Paper 1 key definitions

• 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.
• 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.
• Stopping Potential: The minimum potential difference required to stop the highest kinetic energy electrons from leaving the metal plate in the photoelectric effect.
• Threshold Frequency: The minimum frequency of photons required for photoelectrons to be emitted from the surface of a metal plate through the photoelectric effect.
• Work Function: The minimum energy required to remove an electron from a metal’s surface.
• 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 speed, wavelength, and frequency, as well as there being a fixed phase difference between them.
• Diffraction: The spreading of waves as they pass through a gap of a similar magnitude to their wavelength.
• 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.
• Path Difference: A measure of how far ahead a wave is compared to another wave, usually expressed in terms of the wavelength.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Young Modulus: The ratio of stress to strain for a given material. Its unit is the Pascal (Pa).
• Electromotive Force: The amount of energy transferred by a source, to each coulomb 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.
• 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.
• 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.
• 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.
• 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.