Th e magnitudes of a = mv vector quantity in two perpendicular directions. decay constant Th e constant λ for an isotope that compression A region in a sound wave where the λ air pressure is greater than its mean value. appears in the equation A = N. It is equal to compressive Describes a force that squeezes an object. the probability of an isotope decaying per unit time computerised axial tomography A technique where interval. X-rays are used to image slices of the body in order decibel A logarithmic unit of measurement that to produce a computerised 3-D image. expresses the relative sizes of two powers using P conservation of momentum In a closed system, the formula 10 lg 1 .  when bodies interact, the total momentum in any P2 specifi ed direction remains constant. density Th e mass per unit volume of a material: constructive interference When two waves reinforce ρ = m V to give increased amplitude. contact force Th e force an object exerts on another Unit: kg m−3. with which it is in contact. dependent variable Th e variable in an experiment contrast media Materials such as barium that easily which is controlled by the experimenter. absorb X-rays. A contrast medium is used to destructive interference When two waves cancel to reveal the outlines or edges of soft tissues in an give reduced amplitude. X-ray image. diff raction Th e spreading of a wave when it passes contrast In a high-contrast image, there is a big through a gap or past the edge of an object. diff erence in brightness between bright and digital signal A signal that has only a few possible dark areas. values, often only two.

2 AS and A Level Physics © Cambridge University Press Glossary www.igcse.at.ua www.igcse.at.ua

digital-to-analogue conversion (DAC) Conversion electrolyte An electrically conducting solution. Th e of a series of digital numbers into a continuous conduction is due to positive and negative ions in analogue signal. the solution. dispersion Th e splitting of light into its diff erent electromagnetic spectrum A family of waves wavelengths. that travel through a vacuum at a speed of displacement Th e distance moved by an object 3.0 × 108 m s−1. in a particular direction (measured from a fi xed electronvolt Th e energy gained by an starting point). electron travelling through a p.d. of 1 volt. drag A force that resists movement of a body through 1 eV = 1.6 × 10−19 J. a fl uid. elementary charge Th e smallest unit of charge that ductile Describes a material that can easily be a particle or an object can have. It has a magnitude drawn into wires (e.g. copper). of 1.6 × 10−19 C. dynamics A study of motion involving force emission line spectrum A sharp and bright line of and mass. a unique wavelength seen in a spectrum. e.m.f. Th e total work done when unit charge energy level Th e quantised energy states of an moves round a complete circuit. Unit: J C−1 or electron in an atom. volt (V). equation of state Equation for an ideal gas: effi ciency Th e ratio of useful output energy to the total input energy for a device, expressed pV = nRT or pV = NkT as a percentage: (Also known as the ideal gas equation.) useful output energy effi ciency = × 100% equations of motion Four equations that can total input energy be used to determine quantities such as Einstein relation Th is refers to the equation for the displacement, initial velocity, fi nal velocity and energy of a photon – that is: acceleration. equilibrium An object in equilibrium is either at hc E = hf or E = λ rest or travelling with a constant velocity because the net force on it is zero. elastic Describes a material which will return to errors Inaccuracies when taking measurements. its original shape when the forces acting on evaporation Th e process by which a liquid it are removed. becomes a gas at a temperature below its boiling elastic hysteresis Th is occurs when the extension point. of an elastic material is diff erent in loading exponential decay graph A decaying graph that and unloading. has a constant-ratio property for a given interval elastic limit Th e value of stress beyond which an of time. object will not return to its original dimensions. exponential decay A quantity that has a elastic potential energy Energy stored in an ‘constant-ratio property’ with respect to time. extended or compressed material. extension Th e change in the length of a material electric charge A property of many particles which from its original length. gives rise to a force between them. farad Th e unit of capacitance. 1 F = 1 C V−1. electric fi eld A region in which a charged body Faraday’s law of electromagnetic induction Th e experiences a force. induced e.m.f. is proportional to the rate of electric fi eld strength Th e force per unit positive change of magnetic fl ux linkage. charge at a point. Unit: V m−1 or N C−1. fi eld lines Lines drawn to represent the strength and electric potential Th e energy per unit charge due to a direction of a fi eld of force. charged body’s position in an electric fi eld. fi eld of force A region of space where an object electrical resistance Th e ratio of potential diff erence feels a force; the force may be gravitational, to current. Unit: ohm (Ω). electric, magnetic, etc.

