OCR Physics A Electrons, Waves & Photons (G482) definitions

Definitions for OCR Physics A – G482 Electrons, Waves & Photons

Quantum physics 

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Quantum physics


coulomb – the charge transferred by a 1 ampere current in one second
volt – 1 joule per coulomb
ohm – the resistance when 1 volt of potential difference produces 1 ampere of current
kilowatt-hour (kW h) – the energy transferred by a 1 kW device in a time of 1 hour
electronvolt (eV) – the energy transferred by an electron travelling through a potential difference of 1 V



electric current – flow of charge per unit time
number density – the number of charged particles per unit volume
resistance = \frac{\mbox{potential difference}}{\mbox{current}}
resistivity  = \frac{\mbox{resistance} \times \mbox{cross-sectional area}}{\mbox{length}}
power – work done per unit time

potential difference – energy transferred per unit charge from electrical energy into other forms (e.g. light or heat)
electromotive force – energy transferred per unit charge into electrical energy from other forms (e.g. chemical)
internal resistance – some energy is lost as heat in the battery; it behaves as if it has an internal resistance
terminal p.d. – the potential difference measured across the terminals of an e.m.f. source

displacement – distance moved from equilibrium of a point on a wave
amplitude – maximum displacement
wavelength – the distance between two adjacent peaks and troughs
period – the time taken for one complete oscillation of a particle
phase difference – the fraction of a cycle (measured in degrees) between the oscillations of two particles
path difference – the difference in the distances travelled by two waves from coherent sources at a particular point

frequency – number of wavelengths passing a point per unit time
speed of a wave – distance travelled by the wave per unit time

work function – minimum energy to release an electron from the surface of a metal
threshold frequency – the minimum frequency of electromagnetic radiation that will release an electron from the surface of a metal



conventional current – moves from positive to negative terminals of battery
electron flow – moves from negative to positive terminals of battery
Kirchoff’s first law – sum of current into a junction equals the sum of current out; charge is conserved
Kirchoff’s second law – sum of e.m.f.s around a loop = sum of p.d.s around the same loop; energy is conserved
mean drift velocity – the average speed of charged particles along the length of a conductor
Ohm’s law – for a metallic conductor at a constant temperature, the voltage is proportional to the current

progressive wave – a transfer of energy as a result of oscillations
stationary wave – a wave which stores energy in pockets
longitudinal wave – displacement of particles is parallel to direction of energy transfer
transverse wave – displacement of particles is perpendicular to direction of energy transfer
reflection – the bouncing back of a wave from a circuit
refraction – the change in direction of a wave as it crosses an interface between two materials where its speed changes
diffraction – wavefronts spread out after passing through a gap or around an obstacle
intensity – power per unit area
plane polarised wave – a transverse wave that vibrates in one plane only
superposition – when two or more waves meet, the resultant displacement is equal to the sum of the displacements of each wave
interference – when two waves superpose at a point and there is a change in overall displacement
constructive interference – when two waves reinforce to give increased amplitude
destructive interference – when two waves cancel to give reduced amplitude
coherence – constant phase difference between waves
node – where the amplitude is always zero
antinodes – where the amplitude of the standing wave takes the maximum possible value

photon – a packet of energy
photoelectric effect – emission of electrons from a metal surface when photons are incident on the surface
line spectrum – light emitted from atoms to produce a series of lines against a dark background


Approximate wavelengths of electromagnetic radiation

Gamma – 10^-16 to 10^-10
X-rays – 10^-13 to 10^-8
Ultraviolet – 10^-8 to 4 \times 10^-7
Visible – 4 \times 10^-7 (violet) to 7 \times 10^-7 (red)
Infrared – 7 \times 10^-7 to 10^-3
Microwaves – 10^-3 to 10^-1
Radio waves – 10^-1 to >10^6