Unit 2

• Electric current


  The rate of flow of electric charge
• Unit of electric current

  A (ampere)
• Efficiency of a system

  Useful work (or energy) out / Work (or energy) put in x 100%
• Potential difference (pd)


  The pd between two points is the energy converted from electrical potential energy to some other form per coulomb of charge flowing from one point to the other
• Unit of potential difference

  V (volt)
• Ohm's law


  The current in a metal wire at constant temperature is proportional to the pd across it
• Electrical resistance


  The pd divided by the resulting current
• Resistivity


  The resistance, R, of a metal wire of length l and cross-sectional area A is given by R = ρl/A, where ρ is a constant at constant temperature for the material of the wire
• Superconducting transition temperature


  The temperature at which a material when cooled loses its normal resistance and becomes superconducting. Some materials (e.g. copper) never become superconducting however low the temperature becomes.
• Law of conservation of charge


  Charge can be treated as if it can neither be created nor destroyed (though positive and negative charges can neutralise each other)
• Charge conservation


  Charge cannot pile up at a point in a circuit
• Electromotive force (emf)


  The energy converted from some other form (e.g. chemical) to electrical potential energy per coulomb of charge flowing from the source
• Unit of electromotive force

  V (volt)
• Progressive wave


  A pattern of disturbances traveling through a medium and carrying energy, moving the particles of the medium oscillating about equilibrium
• Longitudinal wave


  A wave where the particle oscillations are in line with (parallel to) the direction of travel (or propagation) of the wave
• Transverse wave


  A wave where the particle oscillations are at right angles to the direction of wave propagation
• Polarised wave


  A transverse wave in which particle oscillations occur in only one of the directions at right angles to the direction of wave propagation
• In phase


  Waves arriving at a point are said to be in phase if they have the same frequency and are at the same point in their cycles at the same time
• Wavelength of a progressive wave

  The minimum distance (measured along the direction of propagation) between two points on the wave oscillating in phase
• Frequency of a wave

  The number of cycles of a wave that pass a given point in one second, or equivalently the number of cycles of oscillation per second performed by any particle in the medium through which the wave is passing
• Speed of a wave

  The distance that the wave profile moves per unit time
• Diffraction


  The spreading out of waves when they meet obstacles, such as the edges of a slit. Some of the wave's energy travels to the geometrical shadows of the obstacles.
• Principle of superposition


  Waves from two sources or traveling by different paths from the same source occupy the same region, then the total displacement at any one point is the vector sum of their individual displacements at that point
• Phase difference


  The difference in position of 2 points within a cycle of oscillation, given as a fraction of the cycle or as an angle, where one whole cycle is 2π or 360°, together with a statement of which point is ahead in the cycle
• Coherence


  Waves or wave sources, which have constant phase difference between them (and therefore must have the same frequency) are said to be coherent
• Stationary (or standing) wave

  A pattern of disturbances in a medium, in which energy is not propagated. The amplitude of particle oscillation is zero at equally-spaced nodes, rising to maxima at antinodes, midway between the nodes.
• Refractive index


  For light, Snell's law may be written: n1 sin θ1 = n2 sin θ2, where θ1 and θ2 are angles to the normal for light passing between medium 1 and medium 2, and n1 and n2 are called the refractive indices of medium 1 and medium 2 respectively. The refractive index of a vacuum is fixed by convention as exactly 1. For air, n ≈ 1.000
• Snell's law


  At the boundary between any two given materials, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant
• Critical angle, θc


  When light approaches the boundary between two media from the 'slower' medium, the critical angle is the largest angle of incidence for which refraction can occur. The refracted wave is then traveling at 90° to the normal.
• Photoelectric effect


  When light or ultraviolet radiation of short enough wavelength falls on a surface, electrons are emitted from the surface
• Work function


  The minimum energy needed to remove an electron from the surface. Unit: J or eV
• Electron volt (eV)


  The energy transferred when an electron moves between two points with a potential difference of 1V between them. 1 eV = 1.60 x 10^-19 J
• Ionisation


  The removal of one or more electrons from an atom
• Ionisation energy


  The minimum energy needed to remove an electron from the atom in its ground state. Unit: J
• Stimulated emission


  The emission of a photon from an excited atom, triggered by a passing photon of energy equal to the energy gap between the excited state and a state of lower energy in the atom. The emitted photon has the same frequency, phase, direction of travel and polarisation direction as the passing photon.
• Population inversion


  A situation in which a higher energy state in an atomic system is more heavily populated than a lower energy state (e.g. a less excited state or the ground state) of the same system
• Pumping


  Feeding energy into the amplifying medium of a laser to produce a population inversion