AS and A Level Physics © Cambridge University Press Glossary 3 www.igcse.at.ua www.igcse.at.ua

First law of thermodynamics Th e increase in ground state Th e lowest energy state that can be internal energy of a body is equal to the thermal occupied by an electron in an atom. energy transferred to it by heating plus the half-life Th e mean time taken for half the number of mechanical work done on it. active nuclei in a radioactive sample to decay. Fleming’s left-hand (motor) rule Th is rule is used to harmonic A wave of frequency n times the predict the force experienced by a current-carrying fundamental frequency, where n is an integer. conductor placed in an external magnetic fi eld: Hooke’s law Th e extension produced in an object thumb → motion, fi rst fi nger → magnetic fi eld is proportional to the force producing it, provided and second fi nger → conventional current. the elastic limit is not exceeded. Fleming’s right-hand (generator) rule Th is rule is hydrogen burning A sequence of nuclear reactions used to predict the direction of the induced in which four protons fuse together to produce a current or e.m.f. in a conductor moved at right helium nucleus: angles to a magnetic fi eld: thumb → motion, 41H → 4He + 2 0e fi rst fi nger → magnetic fi eld and second 1 2 +1 → fi nger induced conventional current. ideal gas equation Equation for an ideal gas: force constant Th e ratio of force to extension for a spring or a wire. Unit: N m−1. pV = nRT or pV = NkT forced oscillation An oscillation caused by an external driving force whose frequency is equal to that of the (Also known as the equation of state.) driving force. ideal gas A gas that behaves according to the free oscillation An oscillation whose frequency is equations pV = nRT and pV = NkT. the natural frequency of the oscillator. image intensifi er A device used to change a low- frequency Th e number of oscillations of a particle per intensity X-ray image into a bright visual image. unit time. Unit: hertz (Hz). impedance matching Th e reduction in intensity of frequency modulation A form of modulation where refl ected ultrasound at the boundary between two substances, achieved when the two substances have the signal causes variations in the frequency of a similar acoustic impedances. carrier wave. impulse Th e product of the force F and the time fundamental frequency Th e lowest-frequency ∆t for which it acts: stationary wave for a particular system. gain Th e voltage gain of an amplifi er is the ratio of impulse = F∆t the output voltage to the input voltage. geostationary orbit Th e orbit of an artifi cial satellite independent variable Th e variable which changes which has a period equal to one day so that the when the dependent variable changes. satellite remains above the same point on the Earth’s induced nuclear fi ssion A fi ssion reaction started equator. From Earth the satellite appears when a neutron is absorbed by a nucleus. to be stationary. inelastic A collision is inelastic when the kinetic gravitational fi eld A region where any object with energy is not conserved; some is transferred to mass experiences a force. other forms such as heat. Momentum and total gravitational fi eld strength Th e gravitational force energy are always conserved. experienced by an object per unit mass: inertia A measure of the mass of an object. A massive object has a large inertia. F instantaneous speed Th e speed of an object g = m measured over a very short period of time. gravitational potential Th e gravitational intensity Th e power transmitted normally through potential energy per unit mass at a point in a a surface per unit area: gravitational fi eld. power intensity = gravitational potential energy Th e energy a body cross-sectional area has due to its position in a gravitational fi eld. Unit: W m−2.

