Paper 1 Definitions

2: Particles and Radiation

Alpha Decay: The process of an unstable nucleus emitting an alpha particle (two protons and two neutrons) to become more stable.

Annihilation:The process of a particle and its antiparticle colliding and being converted into energy. The energy is released in two photons to conserve momentum.

Antiparticle:All particles have a corresponding antiparticle with the same mass but opposite charge and conservation numbers.

Baryon Number:A quantum number that is conserved in all particle interactions. Baryons have a baryon number of +1 and non-baryons have a baryon number of 0.

Baryon: A class of hadron, that is made up of three quarks. The proton is the only stable baryon.

Beta-Minus Decay: The process of a neutron inside a nucleus turning into a proton, and emitting a beta-minus particle (an electron) and a antineutrino.

Beta-Plus Decay: The process of a proton inside a nucleus turning into a neutron, and emitting a beta-plus particle (a positron) and a neutrino.

Electron Diffraction: The spreading of electrons as they pass through a gap similar to the magnitude of their de Broglie wavelength. It is evidence of the wave-like properties of particles.

Electron-volt (eV): The work done to accelerate an electron through a potential difference of 1V. 1eV is equal to the charge of an electron (E=qv).

Energy Levels: Defined and distinct energies at which electrons can exist in an atom. An electron cannot exist between energy levels.

Excitation: The process of an electron taking in exactly the right quantity of energy to move to a higher energy level.

Gauge Boson: The exchange particles that transmit the four fundamental interactions between particles.

Ground State: The most stable energy level that an electron can exist in.

Hadrons: A class of subatomic particle that experiences the strong nuclear interaction.

Ionisation: The process of an atom losing an orbital electron and becoming charged.

Isotope: Same number of protons but different numbers of neutrons.

Isotopic Data: Data from isotopes that can be used for a purpose, such as carbon dating.

Kaon: A type of meson that decays into pions.

Lepton Number: A quantum number that is conserved in all particle interactions. Both electron lepton numbers and muon lepton numbers must be conserved.

Lepton: A group of elementary subatomic particles, consisting of electrons, muons and neutrinos.

Meson: A class of hadron that is made up of a quark and antiquark pair.

Muon: A type of lepton that decays into electrons.

Neutrino: A subatomic particle whose existence was hypothesised to maintain the conservation of energy in beta decay.

Nucleon Number (A): The sum of the number of protons and neutrons in a given nucleus.

Nucleon: A proton or neutron.

Pair Production: The process of a sufficiently high-energy photon converting into a particle and its corresponding antiparticle. To conserve momentum, this usually occurs near a nucleus.

Photon: A packet of energy.

Pion: A type of meson and the exchange particle for the strong nuclear force.

Positron: A positively charged particle that is the antiparticle of an electron.

Proton Number (Z): The number of protons present in the nucleus of a given element.

Stopping Potential: The minimum potential difference required to stop the highest kinetic energy electrons from leaving the metal plate in the photoelectric effect.

Strange Particles: Particles that are produced through the strong interaction but decay through the weak interaction.

Strangeness: A quantum number that is conserved in strong interactions but not in weak interactions. This reflects that strange particles are always produced in pairs.

Strong Nuclear Force: A force that acts between nucleons in a nucleus to keep it stable. It is attractive at distances of up to 3fm and repulsive at separations less than 0.5fm.

Threshold Frequency: The minimum frequency of photons required for photoelectrons to be emitted from the surface of a metal plate through the photoelectric effect. It is equal to the metal’s work function divided by Planck’s constant.

Work Function: The minimum energy required to remove an electron from a metal’s surface.

3: Waves

Amplitude: A wave’s maximum displacement from its equilibrium position.

Antinode: A position of maximum displacement in a stationary wave.

Cladding: A protective layer on an optical fibre to improve the tensile strength of the fibre, prevent scratching and to prevent signal transfer between adjacent fibres.

Coherence: Waves are coherent if they have the same wavelength and frequency, as well as there being a fixed phase difference between them.

Diffraction Grating: A grating with hundreds of slits per millimetre, that results in sharper interference patterns. They are used to calculate atomic spacing and to analyse elements.

Diffraction: The spreading of waves as they pass through a gap of a similar magnitude to their wavelength.

Electromagnetic Waves: Waves that consist of perpendicular electric and magnetic oscillations.

Frequency: The number of waves that pass a point in a unit time period. It is the inverse of the time period.

Fringe Spacing: The distance between two adjacent bright fringes or two adjacent dark fringes.

Interference: The name given to the superposition of waves that occurs when two waves meet. If the waves are in phase they will constructively interfere, but if they are out of phase, they will destructively interfere.

