OCR Physics A Fields, Particles and Frontiers of Physics (G485) definitions

Learn and practice your knowledge of these definitions using Quizlet.


tesla – SI unit for magnetic flux density; equivalent to 1 newton per ampere per metre
weber – SI unit for magnetic flux
farad – coulomb per volt
parsec – distance that gives a parallax angle of 1 arcsecond



electric field strength – force per unit positive charge
magnetic flux densityB = \frac{F}{IL}
magnetic flux – magnetic flux density x area (perpendicular to field direction)
magnetic flux linkage – magnetic flux x the number of turns
capacitance – charge per unit potential difference
time constant of a circuit – time taken for current/charge/voltage of a discharging capacitor to fall to 37% of its original value (equal to the product of capacitance and resistance
proton number – number of protons in the nucleus of an atom (a.k.a. atomic number)
nucleon number – number of neutrons and protons in the nucleus of an atom (a.k.a. mass number)
activity – the rate of decay of nuclei in a radioactive sample
decay constant – probability of a decay of a nucleus per unit time
half life – the mean time taken for half the number of active nuclei in a radioactive sample to decay
binding energy – minimum energy to separate protons and neutrons of a nucleus
intensity – power per unit cross-sectional area



Faraday’s law of electromagnetic induction – induced e.m.f. is proportional to the rate of change of magnetic flux
Lenz’s law – the induced current or e.m.f. is in a direction so as to produce effects which oppose the change producing it
isotope – a version of an element with a different number of neutrons (but the same number of protons and electrons)
Doppler effect – the change in the frequency and wavelength of a wave caused by the relative movement between source and detector
piezoelectric effect – application of a p.d. across a crystal causes a vibration
Compton effect – x-ray photon interacts with electron; the scattered photon has a longer wavelength and the electron is ejected
Olber’s paradox – if Universe is static and infinite, the night sky should be uniformly bright because every line of sight would end on a star
Hubble’s law – the speed of recession of a galaxy is proportional to its distance
cosmological principle – universe has the same large-scale structure when observed from any point (homogenous, isotropic and the laws of physics are universal
3K microwave background radiation – gamma radiation released during the Big Bang has stretched as the Universe expanded, causing it to become microwave with an energy equivalent to a temperature of 3K
critical density – the density for which the universe will expand towards a finite limit (resulting in a flat universe)
open universe – universe will expand forever
flat universe – universe will expand towards a finite limit
closed universe – universe will contract (to a big crunch)

nature of radioactive decay:

  • random – cannot predict when or which nucleus will decay next
  • spontaneous – decay cannot be induced

nature and range of fundamental forces acting on nucleons:

  • electrostatic force – repulsive between protons, no effect on neutrons; long-ranged
  • gravitational force – attractive; long-ranged
  • strong nuclear force – attractive; short-ranged

principals of positron emission tomography (PET):

  • a positron emitting tracer is used
  • positron annihilates with an electron inside the patient
  • this produces two gamma photons travelling in opposite directions
  • patient is surrounded by a ring of gamma detectors
  • arrival times of photons indicates location of tumour
  • 3D image is created by a computer

principals of a CAT scan:

  • x-ray beam passes through patient at different angles, using a thin fan-shaped beam
  • images of ‘slices’ through patient are produced by a computer
  • x-ray tube moves along the patient
  • produces 3D image and provides better contrast between different soft tissues than a conventional x-ray

principals of ultrasound scanning:

  • a piezoelectric crystal is used to send pulses of ultrasound into the patient
  • ultrasound waves are reflected at boundaries of tissue
  • the intensity of the reflected signal depends on the acoustic impedances
  • the time delay is used to determine the depth

principles of magnetic resonance:

  • some nuclei behave as small magnets when placed in a strong magnetic field
  • the frequency of precession is know as Lamor frequency
  • radio frequency pulses are applied which produce resonance
  • when radio frequency pulse is turned off, nuclei relax and emit RF signal
  • RF signal is detected by a coil receiver and processed
  • the magnetic field is non-uniform to locate position of the nuclei in body
  • different tissues have different relaxation times for the hydrogen nuclei

formation of a star:

  • gas cloud drawn together by gravitational forces (gravitational collapse)
  • GPE is converted to KE, causing temperature to increase
  • hydrogen nuclei (protons) fuse to make helium + energy
  • a stable star is formed when the radiation pressure is equal to the gravitational pressure

probable evolution of the Sun:

  • when hydrogen runs out, the outer layers of the star expand
  • a red giant is formed
  • the outer layers are shed, leaving behind a white dwarf

probable evolution of a star much more massive than the Sun:

  • supernova
  • core becomes either a neutron star or black hole

evolution of universe from 10^{-43} seconds after the Big Bang to the present:

  • hot, dense singularity
  • all forces were unified
  • expansion led to cooling
  • quark and lepton soup
  • more matter than antimatter
  • quarks combine to form hadrons (protons and neutrons)
  • helium formed from imbalance of protons and neutrons
  • atoms formed
  • gravitational force forms stars/galaxies
  • temperature becomes 2.7K and the universe is saturated with microwave background radiation

evidence for the Big Bang:

  • spectra from galaxies show shift to longer wavelengths (redshift), which suggests galaxies are moving away from Earth
  • the more distant galaxies are moving faster
  • existence of microwave background radiation; the temperature of the universe is 3K; gamma radiation stretched to become microwaves as the Universe expanded
  • existence of primordial helium
  • temperature fluctuations are predicted and observed