4 AS and A Level Physics © Cambridge University Press Glossary www.igcse.at.ua www.igcse.at.ua

interference Th e formation of points of cancellation lost volts Th e diff erence between the e.m.f. and the and reinforcement where two coherent waves pass terminal p.d. It is also equal to the voltage across through each other. the internal resistance. internal energy Th e sum of the random distribution macroscopic Visible to the naked eye. of kinetic and potential energies of the atoms or magnetic fi eld A force fi eld in which a magnet, molecules in a system. a wire carrying a current, or a moving charge internal resistance Th e resistance of an e.m.f. source. experiences a force. Th e internal resistance of a battery is due to its magnetic fl ux density Th e strength of a magnetic chemicals. fi eld. Magnetic fl ux density B is defi ned as: inverting amplifi er A circuit, involving the use F of an amplifi er, where the output is 180° out of B = Il phase with the input. ion An atom with a net positive or negative charge. where F is the force experienced by a conductor isotopes Nuclei of the same element with a in the magnetic fi eld, I is the current in the diff erent number of neutrons but the same number conductor and l is the length of the conductor in of protons. the magnetic fi eld. (Th e conductor is at right I–V characteristic A graph of current against voltage angles to the fi eld.) for a particular component. You can identify a magnetic fl ux linkage Th e product of magnetic fl ux component from its I–V graph. and the number of turns. Unit: weber (Wb). kilowatt-hour Th e energy transferred by a 1 kW magnetic fl ux Th e product of magnetic fl ux density device in a time of 1 hour. 1 kW h = 3.6 MJ. normal to a circuit and the cross-sectional area of kinematics A study of motion using quantities such the circuit. Unit: weber (Wb). as time, distance, displacement, speed, velocity magnetic resonance imaging (MRI) a medical and acceleration. imaging technique which uses nuclear magnetic kinetic energy Energy of an object due to its motion. resonance. kinetic theory of gases A model based on the mass A measure of the amount of matter within an microscopic motion of atoms or molecules of a gas. object. Unit: kilogram (kg). Kirchhoff ’s fi rst law Th e sum of the currents mass defect Th e diff erence between the total mass entering any point (or junction) in a circuit is equal of the individual, separate nucleons and the mass to the sum of the currents leaving that same point. of the nucleus. Th is law conveys the conservation of charge. mean drift velocity Th e average speed of charged Kirchhoff ’s second law Th e sum of the e.m.f.s round particles along the length of a conductor. a closed loop in a circuit is equal to the sum of p.d.s microscopic Too small to be viewed with the in that same loop. naked eye. Larmor frequency Th e frequency of precession of microwave link A communications system that nuclei in an external magnetic fi eld. uses a beam of radio waves in the microwave Lenz’s law Th e induced current or e.m.f. is in a frequency range to transmit audio, data or video direction so as to produce eff ects which oppose the information. change producing it. mobile phone (cell phone) An electronic device light-dependent resistor (LDR) A resistor whose used for mobile communication by connecting to a resistance decreases as the intensity of light falling cellular network of base stations. on it increases. moderator A material used in a nuclear reactor to light-emitting diode (LED) A semiconductor slow down fast-moving neutrons so that they component that emits light when it conducts. have a greater chance of interacting with the linear momentum Th e product of an object’s mass and fi ssile nuclei. its velocity, p = mv. Momentum is a vector quantity. modulation Th e process of using one waveform longitudinal wave A wave in which particles oscillate to alter the frequency, amplitude or phase of along the direction in which the wave travels. another waveform.

AS and A Level Physics © Cambridge University Press Glossary 5 www.igcse.at.ua www.igcse.at.ua