Laser: A light source that produces a coherent beam.

Longitudinal Wave: A wave with oscillations that are parallel to the direction of energy propagation. Sound waves are an example of a longitudinal wave.

Material Dispersion: Waves of different wavelengths travel at slightly different speeds through an optical fibre and so reach the end of the fibre at slightly different times, causing pulse broadening. The use of monochromatic light fixes this.

Modal Dispersion: Waves enter an optical fibre at slightly different angles, meaning the distance each beam has to travel is slightly different. This leads to the beams reaching the end at different times and so causes pulse broadening.

Node: A position of minimum displacement in a stationary wave.

Optical Fibre: A thin glass fibre through which signals are passed through. Optical fibres usually have cladding surrounding them.

Path Difference: A measure of how far ahead a wave is compared to another wave, usually expressed in terms of the wavelength.

Phase Difference: The difference in phase between two points on a wave. It is usually expressed in radians.

Phase: A measure of how far through the wave’s cycle a given point on the wave is.

Polarisation: The restriction of a wave so that it can only oscillate in a single plane. This can only occur for transverse waves.

Pulse Broadening: The elongation of a signal passed down an optical fibre, commonly due to modal or material dispersion.

Refractive Index: A material property that is equal to the ratio between the speed of light in a vacuum, and the speed of light in a given material.

Snell’s Law: A law linking a wave’s angle of incidence to its angle of refraction, with the use of the refractive indexes of the mediums involved.

Speed: The product of a wave’s frequency and wavelength.

Stationary Wave: A wave that stores, but does not transfer, energy.

Total Internal Reflection: An effect that occurs in optical fibres, where full reflection occurs at the inside boundary of the fibre, meaning no radiation passes out.

Transverse Wave: A wave with oscillations that are perpendicular to the direction of energy propagation. Electromagnetic waves are examples of transverse waves.

Wavelength: The distance between two identical positions on two adjacent waves. It is commonly measured from peak to peak or trough to trough.

Young’s Double-Slit Experiment: An experiment that demonstrates the diffraction of light by passing monochromatic light across two narrow slits and observing the resulting pattern of bright and dark fringes.

4: Mechanics and Materials

Breaking Stress: The maximum stress that an object can withstand before failure occurs.

Brittle: A brittle object will show very little strain before reaching its breaking stress.

Centre of Mass: The single point through which all the mass of an object can be said to act.

Conservation of Energy: Energy cannot be created or destroyed - it can only be transferred into different forms.

Conservation of Momentum: The total momentum of a system before an event, must be equal to the total momentum of the system after the event, assuming no external forces act.

Couple: Two equal and opposite parallel forces that act on an object through different lines of action. It has the effect of causing a rotation without translation.

Density: The mass per unit volume of a material.

Efficiency: The ratio of useful output to total input for a given system.

Elastic Behaviour: If a material deforms with elastic behaviour, it will return to its original shape when the deforming forces are removed. The object will not be permanently deformed.

Elastic Collision: A collision in which the total kinetic energy of the system before the collision is equal to the total kinetic energy of the system after the collision.

Elastic Limit: The force beyond which an object will no longer deform elastically, and instead deform plastically. Beyond the elastic limit, when the deforming forces are removed, the object will not return to its original shape.

Elastic Strain Energy: The energy stored in an object when it is stretched. It is equal to the work done to stretch the object and can be determined from the area under a force-extension graph.

Equilibrium: For an object to be equilibrium, both the resultant force and resultant moment acting on the object must be equal to zero.

Hooke’s Law: The extension of an elastic object will be directly proportional to the force applied to it up to the object’s limit of proportionality.

Impulse: The change of momentum of an object when a force acts on it. It is equal to the product of the force acting on the object and the length of time over which it acts.

Inelastic Collision: A collision in which the total kinetic energy of the system before the collision is not equal to the kinetic energy of the system after the collision.

Moment: The product of a force and the perpendicular distance from the line of action of the force to the pivot.

Momentum: The product of an object’s mass and velocity.

Newton’s First Law: An object will remain in its current state of motion, unless acted on by a resultant force. An object requires a resultant force to be able to accelerate.

Newton’s Second Law: The sum of the forces acting on an object is equal to the rate of change of momentum of the object.

Newton’s Third Law: Every action has an equal and opposite reaction. If an object exerts a force on another object, then the other object must exert a force back, that is opposite in direction and equal in magnitude.

Plastic Behaviour: If a material deforms with plastic behaviour, it will not return to its original shape when the deforming forces are removed. The object will be permanently deformed.

