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

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#### Units

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

#### Quantities

electric field strength – force per unit positive charge
magnetic flux density 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

#### Concepts

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 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