mole Th e amount of matter which contains nuclear model of the atom A model of the atom in 6.02 × 1023 particles. which negative charges (electrons) are distributed moment Th e moment of a force about a point is outside a tiny nucleus of positive charge. the magnitude of the force, multiplied by the nucleon number Th e number of neutrons and perpendicular distance of the point from the line protons in the nucleus of an atom (also called of the force. Unit: N m. mass number). natural frequency Th e unforced frequency of nucleon A particle found in an atomic nucleus, oscillation of a freely oscillating object. i.e. a neutron or a proton. negative feedback Th e output of a system acts to nucleus Th e tiny central region of the atom that oppose changes to the input of the system, with contains most of the mass of the atom and all the result that the changes are reduced. of its positive charge. neutron number Th e number of neutrons in the nuclide A specifi c combination of protons and nucleus of an atom. neutrons in a nucleus. newton Th e force that will give a 1 kg mass an Ohm’s law Th e current in a metallic conductor is acceleration of 1 m s−2 in the direction of the directly proportional to the potential diff erence force. 1 N = 1 kg m s−2. across its ends, provided its temperature remains Newton’s fi rst law of motion An object will remain constant. at rest or keep travelling at constant velocity unless operational amplifi er (op-amp) A high-gain it is acted on by an external force. electronic d.c. voltage amplifi er with diff erential inputs and, usually, a single output. Newton’s law of gravitation Any two point masses optic fi bre A glass or plastic fi bre that carries light attract each other with a force that is directly along its length. proportional to the product of their masses oscillates Another term for ‘vibrates’. and inversely proportional to the square of oscillation A repetitive back-and-forth or up-and- their separation. down motion. Newton’s second law of motion Th e net force parallel A term used when components are acting on an object is equal to the rate of change connected across each other in a circuit. of its momentum. Th e net force and the change in path diff erence Th e diff erence in the distances momentum are in the same direction. travelled by two waves from coherent sources at a Newton’s third law of motion When two bodies particular point. interact, the forces they exert on each other are perfectly elastic A collision is perfectly elastic when equal and opposite. kinetic energy is conserved. Momentum and total node A point on a stationary wave with zero energy are always conserved. amplitude. period Th e time taken by an object (e.g. a planet) noise An unwanted random addition to a transmitted to complete one orbit. Th e period is also the time signal. taken for one complete oscillation of a vibrating non-inverting amplifi er A circuit, involving the use object. Unit: second (s). of an amplifi er, where the output is in phase with phase diff erence Th e fraction of an oscillation the input. between the vibrations of two oscillating particles, 235 nuclear fi ssion Th e splitting of a nucleus (e.g. 92U) expressed in degrees or radians. into two large fragments and a small number of phase Describes the point that an oscillating mass has neutrons. reached in a complete cycle. nuclear fusion A nuclear reaction where two light photoelectric eff ect An interaction between a photon 2 nuclei (e.g. 1H) join together to form a heavier and an electron in which the electron is removed but more stable nucleus. from the atom. nuclear magnetic resonance A process in which photomultiplier tubes Devices used in a gamma radio waves are absorbed or emitted by nuclei camera to change the energy of an incident γ-ray spinning in a magnetic fi eld. photon into an electrical pulse.