Principle of Moments: For an object to be in equilibrium, the sum of the clockwise moments acting about a point must be equal to the sum of the anticlockwise moments acting about the point.

Scalar: A scalar quantity is one that only has a magnitude. Examples include length, mass and temperature.

Spring Constant: The constant of proportionality for the extension of a spring under a force. The higher the spring constant, the greater the force needed to achieve a given extension.

Stiffness: A measure of how difficult it is to stretch a given object.

Tensile Strain: The ratio of an object’s extension to its original length. It is a ratio of two lengths and so has no unit.

Tensile Stress: The amount of force acting per unit area. Its unit is the Pascal (Pa).

Terminal Speed: The maximum speed of an object that occurs when the resistive and driving forces acting on the object are equal to each other.

Vector: A vector quantity is one that has both a magnitude and a direction. Examples include velocity, displacement and acceleration.

Young Modulus: The ratio of stress to strain for a given material. Its unit is the Pascal (Pa).

5: Electricity

Ammeter: A device that measures the current in the loop of the circuit that it is connected in series with. An ideal ammeter is modelled to have zero resistance.

Current: The rate of flow of charge in a circuit.

Electromotive Force: The amount of energy transferred by a source, to each unit of charge that passes through it.

Internal Resistance: The resistance to the flow of charge within a source. Internal resistance results in energy being dissipated within the source.

Light Dependent Resistor: A light sensitive semiconductor whose resistance increases when light intensity decreases.

Ohmic Conductor: A conductor for which the current flow is directly proportional to the potential difference across it, when under constant physical conditions.

Ohm’s Law: The current and potential difference through an ohmic conductor held under constant physical conditions are directly proportional, with the constant of proportionality being resistance.

Parallel Circuits: Components are said to be connected in parallel when they are connected across each other (separate loops).

Potential Divider: A method of splitting a potential difference, by connecting two resistors in series. The total potential difference is split in the ratio of their resistances.

Resistance: A measure of how difficult it is for current to flow through a material.

Resistivity: A quantity that is proportional to an object’s resistance and cross-sectional area, and inversely proportional to the object’s length.

Resistors in Parallel: The potential difference across resistors connected in parallel is identical for each resistor. The current is split between the resistors. The total resistance is equal to the inverse of the sum of the inverses of the resistances of the resistors.

Resistors in Series: The current through resistors connected in series is identical for each resistor. The potential difference is split in the ratio of their resistances. The total resistance is equal to the sum of the resistances of the resistors.

Series Circuits: Components are said to be connected in series when they are connected end to end (in one loop).

Superconductor: A material which has zero resistivity when the temperature is decreased to, or below, the material’s critical temperature. Superconductors can be used to produce strong magnetic fields and reduce energy loss when transmitting electric power.

Terminal Potential Difference: The potential difference across the terminals of a power source. It is equal to the source’s emf minus any voltage drop over the source’s internal resistance.

Thermistor: A temperature sensitive semiconductor whose resistance increases when temperature decreases.

Voltmeter: A device used to measure the potential difference across components. An ideal voltmeter is modelled to have infinite resistance.

6.1 Further Mechanics

Angular Speed: A measure of the speed of an object’s angular rotation. It is equal to the frequency of rotation multiplied by 2π.

Centripetal Acceleration: The acceleration of an object moving in circular motion. Any object in circular motion must have an acceleration since the direction of the object, and therefore the velocity of the object, is constantly changing.

Centripetal Force: The resultant force responsible for an object moving in circular motion. Centripetal forces always act towards the centre of the object’s rotation.

Critical Damping: The form of damping that reduces the displacement of an oscillating object to its equilibrium position in the quickest time possible and without further oscillation.

Damping: The dissipation of energy from an oscillating system. The consequence is that the amplitude of oscillation will decrease. Damping occurs when a force opposes the system’s motion.

Forced Vibrations: Repeated up and down oscillations, at the frequency of a driver. The amplitude of oscillation is small at high frequencies and large at low frequencies.

Free Vibrations: Oscillations that are not caused by a driver. An object will naturally oscillate at its natural frequency.

Overdamping: A type of damping where the system is damped more than required to stop the oscillations. It takes longer for the system to return to equilibrium than for critical damping.

Radian: A unit of angle.

Resonance: Resonance occurs when the frequency of oscillations is equal to the natural frequency of the oscillating system. The rate of energy transfer is at a maximum during resonance.

Simple Harmonic Motion: Motion where the acceleration of an object is directly proportional, and in the opposite direction, to its displacement.

Underdamping: A type of damping where energy is gradually removed from the system and the amplitude of oscillations slowly decreases.