6 AS and A Level Physics © Cambridge University Press Glossary www.igcse.at.ua www.igcse.at.ua

photon A quantum of electromagnetic energy. p = F piezo-electric crystal A material that produces an A e.m.f. when it is compressed. Also, when a voltage −2 is applied across it in one direction, it shrinks Unit: N m or Pa. slightly. principle of moments Th e sum of clockwise piezo-electric eff ect Th e production of an e.m.f. moments about a point is equal to the sum of across the faces of a crystal when the crystal is anticlockwise moments about the same point for a compressed. body in equilibrium. Planck constant Th e constant which links the principle of superposition When two or more waves energy of a photon and its frequency, given by the meet at a point, the resultant displacement is the equation: sum of the displacements of the individual waves. progressive wave A wave that carries energy from one E = hf place to another. projectile Any object thrown in the Earth’s plane polarised Describes transverse waves that gravitational fi eld. oscillate in only one plane. proton number Th e number of protons in the plastic deformation Th e deformation of a material nucleus of an atom (also called atomic number). beyond the elastic limit. public switched telephone network Th e network which plum pudding model A model of the atom in which connects public telephones throughout the world. radian An alternative unit for measuring angles. negative charges are distributed throughout a sphere π π of positive charge. 2 radians = 360° or radians = 180°. point mass An object with mass that is represented range Th e horizontal distance covered by an object. as a point (dot) because its size is extremely small rarefaction A region in a sound wave where the air compared with the separation between objects. pressure is less than its mean value. polymer A material containing large molecules rectifi cation Th e process of converting alternating composed of repeating structural units. current (a.c.) into direct current (d.c.). positron A positively charged particle with mass refl ection Th e bouncing back of a wave from a surface. equal to that of an electron. refraction Th e change in direction of a wave as it potential diff erence (p.d.) Th e energy lost per unit crosses an interface between two materials where its charge by charges passing through a component. speed changes. Unit: J C−1 or volt (V). regeneration Restoring a signal to its original form, potential divider A circuit in which two or more usually removing or reducing noise and increasing components are connected in series to a supply. signal strength. Th e output voltage from the circuit is taken across relative speed Th e magnitude of the diff erence in one of the components. velocities between two objects. potentiometer A circuit which allows the relaxation time Th e time taken for the nuclei to fall measurement of an e.m.f. by comparison with a back to their lower energy state. known e.m.f. relay An electrically operated switch, caused to open power Th e rate at which energy is transferred or the and close by current in a coil. rate at which work is done. Unit: watt (W). repeater An electronic device that receives a signal precession Th e movement of the axis of a spinning and retransmits it. object (proton) around another axis. resistivity Th e property of a material defi ned by: precision Th e smallest change in value that can resistance × cross-sectional area resistivity = be measured by an instrument or an operator. length A precise measurement is one obtained several times and achieving the same, or very similar, values. ρ = RA pressure Th e force acting normally per unit area L of a surface: Unit: Ω m.

resistor An electrical component whose resistance solid to liquid without a change in temperature. in a circuit remains constant. Its resistance is Unit: J kg−1. independent of current or potential diff erence. specifi c latent heat of vaporisation Th e energy resonance Th e forced motion of an oscillator required per kilogram of a substance to change it characterised by maximum amplitude when the from liquid to gas without a change in temperature. forcing frequency matches the oscillator’s natural Unit: J kg−1. frequency. A system absorbs maximum energy from speed Th e rate of change of the distance moved a source when the source frequency is equal to the by an object: natural frequency of the system. rest mass Th e mass of a an isolated stationary speed = distance particle. time resultant force Th e net force acting on an object. Unit: m s−1. resultant Total or net. speed Th e rate of change of the distance moved right-hand grip rule A rule for fi nding the direction by an object: of the magnetic fi eld inside a solenoid. If the right spin A fundamental property of subatomic particles hand grips the solenoid with the fi ngers following which is conserved during atomic and nuclear the direction of the current around the solenoid, reactions. then the thumb points in the direction of the stationary wave A wave pattern produced when magnetic fi eld. two progressive waves of the same frequency sampling Taking the value of a continuous signal at travelling in opposite directions combine. It is regular intervals. characterised by nodes and antinodes. Also scalar quantity A scalar quantity has magnitude known as a standing wave. but no direction. strain energy Th e potential energy stored in an semiconductor diode An electrical component object when it is deformed elastically. made from a semiconductor material (e.g. silicon) strain gauge A device that contains a fi ne wire that only conducts in one direction. A diode in sealed in plastic. Its electrical resistance changes ‘reverse bias’ has an infi nite resistance. when the object to which it is attached changes sensor A device that produces an output (usually a shape. voltage) in response to an input. strain Th e extension per unit length produced by series A term used when components are connected tensile or compressive forces: end-to-end in a circuit. sharpness Th e degree of resolution in an image, strain = extension which determines the smallest item that can be original length identifi ed. stress Th e force acting per unit cross-sectional area: sidebands A band of frequencies above or below the carrier frequency produced as a result of strain = force modulation. cross-sectional area simple harmonic motion Motion of an oscillator where its acceleration is directly proportional to its systematic error An error in readings which is displacement from its equilibrium position and is repeated through out an experiment, either directed towards that position. producing a constant absolute error or a constant solenoid A long current-carrying coil used to percentage error. generate a uniform magnetic fi eld within its core. tensile A term used to denote tension or pull. specifi c heat capacity Th e energy required per terminal p.d. Th e potential diff erence across the unit mass of a substance to raise its temperature external resistor connected to an e.m.f. source. by 1 K (or 1 °C). Unit: J kg−1 K −1. terminal velocity Th e constant velocity of an object specifi c latent heat of fusion Th e energy required travelling through a fl uid. Th e net force on the per kilogram of a substance to change it from object is zero.

tesla Th e SI unit for magnetic fl ux density. Unit: tesla ultimate tensile stress (UTS) Th e maximum stress (T). 1 T = 1 N A−1 m−1. that a material can withstand. thermal energy Energy transferred from one object to another because of a temperature diff erence; UTS = maximum force another term for heat energy. cross-sectional area thermal equilibrium A condition when two or more objects in contact have the same unifi ed atomic mass unit A convenient unit temperature so that there is no net fl ow of energy used for the mass of atomic and nuclear particles (1 u is equal to 1 the mass of a 12C carbon atom). between them. 12 6 thermistor A device whose electrical resistance 1 u = 1.66 × 10−27 kg. changes when its temperature changes. uniform acceleration Acceleration that remains thermocouple A device consisting of wires of two constant. diff erent metals across which an e.m.f. is produced uniform motion Motion of an object travelling when the two junctions of the wires are at diff erent with a constant acceleration. temperatures upthrust Th e upward force that a liquid exerts thermodynamic scale A temperature scale where on a body fl oating or immersed in a liquid. temperature is measured in kelvin (K). vector addition Using a drawing, often to scale, threshold frequency Th e minimum frequency of the to fi nd the resultant vector. electromagnetic radiation that will eject electrons vector quantity A vector quantity has both from the surface of a metal. magnitude and direction. threshold voltage Th e minimum forward bias voltage vector triangle A triangle drawn to determine across a light-emitting diode (LED) when it starts the resultant of two vectors. to conduct and emit light. velocity Th e rate of change of the displacement time constant Th e time taken for the current, charge of an object: stored or p.d. to fall to 1/e (about 37%) when a capacitor discharges through a resistor. It is also change in displacement velocity = equal to the product of capacitance and resistance. time torque Th e product of one of the forces of a couple and the perpendicular distance between them. Unit: m s−1. (You can think of velocity as ‘speed in a Unit: N m. certain direction’.) tracers Radioactive substances used to investigate the virtual earth approximation An approximation function of organs of the body. where the two inputs of an op-amp are taken to transducer A general term used for any device that be at the same potential. changes one form of energy into another. viscous forces Forces that act on a body moving transition When an electron makes a ‘jump’ between through a fl uid that are caused by the resistance two energy levels. of the fl uid. transverse wave A wave in which the oscillation is at voxel A small cube in a three dimensional picture, the right angles to the direction in which the wave travels. equivalent of a pixel in a two-dimensional picture. triangle of forces A closed triangle drawn for an wave A periodic disturbance travelling through space, object in equilibrium. Th e sides of the triangle characterised by vibrating particles. represent the forces in both magnitude and wavelength Th e distance between two adjacent peaks direction. or troughs. turns-ratio equation An equation relating the ratio weight Th e force on an object caused by a of voltages to the ratio of numbers of turns on the gravitational fi eld acting on its mass: two coils of a transformer: weight = mass × acceleration of free fall

Vnss = Unit: newton (N). Vnpp

wire-pair A form of wiring in which the two Young modulus Th e ratio of stress to strain for conductors needed to carry a signal are placed a given material, resulting from tensile forces, close together. provided Hooke’s law is obeyed: work done Th e product of the force and the distance moved by the force in the direction = stress Young modulus strain of travel. work function Th e minimum energy required by a Unit: Pa (or MPa, GPa). single electron to escape the metal surface. zero error A systematic error in an instrument that X-ray tube A device that produces X-rays when gives a non-zero reading when the true value of a accelerated electrons hit a target metal. quantity is zero.